Have you tried to drop the revs down by moving the dizzy?...keep the idle at 1500 or where its happy...rotate the dizzy to see if you can drop the idle down...you could be as mentioned a tooth out which would cause you to see a false reading on the timing.
Also make sure that nothing in the throttle is binding and the throttle linkage is springing back.
BUT this could be the issue it could be what sort of Manifold are you using? If its an Edelbrock and you are using a holley carb I have read this..........(not my words below but I remember seeing this article when I was looking for my carb and manifold set up) hence why I went holley / welland
You are fighting the exact same problem I had. If you will go out, open the hood, and run your fingers around and under the holley baseplate you will see that it overhangs the edelbrock mounting pad, alot, which creates a vacuum leak. Even though there is a gasket under there it is not sandwiched enough to cover the vacuum passages adequately to prevent a leak. You can put a squarebore spacer under the carb and the vacuum leak will go away. When I installed a spacer, some of my problems went away because my vacuum leak went away, then I had to retune the carb.
Now the mechanic, while he has the carb off, will have to adjust the carb so that the primary transition slots are only exposed by about .020 of an inch, and set the secondary the same. Then bolt it on and tune, should go better. Right now he has the curb idle adjusted so high that too much of the transition slots are exposed, just to get it to idle, because of your vacuum leak. Once this is straighten out you may end up with an off idle bog which is hard to tune out on this carb, which is very frustrating.
With that being said the street avenger carb is an emissions carb. The terms "emissions" and "hot rod" should never be used in the same sentence. I don't know how funding comes into the mix here, but if funding is not a factor I would remove the holley, install an edelbrock 600 and be done with it. You don't need a spacer, which is a band aid for a problem created by the holley carb/edelbrock intake. The edelbrock also has a much better idle circuit and is more tunable than the holley, especially when running a C4 tranny. You wouldn't need your mechanic, just remove the holley, install the edelbrock and set curb idle and idle mix and be done with it.
When I ran into this same problem, I borrowed an edelbrock 600 from a buddy, bolted it on and all of my problems dissapeared. There are lots of forums that address major problems with the street avenger series. Being an emissions carb it is tuned very lean which is not good for a hot rod. I am including an article I used for tuning my avenger which helped immensly, got it close but I never got it right and retired it. Print this out and hand it to your mechanic, and if he doesn't agree or will not listen, find another mechanic. If the mechanic is not a hot rodder he is not aware of these problems. If you get this problem straightened out you may not need a stall converter. A stall converter, until you have your idle problems fixed is a very expensive band aid for a carb problem. Now if you do have a very healthy cam its possible you may have to go that route, but I have a buddy with a 66 that has a cam that rocks like a promod and he can idle his motor down to 700 rpm curb idle. He is also running an edelbrock carb/edelbrock intake combo, no vacuum leak.
If you will go out, open the hood, and run your fingers around and under the holley base plate you will see that it overhangs the edelbrock mounting pad, alot, which creates a vacuum leak.
I could spray around my carb but could not detect a leak simply because my idle was so high, just like yours. Then on this very forum, a kind gentleman by the name of Paul Stevens told me my problem. I followed his advice, bought a 1" phenolic spacer and my vacuum leak dissapeared. Then I could actually idle down and my air fuel mixture screws actually did something.
Both Holley and Edelbrock may squarebore carbs but the width/depth of the baseplates are different, butterfly centerlines are the same and each carb will bolt onto each intake. Holley makes the Wieand intake with a wider mounting flange to mate with the Holley carb, and naturally Edelbrock matches width of their intake to fit their carbs. Both carbs are virtually the same depth of measure. You can mount an Edelbrock carb on a Wieand intake with no leaks, but mounting the Holley on the Edelbrock intake creates leaks, been there, done that, got the T-shirt.
Try this short test tomorrow. Start the car, let it warm up....then cup your hands as tight across the airhorn of the carb as you possibly can. If the motor stalls you do not have a vacuum leak....but if the motor continues to run you have a vacuum leak. Think about it....if you are cutting off the flow of air with your hands then where is the air coming from to let the motor continue to run...air leak at the baseplate?
I have an Autolite 4100 on my car now and still have to run a spacer because of the width of the carb vs the width of the intake mounting base. I just measured my adapter, and on both sides of the carb the adapter overhangs the intake mounting base by 1/2". If your avenger is like mine, and should be, on the passenger side, rear there is a vacuum fitting I think for power brakes. The slot in the carb baseplate runs out to within 1/4" of the carb mounting base. This means there is 1/4" of vacuum slot that is not sandwiched between gasket and intake without an adapter.
