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ATV Won’t Start – Mechanics insider tips that actually work

There’s never a good time to break down but not to worry, I’m a mechanic, and very shortly we’ll have this figured out.

6 Common reasons an ATV won’t start, include:

  1. Flat battery – jump or charge battery
  2. Loose battery terminals – tighten terminals
  3. Fouled spark plug – clean or replace plug
  4. Flooded engine – dry out cylinder
  5. Stale gas – drain and refill
  6. Faulty starter solenoid – test & replace

In this post, you’ll learn how to fault find your no-start ATV like a pro. We’ll cover diagnosis and you’ll learn some insider tips and hacks to speed up the process of both diagnosis and repair.

ATV in field

Mechanics advise for fault finding ATV

A mechanic/technician uses two main strategies when fault finding. We typically use the Common causes approach and the top-down approach.

Now it should be noted, that many latest bikes store fault codes in their ECU (on-board computer) and if your bike is fuel-injected and if a problem is flagged by the ECU, that fault code will be stored and is accessible. Reading these fault codes is obviously a great place to start your diagnosis.

As each manufacturer has a slightly different method for reading and interpreting their fault codes, it is best to google your make and model first. Typically, connecting a fault code harness wire causes the ECU to communicate the codes by flashing dash lights a number of times.

All that said you can still use this article to fault find both carburetor and fuel injected bikes without reading the fault codes.

1 Common Causes Approach

ATV wont start Infograph

The common causes approach is self-explanatory. Most technicians look at the symptoms of a problem ATV and using their experience (in our case – common ATV no-start causes) set about testing their hypothesis.

Most often, checking the common causes yields success and for obvious reasons, but every so often you’ll meet an exception, you’ll exhaust all the common causes with no luck and so the diagnosis must take our second approach, the top-down approach.

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2 Top-Down Approach

A top-down approach is a systematic approach to fault finding. A technician still reads symptoms to direct their firepower but thereafter begins the test and evaluation process until the fault yields.

This type of fault-finding is hard work and often time-consuming and that’s why we’ll look at the common causes approach first before moving to test from the top down.

So let’s go ahead and start by checking those common causes right now.

1 Common No Start ATV Causes

Common causes obviously vary by make and model. Many ATVs, for example, employ auto choke, and most late models are fuel injected and so eliminate a ton of common faults associated with carburetor models, but at the same time, open up new common cause territories.

There is some overlap between the carburetor and fuel-injected bikes, but as their fueling system are different, we’ll deal with those systems separately. Remember, the list below isn’t a complete list of no start causes, just the common ones. We deal with a more complete list later in the second approach.

Common No-start ATV causes, symptoms & fix (carburetor & fuel injected)

Common FaultTypical SymptomFix
Flat batteryRepeated clicking & no-startJump or charge battery
Faulty batteryRepeated clicking & no-startTest & replace battery
Loose battery terminalRepeated clicking & no-startTighten battery terminals
Dirty battery terminalRepeated clicking & no-startClean battery terminals
Flooded engineNo-start & stink of gasDry out cylinder
Dirty air filterNo-start & floodingClean or replace filter
Fouled spark plugNo-start or misfiring engineClean, gap or replace plug
Bad gasNo-start, hard start, no powerDrain & refill gas
Faulty starter solenoidSingle click sound, no crankReplace solenoid
Faulty coilCrank, no spark & no-startTest & replace coil
Faulty pick-up sensorCrank, no spark & no-startTest & replace pick-up
Faulty statorCrank, no spark & no-startTest & replace stator

Common (fuel injected) No-start ATV causes, symptoms & fix

Common FaultTypical SymptomFix
Bad gasNo-start & stallingDrain & replace
Faulty fuel injectorNo-start & stallingTest & replace
Faulty fuel pumpNo-start & stallingTest & replace
Faulty throttle position sensorNo-start & stallingTest & replace

Common (carburetor) No-start ATV causes, symptoms & fix

Common FaultTypical SymptomFix
Bad gasNo-start & stallingDrain & replace
Faulty gas valveNo start & floodingReplace valve
Faulty gas capNo-start & stallingReplace cap
Dirty carburetorNo-start & stallingClean carburetor
Faulty carburetorNo-start & stallingReplace
Faulty fuel pumpNo-start & stallingReplace

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2 Top-Down ATV Troubleshooting

If the common causes didn’t work out for you, not to worry the top-down approach and some patience will force a submission. We’ll need a few test tools along the way, but I’ll let you know what they are and, of course, how to use them.

Check a fact is a fact

As a technician, I’ve worked for GM, VW, Landrover, Jaguar, and Volvo dealers, and all their dealer product training employs a very similar concept. When presented with a reported problem – page one, step one – “Check the reported problem is in fact a problem”

In many cases, the error isn’t with the machine, it’s the operator, and so in the next section, we’ll quickly check that all those facts are in fact, facts.

Easy fix No-Start ATV Causes

Let’s start by looking at some of the easy-to-fix causes of ATV no-starts. In many cases, as said, a small oversight by the operator is the root cause. I know you know how to start your ATV; you’ve likely started it hundreds of times. But this step won’t take but a moment, and then we can move on and do some proper diagnosis, promise!

Okay, let’s quickly eliminate some common operator errors as root causes of our no-start.

Check the following:

Filling ATV with gas

Gas level – Gas in the tank is obviously crucial; running out of gas is common and has caught us all out. Yes, even mechanics!

Kill switch – The kill switch, as you know, shuts down the motor. The switch must be set to “Run” in order for the engine to restart.

Check your kill switch is set to “Run”.

ATV kill switch

Choke use – A choke adds extra gas and is required to start a cold engine. Some ATVs employ a manual control and if that’s yours, it will need to be set to the “ON” position.

ATV Choke

Many modern bikes employ auto choke, and if your bike is fuel injected, (lucky you), cold start fueling is automatic; it’s controlled by the onboard computer, and no action is required.

Correct starting procedure – ATVs, as you know, employ a safety lockout procedure, meaning if you don’t follow a set starting procedure, the engine won’t crank over or start.

Typically, the bike transmission needs to be in neutral; some will require a clutch held and some the brake applied.

Check your make, and model starting procedure online.

ATV with Neutral light on dash

Gas tap “On” – The gas tap is also known as Petcock or Gas valve. Most bikes are fitted with a gas tap, when in use it prevents gas flow from the tank to the engine. Commonly the tap is turned off when a bike is parked up.

ATV Petcock

Check Petcock is set to “On”.

OK, that’s the low-hanging fruit out of the way, now let’s do a little detective work.

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ATV No-start Pre-diagnosis Detective Work

ATV no start detective work

I’ve been a mechanic for over twenty-five years, and I’ve found when fault finding you can learn a ton and get a head start on fixing the problem without even looking at the machine.

Simply taking a few moments to listen to the customer explain the issue and asking a few probing questions often sets us on the correct path.

The main question to consider is – What were the factors at play prior to the engine stalling out or not restarting? (whichever is applicable)

For example:

  • Did you just refuel? – gas could be stale or contaminated
  • Bike won’t start when hot? – engine could be flooded by excessive choke use
  • Bike won’t start when cold? – choke or temperature sensor (fuel injected bikes) may be at fault
  • Did you drive through water? – the electrics could be wet
  • Has the bike been laid up for a time (month plus)? – gas tap may be off or the fuel may be stale
  • Have you added mods, like exhausts, free flow air filter or wired accessories, etc.? – some performance mods will require carburetor fuel adjustment
  • Have you been working on the engine when it quit? – carburetor tuning, engine timing, and valve lash are all common no start causes after engine repairs/adjustments/modifications

Having spent a few moments considering the above, we’ll assume for now it wasn’t fruitful, and so we’ll move to the next phase where we’ll attempt to broadly identify and label the root cause of your ATV no-start.

What do you Hear when you Hit ATV Start Button?

When diagnosing, I find it helpful to broadly categorize symptoms and assign them to labeled buckets, metaphorically speaking.

ATV start sound

Within these buckets, I’ll make a list of the various components that are a likely root cause of that buckets symptom (label).

Thinking in these terms helps to exclude a broad range of issues and conversely causes us to focus on a narrower more likely range of possible causes.

Each bucket, as said will contain the most likely components to cause issues and we’ll look at all these in more detail a little later. But for now, let’s go ahead and categorize your ATV starting symptom so that it fits neatly into one of our five buckets.

And to do that successfully, we’ll need to pay particular attention to what noise your ATV makes when we attempt to start the engine, ie crank the engine over. For those that aren’t familiar with the term “crank engine over” – it describes the cycling (also known as turning over) of the engine as the start button is pressed.

Silver bucket

Go ahead and categorize your symptom, assign it to one of the following buckets and begin diagnosing.

Your engine makes one of the following noises when the start button is pressed:

Bucket 1 – ATV Engine makes a REPEATED clicking sound

ATV battery test

If your engine makes a repeated clicking sound when you hit the start button, then consider yourself lucky.

This type of issue is common and is by far one of the easiest faults to diagnose and fix.

The most likely issue is a flat battery, but wow there! Let’s now jump to conclusions, there are some other possibilities, but all the fixes for this symptom are easy, and frankly, I’m jealous!

Faults are listed below in order of how likely they are. Follow each highlighted link, in turn, to diagnose and eliminate it from the list of possible issues, rinse and repeat until you nail it.

Battery terminals looseCheck terminalsTighten
Battery terminals dirtyCheck terminalsClean
Battery flatCheck voltageJump or charge
Battery faultyCrank testReplace

Battery Terminals Loose

Loose battery terminals

Loose terminals are common and really easy to diagnose and fix.

Diagnose – Access the battery (usually under the seat) and wiggle the terminals.

Tools: Adjustable wrench

Fix – If they are loose, tighten them – problem solved.

Battery Terminals dirty

Dirty terminals are another really common issue and almost as easy to fix as loose battery terminals.

An acid weep from the battery combined with arcing causes battery terminal corrosion.

And corrosion causes resistance to the flow of power from the battery to the starter motor. The dirty terminals conversely prevent alternator voltage recharging the battery.

Battery corrosion

Diagnose – It’s easy to spot, it forms a white crusty deposit on the terminal.


  • Disposable gloves
  • Eye protection
  • Baking soda and water
  • Rag
  • Small adjustable wrench
  • Sandpaper or wire brush
  • Petroleum jelly

Fix – The fix is simple, remove the corrosion. But wow, not so fast, acid will burn the skin and eyes, we’ll need to neutralize it first.

The whole process is as follows:

  • Add a tablespoon of baking soda to some water and work into a paste.
  • Using an old cloth and gloves or brush add the paste to the terminal
  • Allow a few minutes for the paste to work its magic before wiping clean
  • Remove battery terminals (need a wrench)
  • Using sandpaper or a wire brush clean the terminals
  • Refit the terminals and tighten
  • Add a coat of petroleum jelly to terminals (prevents corrosion)

Battery Flat

We’ve all suffered a flat battery at some point in time. Sometimes it’s all on us (leaving the ignition switch on), but more commonly, it’s because either the battery hasn’t been maintained correctly or the battery is in the early stages of complete failure.

Either way, we’ll need to check the voltage and charge the battery back up. A flat battery obviously won’t start your engine, but we can’t test a flat battery either.


  • To test the battery we’ll need a DVOM (voltmeter)
  • To charge the battery we’ll need a battery charger
  • If you wish to boost start we’ll need a jump pack or jumper cables

You’ll find all these tools here on the ATV tools page.

Diagnose – A repeated clicking sound from the engine is a pretty good indication we have a battery issue. Of course, no sound at all is another symptom of a totally flat battery. To confirm this, even without breaking out a voltmeter, you could turn the lights on full; if they are dim, you can be sure your battery charge is low.

Next, we’ll use a voltmeter to confirm this; however, if you just want to get rolling, you can jump ahead to battery charging or, for immediate results jumpstarting.

Battery volt check

Checking ATV battery voltage with DVOM (voltmeter) as follows:

  • Set meter to 20 DC
  • Red probe on + battery terminal
  • Black probe on – battery terminal
  • Read meter

A voltage reading below 12.4 volts may struggle to start your engine, especially in cold weather. Batteries don’t like laying idle either, they much prefer being used ie discharged, and recharged, it keeps them vibrant.

If you park your ATV up for weeks on end, I’d advise using a battery maintainer, also known as a smart charger. Fit them to the battery plug them in and forget them. They turn on and off automatically and use little energy. A good charger will save you money in the longer game. You can check out the smart charger I use here on the ATV tools page.

Fix: The fastest way to get rolling again is jumpstarting. Starting the vehicle will use your ATVs alternator to charge the battery. But you should note, if the battery is faulty or the charging system is at the root cause of the flat battery you’ll be visiting this problem again. After charging the battery it’s best to have it tested, and we cover that below.

To jumpstart you will require a donor vehicle, it doesn’t need to be another ATV, any 12-volt vehicle will work. A car, truck, motorbike, tractor mower, RV, etc all run a regular 12-volt system.

If in doubt check the battery casing where it states the battery voltage. (12 volt systems are referred to as 12 volts but actually are 12.65 volts).


The jumpstart process is as follows:


Connect cables as per diagram:

  • Connect 1,2,3 and 4
  • Start engine
  • Remove 4,3,2 and 1

Don’t want to jumpstart, Ok our second option is to charge the battery. Obviously, we’ll need a battery charger for this fix. Note – Many battery chargers employ a fail-safe system that won’t allow the charger to turn on if battery volts are too low.

You can check out the charger I recommend here on the ATV tools page.

Battery Charging

Connect a battery charger as follows:

  • Connect red to battery +
  • Connect black to battery –
  • Plug in charger
  • Charge for 3-4 hours (depending on battery state of charge and charger Amp rating)

To override this connect a spare battery to turn the charger on, the donor may be removed after 30 minutes and continue to charge for approximately a further 3-4 hours.

Battery charging

Battery Faulty

Batteries wear out and die, even well-maintained batteries eventually give up. batteries typically last 4 to 6 years, sure some batteries last longer but they tend to be the exceptions.


  • We’ll need a DVOM (voltmeter) to test the battery
  • We’ll need an adjustable wrench to swap out the battery

Diagnose: Diagnosing a battery isn’t difficult. It is a two-step process. We first establish the state of charge before running the step two crank test. We check the voltage (step one), first because unless the battery is charged to at least 12.5 volts, step two will not give a true state of battery health.

In this test, I use a DVOM, and a helper to read battery voltage. It’s not difficult, but it is laborious when compared to a battery test tool designed for the job. And so if you prefer to use a battery tester, I have listed an easy-to-use plug-and-play type battery tester that uses a simple traffic light system to test your battery.

You can check out the battery tester I recommend here on the ATV tools page.

Crank test is as follows:

  • Set meter to 20v DC
  • Set to MIN/Max
  • Red probe to battery +
  • Black probe to battery –
  • Helper cranks motor over
Battery crank test

A reading below 9.6v indicates a failing battery.

Fix: Replace the battery

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Back to Bucket 1

Bucket 2 – ATV Engine makes a SINGLE click sound

ATV starter solenoid

A single-click sound from the engine when you hit the start button is a common sound to many an ATV rider. It’s the sound of a faulty starter solenoid.

As problems go, a faulty solenoid is an easy fix, but just before we make the mistake of misdiagnosis let’s first test it and if it checks out OK, run the other tests in this list.

Faults are listed below in order of how likely they are. Follow each highlighted link, in turn, to diagnose and eliminate it from the list of possible issues, rinse and repeat until you nail it.

Starter solenoid faultyTestReplace
Hydro-locked engineCheckRepair
Engine restrictionCheckRemove

Starter Solenoid

A failed starter solenoid is a very common cause of a single-click sound. The solenoid is a relay and as you know they are responsible for sending power to the starter motor. Solenoids just wear out. In truth, some of the places solenoids are positioned invite trouble. Some models place them under the fender and surprise surprise they fail regularly.

Motorbike solenoid testing

Anyhow I covered testing the solenoid in greater detail previously right here – ATV will only pull start

You may find this post helpful also – ATV click no start

Hydro-locked Engine

A hydro-locked engine isn’t common but it can happen under certain circumstances. Hydro-locking is where the cylinder fills with fluid and since the fluid isn’t compressible the piston can’t move inside the cylinder.

How did the fluid get in there? Driving into the drink is one possibility, so too is a leaking carburetor float valve or a failed head gasket that allows coolant enters the cylinder.

Removing the spark plug releases the fluid, but it doesn’t fix the root cause.

I’ve covered Hydro-locking in greater detail previously, together with some other causes of an engine that’s stiff or refuses to turn over.

You can check it out right here – ATV won’t pull start

ATV cylinder water

Check out – Can ATV go in the water?

Engine Restriction

A restricted crankshaft, piston, or valvetrain will prevent the engine from cranking over. And by restriction here, I’m thinking of a major mechanical failure.

The result is a single click sound when the start button is pressed, that sound is the solenoid doing its job, but the starter doesn’t follow up by rotating the engine.

We need to find out why.

Dropped valve

Tools: Good socket and ratchet set including a plug socket and breaker bar

Rotate ATV engine by hand


  • Remove spark plug
  • Remove the engine side cover to access the crank fastener
  • If your ATV has a pull starter you won’t need to remove the pull assembly
  • Using the breaker bar & socket, attempt to rotate the engine slowly CCW or for pull starter, attempt to pull over the engine slowly with pull starter.

If the engine is locked tight, suspect engine damage, but before assuming the worst, best to remove the starter motor and repeat the test. A starter motor issue is still possible and starter motor testing is covered below in greater detail.

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Back to Bucket 2

Bucket 3 – ATV Engine cranks over but sounds SLOW

Battery volt check

This is a very common symptom associated with a no-start especially in colder weather and after an ATV has been sitting idle for a time.

The usual root cause is a battery that is low on voltage, a jumpstart from any 12-volt vehicle is the fast fix but you could also charge the battery using a battery charger.

And although this is the most likely of the slow crank, it isn’t the only possible explanation, you’ll find the other in the list below.

Faults are listed below in order of how likely they are. Follow each highlighted link, in turn, to diagnose and eliminate it from the list of possible issues, rinse and repeat until you nail it.

Battery terminals looseCheck terminalsTighten
Battery terminals dirtyCheck terminalsClean
Battery flatCheck voltageJump or charge
Battery faultyCrank testReplace
Starter motorCheckReplace
Oil level overfullCheckRemove
Incorrect oil typeCheck viscosityReplace
Incorrect timingCheckAdjust timing

Oil Level Overfull

An engine is designed to hold a set amount of oil, too much oil can cause problems. An engine requires a void inside the crankcase to deal with negative and positive crankcase pressure caused by the moving crankshaft, piston, and valvetrain.

Too much oil

Overfilling with oil then fills this critical void and causes other issues, a slow or no crank is among them.

Diagnose: Go ahead and check your oil level.

Fix: Remove excess oil

Oil Type

Oil type is important and we should always use the oil specified by the engine maker. That said if your ATV lives and works in extreme conditions you may need to move to a more suitable grade.

In colder temps, a heavier oil grade moves like treacle, and that can as you know cause engine damage, but also a slow crank speed and a no start.

Fix: Move to a lighter oil grade

Oil temp chart

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Back to Bucket 3

Bucket 4 – ATV Engine makes NO SOUND at all

Fuse type and location

This symptom has the potential to be a super easy fix or a troublesome one. I know that’s not the most helpful information but stick with me here and you’ll see what I mean.

Faults are listed below in order of how likely they are. Follow each highlighted link, in turn, to diagnose and eliminate it from the list of possible issues, rinse and repeat until you nail it.

Battery flatCheck voltsJump or charge
Main fuse blownCheckReplace
Kill switch issueTestRepair
Wiring issueTestRepair
Neutral switchTestReplace
CDI box faultEvaluateReplace

Main Fuse Blown

The main fuse is so-called because it’s the parent fuse, meaning all circuit power passes through it.

Fuse Types

If the main fuse is blown, power won’t flow.

Diagnosis: The main fuse is located close to the battery under the seat. Remove the fuse and check the element. Fuses commonly blow because there’s a short in the circuit, other reasons include incorrect fuse size (too small), failed component, or just an overloaded circuit (accessories drawing excessive amps).

Fix: If the fuse is blown replace it, be sure to use the correct fuse amp rating.

Kill Switch Issue

With the kill switch set to stop, power flow to the starter motor is blocked, and if the kill switch fails to engage in the run position, many ATVs will offer the no sound at all symptom when the start button is pressed.

As not all kill switches are wired the same, a wiring diagram would be useful. That said kill switches either open a circuit or close a circuit. So when testing, just make sure the DVOM reading changes when the switch is operated.

Tools: DVOM

Diagnose: Disconnect the switch block connector and check continuity. (Never test continuity on a live circuit)

Fix: Repair or replace the switch

Kill Switch Test

Wiring Issue

Circuit wiring issues are common in older ATVs and the best way to approach a wiring issue is with a wiring diagram and a power probe. Check out the power probe I recommend here on the ATV tools page.

As power must move in a circle, using a power probe begin by checking the power side of the circuit, (fuses, etc) then move to check the ground side of the circuit.

Broken, loose, and corroded wiring or terminals are very common types of electrical issues. So too is rodent damage, mice love wiring insulation.

Neutral Switch

A neutral switch is a switch located on the transmission and is used by the CDI/ECU to identify when the transmission is in neutral. ATVs employ a safety lock-out procedure to prevent accidental in-gear starting.

Neutral switch

The neutral switch plays a critical role, meaning if the neutral switch isn’t working the ECU/CDI may not permit starter motor activation.

Diagnose: Check if neutral light is on the dash panel when ATV is in neutral. I’ve covered neutral switch diagnosis previously and you can check that out here – Neutral light always on

Fix: Test and replace the neutral switch.

CDI Box Fault

CDIs and ECUs are difficult to diagnose. Most technicians will call them bad only after first exhausting all other possibilities. What we can check is power and ground to the modules and we can check the block terminal connector for loose block connectors, water, corrosion, or pin damage.

It should be noted, that a failed Rectifier/regulator will not only fry your battery but is also a common cause of CDI failure. If your ATV is overcharging the battery (over 15.5v) replace the rectifier/regulator.


CDIs and ECUs are pretty durable and are last on this list for good reason.

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Back to Bucket 4

Bucket 5 – ATV Engine cranks over, it sounds NORMAL, but won’t start

The list of possible no-start causes in this bucket is too large to list, and so in order to make it more manageable, this bucket contains 3 subcategories.

These categories, which we’ll call systems don’t contain a list of faults as per previous buckets, instead they contain a list of components.

The 3 systems are categorized as follows:

We use just 3 categories because an engine needs these 3 main systems to work in harmony in order for the magic to happen. If one of these systems isn’t working, you’ve guessed it, you get a no-start.

As said each system contains a ton of components on which that system relies. Below, I’ve listed the main components under each system heading. One of these components within one of the 3 systems has failed and is causing our no-start.

I know the list below is long, but don’t be disheartened, I have a secret mechanics hack to short circuit the diagnosis process, meaning you won’t need to check all 3 systems or indeed all the components within the systems. So let’s check out that hack right now.

Gas Shot Test

The gas shot test is an elimination round and answers many questions very quickly. I use this hack in the workshop all the time. The idea is simple, the results of this test will indicate which of our 3 systems has likely failed. It won’t point to the failed component within that system, we’ll still need to do some detective work. But we will at least be searching in the correct category if you like.

Gas shot test

The test is easy and here it is:

  • Remove spark plug
  • Add a small amount of gas (about oil cap size capful) (gas must be fresh)
  • Refit spark plug
  • Attempt to start the engine

Mechanic top tip – Plug holes aren’t easy to access, a funnel with an attached hose or syringe makes adding gas to the cylinder a ton easier.

Analyzing the gas shot results

With the test out of the way now we’ll need to analyze the results, and the two results most likely are as follows:

Result 1 – The engine started, or fired (fired – meaning attempted to start). Firing may be identified as the engine not running but smoke seen puffing from the tailpipe when cranking.

This result tells us your ATV likely suffers from a fuel system issue. Bad gas, Choke not working, Faulty fuel pump (if fitted), Blocked gas filter, and Carburetor faults are all high on the list of possibles.

Jump to the Fueling system below and begin your diagnosis journey.

Result 2 – The engine doesn’t start and critically makes no attempt to fire. This result tells us you may have an ignition system or a compression system issue. Since ignition system issues are more common, we’ll diagnose the ignition system first before moving on and checking the Compression system, if needed.

Begin your Ignition system diagnosis below.

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Back to Bucket 5

(I) Ignition System

The ignition system is a collection of components dedicated to producing voltage so that the spark plug can do its thing – fire the spark plug. To that end, the ignition system relies on another important subsystem – the charging system which employs a stator to generate voltage. (More on the stator later)

The ignition system isn’t a one-trick pony, it’s also responsible for adjusting the ignition timing. Meaning it adjusts when the spark is actually fired.

Adjusting the timing is important because it takes a set amount of time for an ignition system to generate a spark at the spark plug and as you know that spark must be added to the mix at a very precise point of the engine cycle.

The faster an engine rotates, the earlier the ignition system needs to begin the spark generating process so that the voltage arrives at the spark plug in good time, and remember this is a moving target, meaning ignition timing needs to be constantly adjusted according to engine RPM.

Yep, your ignition system works like a dog.

Components are listed in order of failure rate (highest first)

Batterypowers up the ignition system and powers the starter motorFlat
Spark plugencourages voltage to jump the plug gap and generate a sparkFouled
Coil & capconverts low voltage into high voltageFailed
Statorgenerates the voltage required to fire the spark plugFailed
Pick-upsignals CDI / *ECU with the correct time to fire the spark plugFailed
Safety lock -outprevents unintended in gear startsFaulty
Starter motorcranks over the engine which causes the stator to generate a voltageFailed
Ignition switchpowers the CDI / *ECU and initiates the start processFaulty
Kill switchkills the spark by either opening the ignition circuit or grounding it, depending on the designFaulty
CDI / *ECUreceives inputs, adjusts timing, and fires the plug via the coilFailed
*ECU (Engine Control Module) – fuel-injected engines

The following sections are dedicated to outlining the basic function of the components within the ignition system. We’ll also cover diagnosing and testing the various components.

When faced with a possible ignition system issue makes sense to go straight to testing the spark and we’ll cover that next in the spark plug section.

For many of the following tests, we’ll need a DVOM or power probe. You can find all these tools here on the ATV tools page.

Spark Plug

The spark plug is top of our ignition system troubleshooting list for three reasons, it’s a common cause of no starts, it’s critically important and it’s easy to check.

A plug lives a hard life, consider for a moment what it does and what it must cope with. Thousands of volts pass through it, it must withstand chemical attack, compression pressures, and extreme combustion heat. And yet despite this plugs can last years (we should change them every two years or 1500 miles, of course).

As we suspect we have a spark issue we’ll run an ignition system test. Ideally, we’d use an inline spark tester, that way the whole ignition system is tested, and we’ll cover that below, but first, we’ll check spark MacGyver style.

The MacGyver spark test doesn’t test the whole system conclusively and isn’t a great test for intermittent spark issues. But it works great for no-starts.

The MacGyver spark test is as follows:

  • Remove plug wire
  • Remove plug
  • Examine plug (see plug condition table below)
  • Refit plug wire
  • Ground plug (make contact with engine raw metal)
  • Crank engine over
  • Check spark

No spark means we have a problem, but be sure the plug is grounded securely, otherwise you’ll misdiagnose. Improvise with a jumper lead.

Now let’s check the spark using an inline ignition test tool.

In line ignition system tester


  • Remove the plug cap and fit it to the inline tester
  • Fit inline tester to the spark plug
  • Attempt to start

Checking the test tool window quickly tells us if we have a spark or not.

Reading the plug condition is another useful check.

Plug conditionCheck
Black (Rich)Air filter, Carb
White (Lean)Gas type, Vacuum leak, Gas flow blocked
OilyOil level, Mechanical issue
Wet (Flooded)Plug, Air filter, Carb, Choke, Gas quality

Spark Plug fouling is a dirty plug and it results in misfiring and no-starts. The reasons a plug becomes fouled in the first place are numerous Here are just some of the common causes of plug fouling:

  • Too much oil
  • Wrong oil type
  • Wrong spark plug
  • Bad gas
  • Engine wear
  • Vacuum leak
  • Faulty ignition system
  • Carbon build-up

Diagnose: Remove the spark plug and check

Fix: Clean & gap plug (see below)

Flooding is a condition where unburnt gas accumulates inside the cylinder and fouls the spark plug preventing it from firing. Often it’s the plug itself that is at the root of the problem, but there are tons of causes of flooding.

  • Excessive use of choke
  • Blocked air filter
  • Leaking carburetor float needle

Diagnose: Remove plug and check

Fix: Dry out cylinder, clean & gap plug (see below)

I’ve covered how to diagnose and fix a flooded ATV engine previously – ATV flooded with gas

Spark plugs should be cleaned and gapped at least once every six months. Carbon deposits build up on the plug’s electrode and are, as you know a cause of plug fouling and flooding.

Spark plug gap ATV and Dirt Bike

To clean a plug use a wire brush to clean the business end, if it’s wet use a flame to clean it or allow it to dry naturally.

Use a plug gapper or feeler gauge to adjust the gap, use pliers to either open the gap or tap it closed.

Coil & Cap

Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

Coil test

I’ve covered coil testing in greater detail previously. It’s a dirt bike but the process is identical. I test the coil primary, secondary, and resistor cap.

You can check it out here – Dirt bike won’t start when hot


Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

ATV stator resistance test diagram

I’ve covered stator testing in greater detail previously, you can check that out here – Will ATV start with bad stator


Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

Pickup test

I’ve covered pick-up testing in greater detail previously. It’s a dirt bike but the process is identical.

You can check it out here – Dirt bike won’t start when hot

Starter Motor

The starter motor plays a pivotal role in ensuring there’s enough voltage at startup. Without a strong starter motor, the stator and pick-up may not provide enough voltage to fire the spark plug. I’ve covered starter motor testing in greater detail previously and you can check that out here – ATV will only pull start

Ignition Switch

Ignition switch wiring varies from model to model, an ECU will be quite different from a CDI bike. You will require a wiring diagram for your machine to fault find an ignition system systematically.

That said here are a few tips that will help:

  • Check wiring at the steering column, flexing commonly causes wiring breaks.
  • Try the wiggle test (move the switch loom by hand and see if the condition changes)
  • Check power and grounds
  • Check block connector pins are clean, dry, and tight

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(II) Fueling System

All engines need fuel, but fuel isn’t just gas alone, because gas won’t burn without air (oxygen). And so gas must be mixed with air, problem is engines are very particular as to the ratio of air to gas. We refer to the ratio as the Air Fuel Ratio (AFR). The sweet spot is 14.7 parts air to 1 part gas and this ratio is known as stochiometric.

It’s the job of the fuelling system to maintain this ratio and deliver it to the engine for every given throttle demand.

The fuelling system your ATV employs will dictate the type of fuel system components your bike has. ATV fuelling systems come in 2 main flavors – the old-school carburetor and the latest and greatest electronic fuel injection (EFI).

While these systems have the same goal, they use different components to arrive there. We’ll take a quick look at how each type goes about it right now.

The carburetor

The old-school carburetor is a mechanical device calibrated very precisely to supply gas to the engine in proportion to the volume of air that passes through it. Carburetors are simple and complicated all at once. But don’t be fooled, carbs are well-tried and tested; while they do have some shortcomings they are generally very reliable.

ATV carburetors come in two main flavors – the mechanical slide and the vacuum-activated Constant Velocity (CV) type.

Carburetors work like this:

CV type – The CV is the more common type of carburetor as they offer better all-round performance and are cleaner to the environment. On CV-type carburetors, the throttle cable is connected to the butterfly valve and when opened creates a vacuum which in turn lifts the slide (and attached needle) and draws gas into the engine through carburetor-housed metering jets.

The main downside to CV carburetors – as they are vacuum-operated, they are less responsive to throttle inputs.

Mechanical slide – The second common ATV carburetor type is the mechanical slide. It operates on the same principle as the CV type carburetor, however, there is one main difference.

The throttle cable is connected directly to the slide and so nailing the throttle lifts the slide with the attached needle and feeds gas to the engine. It is a very direct throttle response and is generally fitted to engines that are performance-focused. However, because it’s so direct in how it opens the slide, it commonly sufferers from bogging (lack of gas), until the vacuum catches up and feeds more gas.

In addition to these main differences, some carburetors may employ manual choke operation while others may have an automatic thermo-activated choke.

All carburetors however suffer from some common complaints, they include having to adjust for temperature and altitude, since both these conditions change the quantity of oxygen in the air, it causes a carburetor engine to run poorly. Typically the operator must make an adjustment to the carburetor. Not practical for most, and so the EFI was born.


While electronic fuel injection is not a new idea, it is relatively new to the world of small engines. Fuel injection brings some great advantages, it automatically adjusts for air temperature and altitude and it offers sharper performance since the fuel is delivered quickly and atomized, just the way the combustion chamber likes it.

To accomplish this the EFI system which is largely an electronic system employs a computer, sensors to collect data, and a fuel pump and fuel injector to deliver the payload.

A cool byproduct of EFI means bikes can be mapped (ignition and fuel system optimized) for greater power or efficiency with the touch of a button, something not possible with a carburetor bike.

EFI is a robust system. However, some bikes are susceptible to electrical gremlins caused by loose wiring. Fuel pumps wear out as do fuel injectors.

The EFI works like this:

An air temperature sensor (ATS) fitted to the air intake measures the temperature of the ambient air (important as oxygen density is proportional to air temperature)

Baro sensor feeds air pressure reading to the ECU (important, as a change in elevation changes oxygen levels) Baro and ATS are often integrated.

The throttle position sensor (TPS) reports the current throttle position to the ECU.

The throttle body resembles a carburetor in some ways but they don’t serve identical functions. The throttle body is fixed to the engine and houses the butterfly valve with the throttle position sensor attached and also houses the fuel injector.

The electric fuel pump supplies gas to the fuel injector.

The fuel injector is opened and closed by the ECU.

The engine control unit (ECU) manages the whole show. As it’s a pre-programmed computer it can make tons of calculations and very quickly works out the correct fuel quantity for every given throttle demand. By varying the fuel injector duty cycle (“on” time) it can respond to throttle demands.

Components are listed in order of failure rate (highest first)

GasSource of all powerStale
CarburetorMixes gas with air in proportionDirty
Air filterCleans and uniforms airflowDirty
Fuel filterFilters fine particlesBlocked
Fuel pumpMoves gas from the tank to the carburetorFailed
Tank, cap & ventStores, and allows tank breathBlocked
Fuel tapControls gas flow to the carburetorLeaks
*Electric fuel pumpPumps gas from tank to injector, controlled by ECUFailed
*Fuel injectorInjects measured amount of gas, controlled by ECUFailed
*TPSECU receives Throttle position sensor readingsFailed
*ATSECU receives Air temperature sensor readingsFailed
*BaroECU receives Baro pressure readingsFailed
* Fuel Injected Engines


Ethanol blended gas is everywhere now; we have no choice but use it. It does cause some problems in a small engine kit that lays idle for weeks on end, and so all small engines should now use a stabilizer in the gas.

The stabilizer helps repel moisture that ethanol attracts and prevents carburetor gumming and hard starting misfiring etc.

Stale ethanol gas is right up there on the list of no-start causes especially if your ATV has been sitting idle for a few weeks.

Diagnose: If your gas is older than a couple of months and your bike has been sitting idle, your gas is likely stale

Fix: Drain gas tank and carburetor bowl and fill with fresh gas


Carburetors cause lots of issues as they get older. They are like wind instruments in that they are finely tuned. If they supply too much or too little gas, your engine will protest. They are of course adjustable within a window but if the moving parts of your carburetor wear they allow air to sneak into the combustion chamber unmetered and this upsets the crucial Air to fuel ratio (AFR).

Carburetors tend to be difficult to diagnose, generally, there’s no smoking gun and often it is a process of elimination.

That said the number one cause of most carburetor problems is dirty jets. Contaminated gas is the usual culprit. Dirty gunge blocked up the tiny brass jet holes which starves the engine of fuel.

Diagnose: One of the first carburetor checks is a fuel flow check.

Dirt bike carburetor bowl drain

Check fuel flow as follows:

  • Turn petcock off
  • Open carb bowl drain
  • Turn petcock on
  • Check flow

Fix: A lack of gas flow points to a tank blockage, a fuel tap (petcock) blockage, or a carb float valve issue. (See pic)

ATV carburetor needle valve

If on the other hand, you have good gas flow, suspect blocked carb jets. A carburetor strip down and thorough cleaning will fix the issue.

Air filter

Ail filters should be checked weekly and even more often in dry dusty conditions. Foam filters may be washed in warm detergent but should only be oiled if the manufacturer recommends it and of course only use recommended filter oil.

Diagnose: Remove the air filter and attempt to start the engine

Fix: Clean or replace the filter

You may find this post helpful:

Can I use WD40 on my air filter

Fuel filter

ATV filters may be inline clear filters where you can see the gas and dirt if present which is really useful for fast diagnosis; however, many ATVs use a carburetor mesh screen filter that can’t be viewed so easily. Many petcocks include an integrated fuel filter bowl.

Diagnose: Access the filter and check for blockage (may need to partially remove the carburetor)

Fix: Clean or replace the filter

You may find this post helpful:

ATV fuel filter not filling up

Fuel pump

Not all ATVs employ a fuel pump; for those that do two types are common, a vacuum-operated fuel pump and an electric pump. Carburetor bikes will either have no pump or a vacuum pump fitted.

Common vacuum fuel pump issues include perished vacuum lines and worn-out diaphragms.

Diagnose: Follow the fuel line from the carburetor back towards the tank. If you have a fuel pump fitted, you’ll find it on this line. Check all vacuum hoses and check for leaks.

Fix: Replace lines or pump

Gas tank, cap, and vent

Gas tanks don’t give trouble so much anymore; most are plastic and so are rust-free. However, faulty gas caps and vents may cause a no-start. A tank needs to breathe and so if a faulty cap or vent seals the tank gas stops flowing and the engine stalls and won’t restart.

Diagnose: Remove the cap and attempt to start the engine.

Fix: Replace the cap and vent

Fuel tap aka Petcock

Most ATV fuel taps are manual and don’t cause issues, that said older style petcocks employ a bowl that catches crap and can become blocked; if your ATV sufferers from a fuel blockage, check the petcock bowl. Some vacuum operated and when the internal diaphragm fails it allows gas to leak into the intake system causing the engine to over fuel and flood.

Diagnose: Remove the vacuum line and check for gas

Fix: Replace Petcock

*Electric fuel pump

Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

*Fuel injector

Dirt bike fuel injector

Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.


Run a resistance test using a DVOM to determine if it’s within your manufacturer’s spec.


Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

You may find this post helpful – ATV won’t start when cold


Run a resistance test using a DVOM to determine if it’s within the manufacturer’s spec.

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(III) Compression System

This section is last for good reason, it is the least likely system to cause an issue; however, if your issue does lie within this section there’s a good chance your problem is a serious one. That said there are some easy-to-fix issues within this section and we won’t meet trouble halfway.

What is compression?

Compression is an engine’s ability to contain the pressure created by the piston, as it squeezes and squashes the contents of the cylinder (air & fuel) into the combustion chamber (area at the top of the cylinder around the spark plug).

stroke 2

Compression is important, the squeezing helps warm the air and prepare it for ignition (explosion).

An engine with low compression will therefore struggle to start or likely won’t start at all. The reasons for low compression are covered below together with how we go about testing same.

You should note, compression test readings vary from one engine maker to another and so you will need to know what your engine compression specs are. Generally speaking, a reading below 100 Psi is deemed low compression. That said some engines may have a min far above 100 Psi, so check your specs before condemning.

Components are listed in order of failure rate (highest first)

Spark plugProvides ignition & sealsLoose
ValvetrainAllows fuel in and gases outLash out
TimingMaintains crank & cam timeTiming out
Head gasketSeals combustion chamberLeaks
RingsCreates compressionWorn
Battery & starterCranks engineFaulty

You may find these posts helpful:

ATV has spark and fuel but won’t start

How much compression should an ATV have?

When faced with a possible compression issue makes great sense to just go ahead and run a compression test. Doing so usually points us to the problem component.

Compression Issue

The problem is some engines don’t lend themselves to compression testing and here’s why.

Decompression valve

A compression releases assembly also known as a decompression valve kicks in during cranking to lower compression so as to lessen the workload on both the starter motor and battery.

The problem is, this lowering of compression also skews the compression readings and renders them useless. In fact, many manufacturers won’t offer compression specs if their engines are fitted with decompression valves. In these cases, we use a leakdown tester and so I’ll cover both test types in this section.

Assume you have a decompression valve fitted if you can’t find compression specs for your engine; if that sounds like you, you’ll need to run a leakdown test. Indeed a leak-down test is my preferred way to test for compression issues.

Compression test

To run this test you’ll need some tools, a compression test kit or a leak-down test kit. If you use a leakdown tester you’ll also need access to compressed air. Either way, you’ll find the tools needed here on the ATV tools page.

We’ll run the compression test first. This test is a two-part test. First, we run the dry compression test, and then we run the wet test.

The dry test is as follows:

  • Remove spark plug
  • Ground coil (prevents coil damage)
  • Fit compression tester
  • Choke off
  • Throttle wide open
  • Crank over 6 – 10 times
  • Read and note the highest reading

Now we run the second part of the compression test – the wet test is as follows:

Wet Compression test
  • Add a cap full of engine oil to the cylinder
  • Repeat the dry test as above
  • Read and note the highest reading

Now we need to make sense of the results. One of three results is likely, they are as follows:

1 Readings are the same but compression is low – If the wet compression readings are the same as the dry reading from the first test (or close) and the compression readings are below spec (as per your engine maker), then it is likely you have a top-end issue.

Go ahead and check your spark plug is tight, the valve lash is within spec, and the head gasket is ok.

2 Readings are different and compression is low – If, on the other hand, the readings are very different meaning the wet compression readings are significantly higher. Then it is likely your engine suffers from worn rings or piston or cylinder or combo of all three.

You’ll need to strip your motor for inspection.

3 Readings are the same and compression is normal – If your readings test well above 100 psi and both wet and dry tests are similar, Congrats! Low compression isn’t your issue.

Instead, I’d direct my attention to engine timing.

Leak down test

A leak-down tester is likely a faster and more conclusive way to both test and pinpoints the root cause of a suspect compression issue. In this test, we’ll fill the cylinder with compressed air and we’ll use the test tool to measure the cylinder’s ability to hold pressure over a given time period.

See the leak-down tester I recommend here on the ATV tools page.

TDC ATV engine

But before we do that we’ll need to set the engine to Top Dead Center (TDC).

I’ve covered setting the engine to TDC previously and you can check that out right here – ATV valve adjusting

The bonus of the leak-down tester is its ability to point to the area at fault. In a problem engine, the compressed air will be heard to leak from the cylinder. All we need do is identify where it’s leaking from.

Leakdown tester

Air leak diagnosis is as follows:

With the leak-down test complete, we should now have a handle on the component at fault. See the list of likely culprits below and what we’ll need to do to fix them.

I will include links to tests and tools as needed to test, repair and or replace components.

Spark plug

If you are here because the leak down test pointed to a leak around the spark plug, you are lucky, this is an easy fix. A spark plug provides a spark you already know that, but it has another job, it must seal the combustion chamber.

A loose plug will allow compression escape. Truthfully a loose plug may not be the most common cause of low compression but it is easy to check and a super-easy fix, and that’s why it’s first on our list.

Tools needed – for this we’ll only require a plug socket and ratchet. You’ll find one here on the ATV tools page.

Diagnose – A loose plug will allow compression to escape which may in fact be heard as the engine cranks over. A damaged spark plug washer is also a possible reason a spark plug fails to seal. Remove and check the washer is ok.

Spark plug fitting guide

The fix – Check the plug is tight.


The valvetrain comprises various components but the two most common valve train issues associated with low compression are damaged valves and poor valve adjustment. Valve adjustment is also known as valve lash.

Dropped valve

If the leak down test pointed to the exhaust or carburetor then it’s the respective valve that is at issue. A valve that’s too tight, meaning it’s held open will lower compression and cause a leak when tested. That’s one explanation for a valve leak and the other is a bad valve face or seat.

We can eliminate one of these easily by testing valve lash and I’ve covered that below, but remember if valve lash is within spec, it’s likely a bad valve seat or valve face that is damaged and it will need to be replaced.

Lash should be checked before assuming we have valve damage.

Valve lash

Valve lash is set at the factory but as an engine wears as you can imagine lash moves out of spec. For most ATVs lash should be checked every year or about 1500 miles, most aren’t checked. However, overhead cam engines require less attention.

While excessive lash could cause a no-start it won’t cause a low compression issue. However, a tight valve will.

Valve too tight

A tight valve is less common than excessive lash but it remains a very good explanation for low compression and so well need to check and adjust it as necessary.

I have covered this exact topic in greater detail previously and you can check out that post right here – ATV valve adjustment in 6 steps.


The timing of your engine is obviously important. The top half of the engine (cam and valves) are very precisely timed to the crankshaft and piston. If the valves aren’t opening when they should, the engine won’t run or may run but run poorly. This will also lead to poor compression readings.

ATV Timing Check

Not only that, an interference engine may cause your valves and piston to collide, that’s never a good outcome. An interference engine means the valves and piston use the same space, all be it at different times, so you can see how important timing is to an interference engine.

If you suspect a timing issue, we’ll need to get that checked, I’ve covered checking engine timing previously and you can check that out here – How to tell ATV jumped time.

Head gasket

A blown head gasket will cause a drop in compression. The head gasket is a graphite material sandwiched between the cylinder head and the jug. It helps seal and prevents the mixing of oil, coolant, and combustion passages.

Gaskets fail with age but also may fail as a result of an overheating issue or lack of proper coolant system maintenance. Straight water in coolant systems causes corrosion and that’s a common cause of premature gasket failure.

ATV head gasket

Head gasket failure is common however, you should have some additional symptoms prior to the no-start. Common symptoms include coolant loss, overheating, hard hot starting, white smoke from the tailpipe.

A leak-down test is great at exposing head gasket failures, and also showing where it has failed. Bubbles in the coolant system mean as you know the gasket has failed between the coolant passage and combustion chamber.

Sometimes of course it may not be conclusive as the engine may need to be hot before the leak presents itself and so if I suspect such a gasket failure but can’t identify it with the leakdown test ill use a chemical combustion leak test kit.

You’ll find a combustion leak test kit I recommend here on the ATV tools page.


Worn rings are the last thing any ATV owner wants to hear. Common symptoms include hard starting, long crank, oil consumption, blue smoke, lack of power, spark plug fouling, and misfiring.

ATV piston and rings

If your leak-down test blows air through the dipstick or the wet compression test shows a large difference when compared to the dry test, it likely means an expensive tear-down and a rebuild.

You are also looking at a couple of weeks of downtime given parts lead time and workshop schedules, etc. It is possible to DIY it but you’ll need a workshop manual and some special tools like a torque wrench, ring compressor, and feeler gauge.

Battery & Starter Motor

A strong battery and starter motor are important components for easy engine starting. Both components are key players and directly affect two of the three systems critical for internal combustion engines to run.

A battery and starter motor together provide all the power and muscle to crank over the motor, but critically they must do so with sufficient speed to cause two important things to happen.

1 the engine must crank over fast enough (Min 400 rpm) in order to excite the stator and create a voltage strong enough to fire the spark plug.

Kill switch wiring diagram

The pick-up also relies on a spinning crankshaft to produce a voltage which is used to signal the ECU/CDI and fire the spark plug.

2 the engine must crank over fast enough to create sufficient compression to prepare the air/fuel mixture for ignition.

Stroke 3

Any issue that prevents the engine from cranking over fast enough may prevent the fuel igniting.

A fast way to eliminate this possibility is to try jumpstarting; if the engine starts, you know you have a battery issue, which is more likely than a starter motor issue. I’ve covered battery testing at length previously in this post, you can jump to it here battery testing.

ATV starter motor

I’ve covered starter motor testing in greater detail previously and you can check that out here – ATV will only pull start

You’ll find all these tools here on the ATV tools page.

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I’ve covered a ton of other common ATV issues and you can check them out on the ATV maintenance & troubleshooting page

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