So sometimes my car is running great, other times at WOT it stumbles or has rough idle. Ive had the carb rebuilt, the engines only got 2000 miles on it (performance build). Is it possible since its still the gm HEI distributor that the vacuum advance could be sticking after all these years? Shop keeps timing the engine to try to get it right but it only lasts for so long before it goes back to bogging etc.
(84 trans am)

 

Is it a stock distributor? They were only computer controlled.

 

Since you list a 350 as your engine I'm guessing it's not stock and doesn't have computer controls any more?

If it's got a vacuum advance distributor you can test it. Just apply vacuum to the hose fitting on the vacuum advance canister while you watch the timing advance with your timing gun. If you have a hand vacuum pump with a gage on it, that's even better. Most cans start to supply advance around 3" of vacuum and top out around 9-10" of vacuum. It should NOT leak vacuum (ruptured diaphragm) and it should move smoothly through the advance curve as you apply more vacuum, snapping back to full retard the instant vacuum is removed.

A typical stock can will supply a maximum of about 20* of advance between the full-retard and full-advance limits.