Holley Street Avenger on 1977 350. adjusted air/fuel screws the way Holley says to but still running rich....

How do you know you're running rich?

smell and see dark fuel smelling exhaust on start up and while running for few minutes. tail pipe are black inside. plugs look like they are getting hot

Start at the basics...

Base timing?
Timing advance at idle?
Set the idle mixture screws?

My guess is that you have too little timing advance and therefore, have to open the throttle blades too far to get it to idle. This over-exposes the transfer slots and leads to a real rich idle and cruise.

timing is at 45 degrees. idle mix screws are set to as turning in (clockwise) to max vacuum off manifold like holley says

Check to make sure the float levels aren't too high (gas will dribble out of the boosters). Too high fuel pressure can cause problems also.

Not uncommon for an out-of-the-box Holley to be rich. They have a universal setup in them. If you turn the idle mixture screws all the way in, does the engine stall?

float levels are fine. if i turn screws in it stalls. someone said to try repalcing the power valve.

After a cold or a hot start?

mostly cold. has happened when hot

How is the choke set up?

electric choke

Yes, how is it set up?