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


Hot starting issues are so annoying and can be a 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 5 reasons a dirt bike won’t start when hot, how to diagnose them and how to fix them.

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 there 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 design. Each plug has an optimum operating temperature, which is matched to your engines operating temperature. The plug regulates it’s temperature by design and materials.

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 breakdown when it overheats.

How to test the plug?

It’s a good idea to begin your diagnoses 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.
Fault present, check plug cap is tight, a loose cap will cause problems. Now go ahead and remove the plug, check condition and gap the electrode.

You can check the gap spec with your engine maker.

  • 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 the new plug makes no difference, move on and check coil.

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 to failing when hot, so it’s close to top of our list. A coil usually lives close to the action, because short plug wires cause less issues. The heat from the engine transfers to the its surrounding components, called heat soak and it may be the cause of a faulty coil.

The coil is attached to the frame of your bike, you may need to remove some components like 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.

The coil is a solid state unit, meaning we can’t repair it, we test it and pass it fit or fail it and swap it out.

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

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 stupid simple, really.

Primary coil test:

  • Remove plug wire
  • Remove both wires from the coil primary (usually single connector)
  • Set Volt meter to resistance reading
  • Probe both wires and note the reading, expect in the region of .20 – .30 ohms. But check your manufacturers specs and limits.

A reading outside this window will mean the coils primary coils are faulty.

The secondary test:

  • Remove plug wire cap (usually screws off) and set aside
  • Set the meter to resistance
  • From the pair of wires, probe the coloured wire of the coil ie (not the black wire)
  • And probe the plug wire
  • Note the readings, should be in the thousands, 1112 – 12222. Check the spec of your manufacturer.

A reading out side this window means the coil is faulty.

Resistor cap test:

  • Set the DVOM to resistance
  • Simpy probe 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 a clever piece of kit, and has made bike ignition systems a lot more reliable. Checking the box can be tricky without the correct test kit.

So instead, diagnoses goes along the lines of elimination. All possible ignition components are tested and passed fit – therefore it must be CDI box failure.

Try swapping the CDI box for a know good module, also 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.

4 Choke Fault

Over fuelling on a hot start is a real problem with some bikes, obviously choke isn’t needed when hot. Too much gas can 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 the tail pipe.

Vapour 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 vapour locking, the condition will fix it’self as the bike cools. But fuel lines may be excessivley hot, check fuel filter and line routing.

Some bikes are fitted with a lever specially for hot starting.

Check if your bike has one fitted and if it working correctly, lots operated by cable which will need adjustment from time to time.

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 to charge the coil and create a spark strong enough to start the engine.

The stator comprises of coils of wire (3 usually) surrounded the flywheel which is fitted with magnets. As the magnets pass the coils they produce alternating current A/c. The stator is fitted behind the fly wheel on the side cover of the engine.

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

How to diagnose: Follow the wiring loom that exits the side cover. Disconnect the plug connector. Most stators are 3 phase and will have 3 identical wires grouped together.

There’s two ways to test the stator, the static resistance test and the dynamic a/c voltage test, both will require a volt meter.

Static resistance test:

As your bike is only causing the problem hot, it makes sense to test when hot and fault present.

Set your meter to read resistance and with the connector unplugged, lets just name the 3 phases (Wires) a, b and c. Using your meter, read resistance between a and b, b and c and finally a and c.

We can also check for short to ground by placing the ground probe on chassis ground and probing a b and c in turn, all should read open circuit. Any reading on the meter means you have a faulty stator.

All readings should be between .1 and 1 ohm, an open reading is a faulty stator phase.

Dynamic voltage test:

Bike running and fault present. Set your meter to read a/c voltage, place the black probe on chassis ground and the positive probe on each wire a,b and c in turn.

A reading of 15 to 30 volts a/c can be expected, less or none, suspect a faulty stator.

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, the gap between the rockers and valve tip is known as valve lash.

Lash naturally moves out of spec with engine wear and tear. Lash should be checked periodically, but often goes unchecked as access can be a pain in the ass.

Two valve lash conditions are common, too loose or too tight. We are concerned with the valves being too tight.

As metal heats, it expands which can cause already tight lash to push on the valve and open it slightly. An open valve will drop engine compression and the engine may not start until the valve train cools and contracts.

Two valve train setups are common rockers and OHC with Buckets/Shims.

To check for this condition, run a compression test, or leak down tester. Check when cold and recheck when hot and condition present. The readings should be close, if not suspect valve lash adjustment.

If you don’t have compression tester, go ahead check your valve lash I wrote a whole article on it here.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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