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Dirt Bike Won’t Start When Hot (Top 6 reasons why)

Hot starting issues are so annoying and can be a real pain to diagnose, but not to worry. Together in this post, we’ll get it figured out.

Top 6 reasons a dirt bike won’t start when hot include:

  1. Faulty spark plug
  2. Faulty coil
  3. Faulty CDI box
  4. Choke fault
  5. Stator fault
  6. Valve adjustment

In this post, you’ll learn about the top six reasons a dirt bike won’t start when hot, how to diagnose them, and what you’ll need to do to fix them.

Ignition coil

1 Faulty Spark Plug

A spark plug is a very simple component but does a very important job. Without the spark plug, we’re going nowhere. All plugs look the same, but they’re not. Apart from some having obvious differences like thread size, plugs come in different heat ranges.

Plugs have different heat ranges because engines operate at different temperatures depending on the design. Each plug has an optimum operating temperature, which is matched to your engine’s operating temperature. The plug regulates its temperature by design and materials.

Spark plug

Fitting an incompatible plug can cause hot start issues. So although your bike may run just fine cold with an incorrect plug, the plug may break down when it overheats.

How to test the plug?

It’s a good idea to begin your diagnosis by checking the plug type. The code is printed on the plug. Check with your manufacturer and only fit the recommended plug type. To test the plug, replicate the fault by getting the engine hot.

With the fault present, shut down the engine and check the plug cap is tight; a loose cap will cause problems.

Spark plug gap ATV and Dirt Bike

Now go ahead and remove the plug, check the condition, gap the electrode, and clean it.

You can check the gap spec with your engine maker.

Spark plug fitting guide
  • Refit the plug wire to the plug and ground the plug
  • Crank over the engine and check spark
  • Try testing spark using a new plug and note any difference

If gapping and cleaning/replacing the plug make no difference, we’ll move on and run a complete ignition system test using an inline test kit.

Inline Ignition System Test Tool

In line ignition system tester

The inline tester loads up the system while allowing the mechanic to view spark quality in the tools inspection window (engine running). It’s perfect for hot start issues.

Observing no spark in the test tool window when the no-start condition is present rules out a fuelling system fault and points to an ignition system component failure.

Observing spark in the test tool window while the no-start condition is present suggests a spark plug issue or possibly a fueling issue.

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2 Faulty Coil

Your coil is responsible for producing the 30k plus volts needed to fire the plug. Coils give lots of problems, and it’s not surprising, given the stress they’re under.

Coils are famous for failing when hot, so it’s close to the top of our list. A coil usually lives close to the action because short plug wires cause fewer issues. The heat from the engine transfers to its surrounding components; it’s called heat soak and is a common cause of hot start issues.

Dirt bike coil

The coil is attached to the frame of your bike. You may need to remove some components, like the seat and tank, to access it. You can find it easily by following the plug wire.

Check that the wiring to the coil is secure and corrosion-free. Loose wiring here will cause problems. A coil is a solid-state unit, meaning we can’t repair it. We test it, pass it fit, or fail it and swap it out.

Testing the coil is simple, but we’ll need a Voltmeter, aka DVOM. These guys aren’t expensive, and you’ll find lots of uses for them if you plan on fixing your own bike, car, etc.

We’ll run three tests, a primary coil test, a secondary coil test, and a resistor cap test. Stay with me here because these tests are stupidly simple.

Primary coil test:

Coil test
  • Remove plug wire
  • Remove both wires from the coil primary
  • Set the voltmeter to resistance Ω reading

Probe both wires and note the reading; expect a low resistance reading; each manufacturer’s coil will be different, so check your specs. Your manufacturer will offer a range and if you are within the range, you are good. A reading outside this window, conversely, will mean the coil’s primary coils are faulty.

The secondary coil test:

Secondary coil test
  • Remove plug wire cap (usually screws off) and set aside
  • Set the meter to resistance
  • probe the wires as per the picture

Note the readings should be in the thousand. Your coil specs will be different, so check your specs. Your manufacturer will offer a range and if you are within the range, you are good. A reading outside this window, conversely, will mean the coil’s primary coils are faulty.

Spark plug resistor cap test:

Spark plug resistance test

Set the DVOM to resistance
Simply probe on either side of the cap.

The resistance will be in the thousands, but you’ll need to check with your engine manufacturer.

3 Faulty CDI

The CDI (Capacitor Discharge Ignition) is clever kit and has made bike ignition systems a lot more reliable. While modern bikes may use an ECU (Engine Control Unit), most will use the tried and tested Black box, aka CDI box.

Diagnosing a faulty CDI

Motorbike CDI box

Checking the box can be tricky without the correct test kit. CDI are solid-state components that can’t be opened or repaired. So here are a few checks we can perform and a strategy for testing:

Visual – Check for loose wiring at the CDI terminal.

Heat gun – A heat gun can help diagnose a failed CDI box. With the bike cold, heat the CDI box using a heat gun (careful of looms and gas) and see if the condition presents itself.

Swap it out – If you have access to a spare CDI (maybe somebody you know has the same bike), great! Try swapping the CDI box out and testing the bike to eliminate the CDI.

Elimination – If none of the above works for you, then continue testing components as per this article; if all checks out OK, circle back and suspect the CDI failure. However, we’ll check all possible ignition components first before concluding we have a CDI fault. CDI boxes are generally pretty durable, but obviously, they do fail.

4 Choke Fault

Over fuelling on a hot start is a real problem with some bikes. Obviously, a choke isn’t needed when hot. If your bike has an auto choke, check the thermo works without issue, i.e., it’s fully off when the bike is hot.

Un flood dirt bike

A sticking choke will add too much gas, which may flood the engine. If you suspect flooding, open the throttle wide and crank over the engine a few times. This helps dump the excess gas out of the tailpipe.

I wrote a post about drying out a flooded engine which you may find helpful – Start a flooded 4-stroke bike

Vapor lock is another common condition. Gas in the fuel line gets too hot and turns to a gas, which starves the bike fuel. If your bike is vapor locking, the condition will fix itself as the bike cools. But fuel lines may be excessively hot.

Check fuel filter and line routing; I wrote a post about inline fuel filter issues which you may find helpful – Fuel filter not filling up

5 Stator Fault

The stator or generator provides the power to charge your battery and run your lights (if fitted). It also provides the voltage which powers the coil and CDI box, which, as you know, are needed to create a spark strong enough to start the engine.

TV engine stator location

The stator is fitted behind the rotor (flywheel) behind the side cover of the engine.

The stator comprises coils of wire surrounded by a rotor (flywheel) which is fitted with magnets. As the engine cranks over (kickstarted), the magnets pass over the coils exciting them and producing a voltage known as alternating current (A/C).

The A/C voltage is then sent to a rectifier/regulator, which converts it into direct current (DC), which our bike uses to power the coil and the CDI box.

Common stator faults include shorting, excessive resistance, and grounding. The stators may fail or fail intermittently when hot.

How to diagnose stator:

To complete these tests, we’ll need a DVOM, and we’ll need to disconnect the stator block connector.

Stator wiring loom terminal location

Follow the wiring loom that exits the side cover. Disconnect the plug connector.

There are two common ways to test the stator, the static resistance test and the dynamic AC voltage test. When testing resistance, we always check on a dead circuit, meaning we disconnect the component to isolate it. Attempting to check resistance on a live circuit risks meter and component damage.

Stator loom

Most stators are 3 phases and will have three identical wires grouped.

Some will place them in their own dedicated terminal block, or they’ll be integrated with the pickup wiring. Both types are seen in the picture.

Static resistance test:

As your bike is only causing the problem hot, it makes sense to test when hot and fault is present if you can try and replicate the problem before testing. With the stator block connector located, disconnected, and wiring identified – let’s name the 3 phases (Wires) A, B, and C. (it doesn’t matter how you name them)

ATV stator resistance test diagram

Set your meter to read resistance (Ω) (also known as Ohm’s), and with the connector unplugged, read resistance between A and B, A and C, and finally, B and C.

All readings should be between .10 and 1 Ohm.

Open reading is a faulty stator phase.

We can also check for short to the ground while we have the meter set to resistance (Ω) by placing the ground probe on the chassis ground and probing A, B, and C in turn.

ATV Stator short to ground test

All should read open circuit – O/L (Open Loop). Getting a reading other than open (O/L) means you have a faulty stator.

Dynamic voltage test:

For this test, your bike will need to run about 3000 rpm; a helper will be useful. Set your meter to read AC voltage (~), place the black probe on the chassis ground (metal bodywork or engine metal) and the positive probe on each wire A, B, and C in turn.

Dynamic stator AC voltage check

A reading of 15 to 60 volts AC can be expected, less or none – suspect a faulty stator.

Reading will vary from model to model and according to engine RPM. Check readings against your bike’s spec.

Testing the Pickup

The pickup is usually stator integrated, and while they do different jobs, the pickup, like the stator, can also suffer from heat soak and become temperamental when hot.

Kill switch wiring diagram

The pickup identifies TDC and signals the CDI or ECU, and the CDI or ECU uses the signal to activate the coil.

And so you can see if we have a pickup issue, it follows we’ll have a spark issue.

Pickup terminals

The pickup wiring is integrated with the stator block terminal and is usually a pair of wires.

Pickup test

Check resistance across the terminals. A typical resistance value is 220 – 350 Ohms range, but check the specs for your bike; all components and manufacturers’ values will differ.

6 Valve Lash

Your engine contains mechanical valves that open and close sequentially. They allow fuel mix in and spent gases out. The whole assembly is known as the valve train, and it’s timed and driven by the crankshaft. The valves are opened by the rockers or camshaft lobes, depending on the setup.

The gap between the rockers/buckets and valve tip is known as valve lash. Lash naturally moves out of spec with engine wear and tear and should be checked periodically but often goes unchecked as access can be a pain in the ass.

Two valve lash conditions are common, valves that are too loose or too tight.

Valve too tight

We are concerned with the valves being too tight.

As metal heats, it expands, which can cause an already tight valve to push open slightly and remain open.

An open valve will drop engine compression, and the engine may not start until the valve train cools and the tight valve contracts and closes. Two valve train setups are common rockers and OHC with Buckets/Shims.

Leakdown tester

To check for this condition, run a leak-down test. Check when cold and recheck when hot and the condition present.

The leakdown tester uses shop air to compress the cylinder with a piston at Top Dead Center (TDC). The leakdown tester uses gauges to measure pressure loss over a given period, but it usually indicates where the fault lies by offering us a tell-tale leaking air sound.

If we have a valve issue, expect to hear air leaking from the exhaust system or carburetor/airbox. If using the gauges, measure and note readings with the engine hot and cold to help diagnose the root cause. The hot and cold readings should be close; if not, suspect valve lash adjustment is needed.

You may also find these posts useful:

I’ve written a ton of Dirt bike troubleshooting posts. Hopefully, you won’t need them, but if you do, we have you covered.

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