I have a '86 IROC, LG4, headers, Edelbrosck performer RPM manifold, Holley street avenger, non-computer Accel distributor. After swapping in the carb and distributor I am trying to set the timing. I can't get the engine to idle below 1200 rpm so I"m having trouble setting initial timing. The choke is open and the car is warmed up. What could be the problem? If I retard the timing to the point where the idle is below 1000 rpm the engine stalls. If I leave it at 1200 rpm the engine backfires through the carb when I shut it off.

 

First disconnect the throttle cables and make sure you aren't holding it open because of that.

Next check for vacuum leaks. Verify all of the vacuum ports are used or plugged.

Then carb binding, such as a misaligned base gasket.

Finally, the fast idle cam. It's on the passenger side. If it's turned in too much, or the cam isn't dropping down with the choke, you'll get an open throttle condition.

That's where I would start.