As a motorcycle mechanic I constantly have customers complain that their motorcycle won’t start when hot. Usually it worked absolutely fine for a few months before this problem started happening. Others had their two wheeler for a couple of years until it happened.
In either case, it’s important to diagnose the issue. In this case there can be several problems. It’s either electronics, fuel or something to do with compression. Which is why my first suggestion is to visit a mechanic.
But if you have some time on your hands and want to save a hundred bucks, these are the main things to look for:
1. Malfunctioning electronics
Electrical components can get hot and stop working. This is the first thing to check. Here’s an excellent step-by-step guide a gentleman motorcyclist shared on a forum I frequent:
Find out if the spark dies out. When it dies pull a plug wire off, stick a known good plug in the end and hold it close to the engine while someone cranks the engine over.
DO NOT hold on to anything metal, just hold the spark plug boot and put the ground strap of the extra plug against the engine, if you do it will give you a jolt. You should see a nice big blue spark between the spark plug gap. You can use a spark checker from an auto parts store thats a bit safer to do.
After you determine that the spark does indeed crap out when hot then you can start checking parts to see what’s wrong. You’ll need a service manual and an ohm meter.
2. Loss of compression from tight valve clearances
Valve clearances that are too tight when cold will get even tighter when the engine gets hot. When this happens, the valve stays open a little bit, making the compression go away. Then the engine can’t run! In order to check valve clearances, you’ll need:
- feeler gauges
- service manual for the specs and how to do it
While every motorcycle is different, this video provides a good overview on the importance of valve clearances and how to deal with problems that can arise from their malfunction:
3. Worn out ignition coils
If the spark plug and the gas line are both doing fine, it could be the ignition coils. Of course it could be all three or the ignition coil AND the spark plug that’s causing the problem. But if an ignition coil is worn out it often won’t work when it’s either cold or hot. Here’s how to check your ignition coil:
In case of a malfunction, replacing your ignition coil with a new one will fix the problem. You can find ignition coil and spark plugs on Amazon for many motorcycles. As far as the actual replacement process goes, here’s a solid step-by-step video guide on that as well:
4. Fuel line and vapor lock
Fuel lines should go downward from the tank to the carbs, never having an upward turn. If they turn up and go down again, a vaporized gas bubble can form that will block the liquid gas flow.
One person found a temporary solution in simply opening the gas cap every ten minutes or so when riding, to relieve the pressure. Another person found that using 91 octane instead of 87 octane completely fixed the problem for them.
5. Dirty carburetor
In other cases what seemed like a fuel line problem had all to do with a dirty carburetor. The symptoms of these two often interlap. Here’s a great video tutorial on cleaning a motorcycle carburetor:
Final Word: Motorcycle Won’t Start When Hot
Electronics, ignition coils, compression, fuel line.. these are the most common reasons why motorcycles won’t start when hot. If you have solid mechanic skills, you might be able to pinpoint the problem yourself and actually fix it. Definitely give some of these tips a try and see if they work or not.
If you don’t have the time and willingness to diagnose the problem yourself, it’s always best to visit a mechanic and get a full assessment of the situation. Hope this helps!