I used to run an rs4 3 evo with an the .18 cv-rx and the stock HPI pipe and never had a problem like the one you describe. Make sure the pressure line from the pipe to the fuel tank is at least 6 inches long, this reduces the effects of the exhaust pulses hitting the tank. I would also check to make sure the pipe is not jammed with anything, because it could be forcing all the exuast through the pressure line if the stinger is restricted.