JLO 400 won't start when hot.

[RocketHurricane]

My Attex Chief with the JLO 400 engine starts when cold and runs strong, but if it is shut off when hot it will not restart again until it cools down. While still hot we have tested spark (good), compression (120 in both cylinders). Any suggestions what could cause this behavior.

[Insomniac]

I would suspect the fuel pump diaphragm. Its common when they get worn out they get warm and don't pump fuel properly.

[RocketHurricane]

I would suspect the fuel pump diaphragm. Its common when they get worn out they get warm and don't pump fuel properly.

The fuel pump has a new diaphragm in it.

[jpswift1]

I'm going to take a guess and say it could be in the ignition. Try replacing the points and condensers.

[Insomniac]

If you have spark, your compression is adequate, fuel pump is new or rebuilt then crankseals would be my next guess, not enough crankcase vacuum to run the fuel pump. If you squirt fuel into the carb will it start up?

[RocketHurricane]

Could this be a case of vapor lock? Once the hot motor is shut off, the fuel that is sitting in the carb heats up and when we try and restart it will not feed fuel until everything cools down again?

[Joe H]

That is what I would suspect, Lewie72 had his racer vapor lock I think he got it started by pinching the fuel line. I really doubt it is the crank seals it wouldn't run right if it was that was the case. Check the tension on the axial fan belt it may be slipping and not cooling the motor or try jetting up a little to get the engine running cooler.

[Insomniac]

Is your fuel tank vented properly? or is it obstructed?

[RocketHurricane]

I put exhaust wrap on the exhaust last night and took it out for a drive. I drove for about 20 minutes so the engine would get nice and hot. It ran great for this period, and idles nicely. Immediately after shutting it off I tried to restart. No luck. It just cranks with no hint of starting. Even starting fluid will not get any result. I felt the carb and it does not feel hot at all. The exhaust wrap worked well, it is just warm to the touch. I also cracked the fuel cap open but that had no effect. Pulled the plugs and they are dry and clean. Stumpd at how it can be running so strong and a second later it won't start. The joy of old 2-strokes...

I'm going to pull the carb and pump and check them out unless anyone has other ideas.

[Joe H]

Try putting a vacuum guage on the crankcase where the pulse line for the fuel pump goes, check the amount of pulse from the crankcase when it is cold and then again after it gets hot. If the pulse weakens after it is hot I would suspect the crankcase divider or whatever its called on the center of the crank is bad and canceling your pulse and messing up the combustion ratio.