Buddy of mine bought a '71 'Cuda 383 4-speed, and we're running in to some odd issues with tuning her up - thought I'd throw a couple questions out.

First off, engine specs are unknown. Seems to be a mild performance cam. Don't know compression. Carb is a Holley 750 double pumper.

Baseline timing is set at 5 BTDC (vacuum advance off, vacuum port on carb plugged). All in at 2,500 RPM, timing is about 33 BTDC (again, vacuum advance off).

Idle is set to 900.

Vacuum at idle is about 12 Hg.

Idle screws on the front bowls are about 1.75 turns out. Idle screw on rear bowls are about 1.5 turns out.

When hitting the pump shots, idle drops.

Car seems to like where it is set now (without a test drive, just warmed up), but here is where it becomes tricky: When hooking up the vacuum advance, idle shoots up to about 1,500, then surges up and down between 1,500 and 600, and it runs rough. When checking all-in at 2,500 with vacuum advance hooked up, timing is 40 BTDC.

Car is running an MP distro, and a chrome box.

Have not run in to this before, so need some advice.

Thanks!