[10mikeymike]

I just got my 355 SBC back together and can't set the idle. The engine has and Edelbrock 2 X 4 RPM Air Gap manifold with 2-500 cfm Thunder Series AVS carb's. It has a Comp 280H grind cam with an MSD 8570 small cap distributor, 6A box and no vacuum advance. It fired right up on first start and still does, but wants to idle at 2000 RPM. If I back the timing off to 0 degrees initial advance it will idle down to 800-900 RPM, but the headers soon begin to glow. If I bump it up to 10 degrees, it runs right up 2000 RPM. There are no vacuum leaks at the carb or on the manifold. The linkage is properly adjusted and there are no kinks in the throttle cable. The idle speed screws are backed out and off of the cams. The idle mixture screws on both carbs are out about 1 turn. It has an electric choke on the primary carb and the butterfly is closed on cold starts. The butterfly on the secondary carb is fixed in the wide open position per Edelbrock's instructions. It appears neither carb has a fast idle cam or means of fast idle adjustment like their Performer carbs. I called Edelbrock tech support from work and all they reccomended was 16 degrees initial advance, but I'm afraid that much advance will have it idling at 3000 RPM.

 

[lg1969]

Either you have a stuck throttle shaft that could cause the blades to stay open or a real bad vacuum leak. Is the vacuum port for the PVC connected?
For a motor to run 2000RPM that is a lot of air getting sucked in somewhere else.

 

[66GMC]

The butterfly on the secondary carb is fixed in the wide open position per Edelbrock's instructions.

Huh?

I was going to suggest that you look under the base of both carbs to make sure that the secondary butterflies were in the 100% CLOSED position. Holleys, for sure, do have an adjustment screw for the secondary throttle plates that is only visible /accessible from underneath.

Are these carbs new or used?

Sometimes it is neccesary to drill a small hole in the primary butterflies in order to address mixture screw problems. (Mixture adjustments are completely ineffective when the throttle blades are above the transfer slots.) If these are used carbs that have had this done, and your engine doesn't have this requirement ... that would cause a "too fast" idle.

Thinking more about the fact that the Edelbrock carbs are of the "AVS" (Air Valve Secondary) design ... maybe the WOT thing makes a bit more sense.
Perhaps an adjustment to the tension on that AVS spring is all you need. Perhaps it's set too weak, allowing it to open at too low an RPM.

Somethings obviously letting too much air in if the primary speed screws are both completely off their seats, and it's still over-speed.