Aragh I know the feeling. There’s never a good time for a mechanical fault. Not to worry, we’ll get it figured out.
The top 4 reasons an ATV dies when you give it gas:
- Not enough gas (clogged main jet)
- Too much gas
- Ignition fault
- Plugged muffler
In this post, you’ll learn about the most likely causes of a stalling ATV and what you can do to fix them.
In my experience, three common faults cause this condition, dirty main fuel jet, incorrect fuel mix setting, and incorrect needle clip position. We’ll cover all of these in this post and other possibilities too.
Fuel System Overview
Most stalling issues are caused by a fueling issue, either too little or too much. Before we get into the detail of your problem, it may be helpful to have an outline of how an ATV fuel system works and the fuel ratio it likes.
Fuel-injected fuel systems – Some modern ATVs employ fuel-injected ECU-controlled fuel systems. This guide doesn’t cover those types of models. If you have a fuel-injected type system, begin your diagnosis by reading DTC’S. (Diagnostic Trouble Codes).
A common cause of a fuel-injected bike dining when you give it gas is a throttle position sensor fault and fuel injector issue.
Check the wiring terminals are secure and undamaged.
Fuel-injected bike store fault codes and so that’s the smartest place to begin diagnosis.
Carburetor fuel systems – As most ATVs run carburetor-type fuel systems, this guide will cover those.
Your ATV carburetor is a precise piece of kit and is tasked with three distinct jobs.
- Mix air and gas together to a precise ratio
- Supply the correct quantity for the given engine load
- Hold a sufficient store of gas in reserve
Gas engines like to run with an air-fuel ratio (AFR) of 14.7 parts air to one part gas. This is the optimum ratio and is known as “Stoichiometric.” When a technician tunes your carburetor, this is the ratio they are chasing. As you’ll learn, lots of things affect the AFR, most can be controlled by the rider, but some can’t.
In most cases cleaning the main jet will fix your problem, that said it won’t hurt to run a few quick tests first to help confirm our suspicions.
Does your condition improve if you apply the choke lever? If it does, you have a lean condition. See adjusting your carburetor below.
Does the engine only die if you open the throttle quickly? If it does, your accelerator pump may need attention. See adjusting the accelerator pump below.
Does your engine die if you open the throttle both quickly and slowly? If it does, your main jet may be clogged. You’ll need to remove and clean your carburetor.
What’s the spark plug condition? A black plug indicates a rich running engine, and a grey/white plug indicates a lean condition.
Have you made any mods like a bigger air filter or exhaust or a change in altitude or temperature? These types of changes will require a fuel adjustment and possibly a re-jetting of the carburetor.
All diagnoses should begin with fresh gas in the tank and a clean air filter. You should also know that clogged carburetor jets cause the most running issues. It never hurts to remove and clean your carburetor thoroughly.
The answers to the above questions will put you on the correct path.
When your engine isn’t receiving enough gas in proportion to air, it’s known as running lean. Or another way to look at this situation – the engine is getting too much air.
Symptoms of a lean engine vary by how lean it’s running, but here are a few of the more common ones:
- Hanging idle
- Starts and dies
- Only runs on choke
- Dies when give it gas
The causes of a lean engine are just as varied. Here’s a list of the more common ones and the fix:
- Cold ambient temperatures or higher altitudes – adjust mix
- Mix out of adjustment – adjust mix
- Stale gas – replace with fresh gas
- Blocked carb jets – remove and clean carb
- Low bowl fuel level – adjust float
- Engine mods – mods like bigger air filters or quick flow exhausts will likely require re-jetting the carburetor
- Vacuum leak – check carb bolts are tight and carb gaskets and manifolds for leaks
- Faulty fuel pump – check fuel flow
- Valve lash out of spec – adjust
A very common cause of stalling when giving it gas is an accelerator pump fault. The pump is only used when you gun the throttle from idle. Its purpose is to inject a gas shot into the carburetor, which helps counteract the lean condition caused by the rush of air as you nail the throttle wide open.
How does it work? The system employs two main components:
- Leak jet
The pump consists of a rubber diaphragm, fuel orifice, and adjustable cam. When activated, the throttle assembly pushes on the pump cam causing the diaphragm to shoot gas through an orifice directed at the venturi. (Worth noting, not all systems will work exactly like this but this type is typical.)
Adjusting the point of contact between the throttle and pump cam affects the timing and duration of the gas shot. A good gas shot should last about one second and be timed to barely miss the slide as it opens.
If the gas shot hits the slide, its effectiveness is reduced. You’ll need to adjust the cam so that the shot arrives just after the slide lifts.
Although I’ve shown the AP here on a CV-type carb, it’s really only commonly found on a mechanical slide carb.
Problems with the pump include:
- Timing off
- Worn diaphragm
You can check your pump operation by removing the intake ducting and operating the throttle quickly, and observing the gas shot.
The Leak jet removes gas when not needed by the pump. When the throttle is opened slowly, the gas simply passes through the leak jet and back to the carburetor bowl.
The quantity of gas supplied by the pump is directly affected by the size of the leak jet. A smaller leak jet supplies more gas to the pump, and a bigger leak jet reduces the volume of gas to the pump.
Problems with the leak jet include:
- Incorrect sizing
The volume of gas supplied by the accelerator pump is directly affected by timing, duration and the leak jet fitted. You can eliminate this as a possible cause by opening the throttle slowly. If the problem isn’t present, your accelerator pump needs attention. If the problem persists, try adjusting the carburetor. See below.
Your engine is said to be running rich (aka fat) when it’s receiving too much gas in proportion to the volume of air it’s receiving. Or another way to look at this – your engine isn’t receiving enough air.
Symptoms of a rich running engine include:
- Black smoke
- Wet plug
- Black plug
Common causes of a rich running engine include:
- Mix out of adjustment – adjust
- Dirty air filter – clean or replace
- Leaking float needle – replace
- High bowl float level – adjust
- Faulty petcock – replace
- Faulty carb – replace
- Incorrectly jetted carb – re-jet
Adjusting Carburetor Fuel Mix
Adjusting your mix is a little technical but not brain surgery. Most ATV carburetors will have three fuel circuits.
Idle Circuit (Pilot jet)- supplies gas at light to mid throttle stage, but is solely responsible for idling. 0 to 1/4 throttle
Midrange Circuit (Needle jet) – supplies gas between 1/4 to 3/4 throttle
Main Circuit (Main jet) – supplies gas to needle jet circuit, from 3/4 to full throttle
A standard adjusting procedure looks like this –
Begin with a warm engine. Check how the throttle responds throughout the range. Turn the engine off, find your base setting by turning the pilot circuit screw (mix screw) all the way home (clockwise) until it seats, count the number of turns. (1.5 to 2 is normal).
Return the screw to its original setting.
Start the engine and turn the mix screw clockwise until the engine stalls. (note the amount turned and return after stall)
Restart the engine, now turn the screw anti-clockwise until the engine stalls.
Restart the engine and turn the mix screw clockwise until the engine rpm is at its highest.
Now reduce the idle screw to where the idle is smooth, anywhere from 1000 – 1500 rpm.
If, after this procedure, your mix screw is set to more than 2 turns out or less than one turn out, it’s a strong indication your carburetor needs re-jetting.
Your mix screw is now set correctly. The throttle should respond sharply and without hesitation or bog.
If it does bog, your:
- Needle clip position may need adjusting
- Carburetor main jet is dirty
- Carburetor needs re-jetting
Needle Clip Position
Your needle lives in the heart of your carburetor, and to access it, you’ll need to remove it from the bike. The needle moves up and down inside the emulsion tube in response to throttle application. The higher the needle inside the emulsion tube, the more gas the engine gets.
A clip at the top of the needle sets the base height of the needle, it has five positions. The clip is usually set to the 3rd position down. Moving the clip down one position will hold the needle higher up and help supply more gas (Richer). Conversely moving the clip up will lean out the mix.
Adjusting the clip position is a trial and error procedure. A systematic approach works best as removing and fitting some carbs can be a pain in the ass.
An ignition fault can allow a bike to idle but fail at higher rpm. Here are some of the checks you can make. Some bikes will run without a battery, while others require a strong, fully charged battery. Use a voltmeter and check your battery.
A faulty spark plug or the wrong plug type is a common cause of intermittent performance issues. Best to confirm the plug code by checking your manual.
Check also that the battery is charging at idle. Anything over 12.65 indicates it’s charging. Visually check the condition of your plug, cap, plug wire, and coil.
Using a simple in-line spark tester, fit the tester and check the spark as you give the bike gas.
If the spark in the inspection window fails, you’ll need to check the resistor cap and run a resistance test on the coil, stator, and pickup, also possible for the CDI box to cause a stalling issue.
This isn’t a hugely common cause of stalling, but it does happen. The spark arrester inside the exhaust becomes caked in soot, so thick that exhaust gases can’t get out, and it causes the engine to stall at anything above idle.
With the engine idling, hold a shop towel firmly over the tailpipe using a gloved hand. Release it after 3-4 seconds. The pressure built up should be great. If it’s weak, it suggests a partial blockage.
Go ahead and remove your spark arrester and clean it.
You may find the following posts helpful: