back in the late '60's nhra had a pure stock class (not to be confused with todays pure stock). a lot of us guys would run that class. there were a lot of 383 road runners and super bees and they all suffered from what we thought was vapor lock. truth was that the crummy 2 valve factory pumps rated at 3-5psi were usually around 3-3.5psi. they'd drive ok and street performance didn't really tell the story of how bad they were. when at the track the pumps would get hot in staging and then there wasn't nothing but crummy starting line and total track performance. we were doing everything including the clothes pin trick to solve the problem but the only solution was a better pump.

there was something else i've noticed in the past several years with a rash of 8:1 compression 383's; performance is dismal. i worked on a '70 road runner, basically stock, 8:1 with a 3.91 gear and automatic. even after a good tune-up that turd couldn't spin the tires! 383's aren't good torque engines and need a good ignition tune-up, not be over cammed, and do like a little gear. i've always used 2 1/2 turns from the air valve just seated on the carb top. float level should be looked at and make sure you don't have a fuel filter on the suction side of the pump.