[TA-71]

I have a 72 T/A 455HO/TH400. Engine was rebuilt by the previous owner. Now running a Comp 268H cam. Heads were massaged to get compression to about 9.2:1. Stock converter. Stock points style distributor with Pertronix internals. Just had the Quadrajet rebuilt by Ray Klemm, highly recommend Ray if you need your carb rebuilt.
After reinstalling the carb, it runs much better, but I cannot seem to get it tuned and dialed in. With the vacuum advance unplugged I set the initial timing at 11*, it seemed to be happy there, with the idle at about 1100rpm. When I plug the vacuum advance in the timing jumps up to almost 30* and the idle jumps up by 300-500rpm and the idle seems to bounce around a little.
I checked the vacuum, it pulls about 15"hg at 1100rpm, with the vacuum disconnected. With it connected it bounces around from 12-15".
With the vacuum advance connected to full ported source, I have to have the idle set around 1300 rpm to maintain enough to idle at 750 in gear, and even there it surges a little.

Any suggestions on what the issues are?

[25stevem]

I would check your cranking compression.
It sounds like your cam is in retarded.

I have run a Isky 270 mega cam in a bone stock 77 400 with only 8 to 1 compression and not had to idle the motor that high.

[Cliff R]

Put a timing light on it and make sure the timing is rock solid in and out of gear. I'm suspecting that at 1300rpm's you are off the idle circuit and some of the mechanical advance is in as well. This will make idle tuning difficult if not near impossible. Idle speed should be down around 650-800 rpms, not up at 1100-1300.

The cam is small and poorly chosen for the big 455HO so it's going to give you some grief at idle as well. You are seeing some of that when it "skips" when the vacuum advance is hooked up and timing jumps to 30 degrees.

I would do two things before continuing. Check the distributor springs and mechanical advance to make sure they are strong enough to keep the timing from advancing at idle speed.

Reduce how much timing the VA adds back to about 12-14 degrees and start over with the tuning.

You never mentioned total advance or all-in w/o the vacuum advance, but I'd start around 30-32 degrees and work from there.......

[TA-71]

Does anyone know if there is a vacuum advance that has full advance at 8” and also adjustable?

[TA-71]

Originally Posted by 25stevem

I would check your cranking compression.
It sounds like your cam is in retarded.

I have run a Isky 270 mega cam in a bone stock 77 400 with only 8 to 1 compression and not had to idle the motor that high.

I checked with the previous owner and the cam was installed straight up.

[Kenth]

Do you have full battery voltage/current to the ignition coil for the Pertronix?
What carb-#?
Is the idle/lowspeed circuit in carb adjusted/modified for the cam/today fuels?
Are the S.C.S. and C.S.C. systems intact and functional?

[25stevem]

All to many times folks have thought they had the cam in straight up only to find they did not.

Take a 1/2 hour and do a compression test, you should have it for your records anyway to keep track of the motor as it ages.