Fuel delivery(?) problem

[jglee]

My '66 with a 390 has developed an annoying problem. Going down the road and all is right with the world. The truck then acts like it's out of gas and dies. But if I unscrew the gas cap, pump the pedal a couple of times, and try again it cranks right up. When the cap is unscrewed you can hear the air coming out like it's under pressure. I put a new vented cap on it a year ago but this has just started happening recently.

Any ideas? I appreciate it.

[cooter]

when you open the cap do you have air rushing out or rushing in? What happens when you leave the cap off while driving? Im thinking your cap is not venting and you are creating vacuum in the tank, its like pouring a pop bottle upside down.

[jglee]

Hey Cooter, I'm thinking the same thing you are because that's the way it acts. The air is coming out instead of in. If I leave the cap off or even just tighten it barely enough to keep it from falling off it does ok. Reckon it's as simple as the cap? Only I could have a bad gas cap.

[Uncle Skip]

Unless you have an original gas cap, you're probably dealing with one that is un-vented. You're pulling a vacuum on the tank and starving the motor. I don't know if anyone even makes a vented cap any more because of all the EPA crap about emissions. Experiment and find a way to vent the cap. Something as simple as drilling a small hole in your cap would probably cure the problem.