Idle drops when in drive

With issues like this, I like to make diagnosing as simple and quick as possible. So far the convertor and carb have been switched out and there's been some discussion that the vacuum advance has no effect on the idle.

I would simply disconnect EVERY vacuum hose on connected to the engine and CAP OFF all the vacuum ports on the engine. Including the brake booster hose and vacuum advance. All hoses. So its just the engine with nothing connected to it.

Now we should have just a running engine with nothing hooked up to it. Does it run any better? Engine rpm still going up and down?

If not,

I'd set the timing to 0 degrees with a timing light. Then I'd determine top dead center on #1 cylinder. When your at TDC, the timing mark on the balancer should be at 0 degrees. Now we know that your balancer hasnt slipped and its correct. Restart the engine and set your timing where you like it.

Now I would grab a can or two of carb cleaner and start spraying all around the base of the carb and all sides of the carb. Especially at the throttle shaft. This would be to check for vacuum leaks. When the cleaner gets sucked into a vacuum leak there will be a noticeable change in engine rpm. If you have a "T" for the brake booster vacuum on the back of the manifold, spray around it as well while its capped off.

If there was no engine rpm fluctuation from the carb cleaner, then you can assume you dont have any external vacuum leaks. The issue is then in the carb itself.

My reading about Holley carburetors shows a slot cut into the body that is to aid in the transition into the secondaries and when tuning the secondaries you are supposed to adjust to where that slot is barely visible. In my mind it seems right to have the valves open just barely because if they were shut completely I don't think they could overcome the vacuum seal to open at all. Someone with more carb knowledge please chime in because I am also still a noob at this.

So yesterday I readjusted the secondaries completely closed again to eliminate them as part of the problem and I replaced the stock 65 primary jets with 62s. I also disconnected and plugged all vacuum lines, even those not connected to the carb.
It cranked and ran well, idling high but smooth. So after it warmed up I checked timing again and it's dead on 11° and advances as you give it gas. I still haven't figured out the vac advance but it still does nothing at idle.
After checking that and tuning the carb a little it smelled rich still so I changed the jets to 60s and reconnected the vacuum lines.
It's not modulating up and down like it was the other day but idle still drops a few hundred rpm when I put it in gear, and with a hard thump. It's also hard to actually drive because of the dead spot off idle. I need to fix that to actually be able to drive it.

The secondary plates of a carburetor are designed to be flush against the bore when not in use. The plates are actually a slight oval shape and the edges are bevel cut so the plate is not at a 90 deg. angle. You might not be able to tell on the Holley without some precision measuring tools, but you can see this on a Rochester's huge secondary plates.

Your last post is helpful. The idle fluctuation is not due to weak advance springs. The very next thing to do is hook the vacuum advance to full manifold vacuum. You should now have about 30-32 degrees of advance. The engine will increase in idle speed also. This is normal. Just adjust the idle speed back down to where you want it. It should now have the proper ignition timing set and be steady idle speed. The vacuum advance helps keep the rpm from dropping so much when it goes into gear.

Once the timing is set, only then do you do carburetor adjustments. I like to use a vacuum gauge to get the idle mix set, but it isn't necessary.

Get the mixture screws turned out an equal number of turns, usually 3-3 1/2 turns is what most guides recommend. You can turn the screws in 1/2 turn at a time. I like to turn both mix screws in, then adjust the idle back down.

As long as the idle speed increases, you are on the right course. Once the rise in RPM is diminished or actually decreases due to the idle mix adjustment, just return the mix screw to it's previous setting and adjust the idle speed to where you want it to be. This is where using a vacuum gauge comes in handy to finely adjust the mix screws for the highest vacuum reading at the desired RPM.

At this point you can diagnose the other carburetor issues, and add the other vacuum accessories to find out if those are even an issue.

Tuning a carbed engine just takes patience and following a set procedure. You always want to verify the ignition settings and condition before the carburetor because ignition problems can mimic a carburetor problem.

Good luck!

After reading your post, bigred, I decided that I was going to return the carb to stock, and go over everything one more time just to be redundant before I go down the list you made. So I started with checking for vacuum leaks again, before I took the carb off to put the jets and secondaries back to stock settings. Well I found that my intake manifold is leaking on the drivers side in the middle, which sucks because its got a brand new gasket and has been torqued down twice as per spec. Then I took off the carb to go over it and found a shocking amount of play in the secondary throttle shaft. It is an obvious vacuum leak and makes setting the throttle plates impossible. Thats what I was doing when I noticed it.

So now I have to take the intake off and reseal it, and fix the carb. Does anyone have experience with the bronze bushings they sell? I also found nylon ones that you can install without disassembling the baseplate. And any tips for a leaking manifold?

Sometimes manifolds need the gasket surfaces refinished. My engine builder encountered that with sbc vortec intakes. They would be fine out of the box for initial install, but if it was removed and re-installed, it warped just enough to cause a vacuum leak. When you remove the manifold, just have a machinist check the flatness of the sealing surfaces and repair if necessary.

I've never repaired worn throttle plates, but there are plenty of rebuilders that can do this for you with a reasonable turn around time. A loose shaft like you describe could cause the idle problems you've been having.

Hopefully that is the actual problem but the frustrating thing is that is the problem my edelbrock carb had that prompted me to replace it.