I have a '71 340 that I rebuilt that is running poorly and has very low vacumn. The block is 30 over, has a mild MP purple shaft, X heads, stock intake and thermoquad. I've checked the cam centerline and compression which are to specs. I've changed carbs, distributor, coil and wires and no improvement. I've checked for vacumn leaks but can't find anything obvious. Timing is advanced 10 degrees at idle. If I advance more than this it won't start.

Any recommendations? If its a vacumn leak, what are some of the likely areas to check?

How much cranking compression does it have? Advancing the timing shouldn't keep it from starting and it's not uncommon to run more initial to help idle quality. Which MP cam is this?

Whats your idea of mild purple shaft ?
whats the actual idle vacuum and at what prm ?
What pistons did you use ?
Whats your compression readings ?
Whats your timing set at?
Auto ? Stock converter ?


Even the MP 280-474" cam will have a pretty choppy idle.