Some more infos:
I checked the dwell angel, it's about 31°
Ignition timing at idle (without vacuum advance) is about 15° BTDC.
Is this too much advance timing ?
(Motor setup: 400 cui engine from 1970, heads 62 from 1968. Tripower carb, Cam: don't know...).
Vacuum at idle: 7 inHG (needle steadily), at higher rpm: >20 inHG, no vacuum leaks recognizable.
Hope, someone can help....
OK, more info. Dwell is OK, factory is 30.
Advance is a funny thing. Each engine can be different, especially once you deviate from stock. If it fires right up and doesn't struggle, it should be OK. Too much and the engine will turn over hard.
First off, if you are running the #62
heads along with stock pistons, you have high compression - probably 10.5.
10.5 compression/iron heads typically requires race gas and/or octane booster - UNLESS you have dished pistons to lower compression in the 9.0-9.3 range.
7" of HG (at what idle speed? 1,200 RPM?) is low. The best I can do is quote from my diagnosis testing from my Chilton's Manual on the 1968-73 A-Body series. Tghis is for a stock engine just for reference. Big cam changes everything. 17-22" HG is normal at idle. Between 10-15" HG, steady but low - late ignition or valve timing, or low compression. 4-9" HG - vacuum leak.
If you are not experiencing "pinging" and you know you have 10.5 compression, then you may have a big cam with a lot of overlap to reduce the "Dynamic Compression," ie cylinder pressure. Big overlap cams reduce engine vacuum.
Big cams will idle higher than stock (Ram Air IV idle was a factory 1,000 RPM's) and will need a higher stall converter so it does not cause you to have to stand on the brake pedal to keep the car from moving when you put it in Drive at the higher idle RPM's. Trying to drop the idle too low for the cam will cause the engine to quit when you put it in Drive.
Next, you probably have a few carb issues that go with the big cam. The carb should have a "power valve" that has a spring designed to open it based on a vacuum signal. If the vacuum is too low, it is not pulling the "power valve" closed as it should and the carb is running rich adding more fuel to the engine. You can get a lighter spring. I think if you check around with carb shops you can find one.
Then, I suspect you probably have to adjust your idle screw to keep the engine running at the higher RPM's so it does not stall out. Doing this opens the throttle blades and exposes the idle circuit slots more than they should be, so adding more gas and more air, and in turn raises the idle.
There are also idle tubes in the carb that may need to be enlarged. Read through this post as it may help: https://www.gtoforum.com/f170/tri-po...stions-120865/
So if you have the high compression and the engine doesn't "ping" when you stand on it, and if you have the big cam that only produces 7" HG at idle, then you may have to focus on the carb and see if you can make a few adjustments/modifications to get the engine to idle lower.
That's my guess from where I am seated.