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!

Vac adv capped then bump the initial up to 18-20 & slow the idle speed down as low as you can get it to idle decently. Then plug the vac adv in to a ported carb port & hopefully the throttle blades are now closed enough to where the ported port you are using has no vac at idle. On the dist with a vac pump (eng off) pump up the vac can gradually & at the vac level where the gap is the narrowest set the reluctor gap at .008". check rotor phasing. After bumping the initial up check (& post) the total & ~38 (checked with vac adv capped) is in the ballpark. That'd be a start. EDIT What springs are in the dist?