Mine is doing the same exact thing. So far I´ve narrowed it down to the automatic shut off valve thats located on the right hand side on top of the carbs. This part is discontinued. It was designed to shut off fuel in case of an emergemcy but pumps fuel when the motor is running.Try pullng the pump apart and see if the diaphram is balloned into the gas passage,if so the pump is bad and you can´t order a rebuild kit. I removed my valve and it still acted up. I assume the fuel is still needing pressure so I am going to try instaling a vacume fuel pump. If you have found out something e-mail me at irishdaddy1977@aol.com or if you have any other questions, thanks