Motorcycle Repair: leaking carbys, small engine mechanic, fuel vapors

Question
i've recently purchased three pocket bikes with butterfly carby's on them for "extra power" but after the bikes have been running for a few minutes and then turned off, fuel leaks out of the carby's everywhere.i took all three to a small engine mechanic, the next day i picked them up, ran them for 10 mins and the same thing happened, i took them back and the needle and seats were replaced, still leaking,the diaphram and gaskets were replaced,still leaking, so all three were replaced with brand new carby's, but after ten mins of running they too leaked. then the fuel lines were replaced with smaller hoses and fitted with restricters to slow the fuel down but that didn't help,the spring under the needle arms was replaced with stronger springs, no good, we are out of ideas,can you suggest anything else i could try short of putting another type of carby on,they really do make a difference
thank you
mick

Answer
Hi Mick,

Everything has been done that I would normally check as the source of the problem. The problem persists with new carbs on all three MC's. The only thing that comes to mind is the following:

Check that the fuel tank cap vent is unobstructed and capable of adequately venting fuel vapors.

Fuel vapor pressure in the tank could be increasing fuel head pressure in the hose and causing needle valve to give way under higher pressure.


Turn off fuel valve/petcock immediately after riding.


Petrol in the USA is more expensive than ever @ $2.30 per gallon. Hope the mini's don't spill too much.

Respectfully,
Mark Shively