Vapor lock?

[gymrat1964]

1981 Yamaha 400 Special II. It starts easily when cool, but if I have ridden somewhere and try to start it after it is hot it won't start again. Cool it down and it starts up fine. The only problem is that vapor lock, as i understand it is from gas getting too hot in the fuel line. The fuel line on this bike is very short and runs from the gas to the carb no where near the engine to get it hot enough to cause vapor lock. (in my opinion). Is there something else that could be causing this problem?

[vicshere]

i thought a vapor lock was from the tank not having a air outlet

[mydlyfkryzis]

TEchnically, vapor lock is when, during high temperature conditions (usually under the hood of a car) combined with the suction of the fuel pump, create a vacuum in the fuel line. The fuel, at the lower pressure than atmosphere, boils in the fuel line. The fuel pump, instead of receiving liquid fuel, receives only the vapor coming from the boiling fuel. This prevents any liquid fuel from being pumped into the carburetor.

Since your motorcycle is gravity fed, it cannot have vapor lock, at least in the strict definition. Newer vehicles, with the fuel pumps in the tank, cannot vapor lock either.

However, a bad vent on the fuel tank, an internally collapsed line, a stuck float valve, clogged screen, a bad petcock (A vacuum actuated petcock with a bad diaphragm too) can all stop the flow of gasoline to your carburetor.

Many people call these type of problems vapor lock incorrectly, but they do have similar symptoms.

When the bike won't start hot, open the gas cap. If the bike restarts then, it may be a venting problem.

You can remove the gas line from the carb when having the problem, and see if gas flows. Don't let gas hit hot engine parts, though, as it is a fire hazard.

Good luck...