True Timing
 

I have a 77 Trans Am with a 400, 670 heads, milling spc 8 cam, distributor was rebuilt with lit mechanical advance springs, New plugs and wires.
Idle timing started at 16 btc.
With 32 or 36 retard (didn't change) the mark at TDC up to 2500rpm. After 2500 the marks jumped much higher then TDC.
I have 91 oct the have (most have) 175 compression.
After adjusting my idle timing was 18 BTC.
🤔🤔🤔

Unplug vac advance before measuring. Set timing at 34-36 btdc at the rpm it stops changing, then measure idle advance. Do all of that before plugging vac advance back in and then set idle speed with it plugged in.

cool set up what gears you running?