Motorcycle Repair: honda 85 CM250C Stalling, honda 85, vent hole

Question
Hi i seam to be having a problem with Honda 85 CM250C Stalling after being riding for about 20 to 30 minutes. you turn it off and let is sit for about 5 minutes restart it and it will run fine with no problems at all. FYI it will start with or with out the choke, cold or hot, first time every time.
i have fitted modified mufflers and a foam air filter. on it i have shimmed up the needle in the carby to increase fuel.
Mike any help you can give is much appreciated
Thanks Mat

Answer
Mat, we need to determine if it is an electrical or a fuel related problem before figuring out how to fix it. Next time the bike stalls, check to see if you have spark at both spark plugs.

If you have spark at the plugs, check to see if you have gas in the line going from the tank to the carb. Pull the hose off at the carb end - being VERY careful not to spill any fuel on the hot engine - and see if there is sufficient fuel flow. If not, then the problem could be vapour lock which is usually caused by the fuel line sitting too close to a hot motor. It could also be a plugged vent hole in the gas cap - check to see if there is a "whoosh" noise when you unscrew it.

As you have fitted modified exhaust and a low restriction air filter, my guess id the motor is running much hotter then normal because the fuel mixture has leaned out. Shimming the needle in the carb will only help in the mid to upper rev range of the motor, you still need to increase the idle jet size and retune the carb to suit.