This is my first post to this site, although I've been looking around for awhile and am impressed with the amount of knowledge floating around out there.  Now I'm hoping that knowledge can help me.

[QUOTE=daveo76]Just rebuilt the Q-jet thinking it was plugged idle passages or a carb vacuum leak. If anything, problem is now worse...[/QUOTE]

You probably answered your own question...
The carb may have a warped base plate, or top. At the speed you get the mis-fire(1500-2500), it should have already transitioned from the idle circuits to the main jets. If it is actually running that lean, that can/will cause an exhaust backfire under decell.
I'd suggest pulling the plugs and "reading" them. Look for really bright white insulators for a lean condition.
The EGR not functioning could create what would appear to be a vacuum leak if it is stuck open a little, and could also cause a backfire, as it is allowing extra air into the exhaust...
How about the cap/rotor? How do they look? If the cap has "lightning bolts" on the inside, it is bad...same with the rotor. Turn the rotor over, and lok at the bottom of it. If you see any lightning bolts, or dark lines going from the center piece out towards the two screws, it is bad. The plastic the rotors are made from doesn't always stand up to the high voltage, and it will actually allow the voltage from the center piece to arc over to the screws...

I'd suggest taking a look at the cap/rotor first, then the plugs, and go from there...
hth

Thanks for the response.  Hmmm, warped base plate.... is that common for a Q-jet?  What about the manifold?  (original)   One question I had regarding the carb to manifold interface:  when I took the carb off, there was only a 1/4" thick black spacer as a gasket.  The rebuild kit had the spacer as well as a cardboard gasket with the four holes to match the bores in both manifold and carb.  I only used the spacer gasket, should I have used both?  If so, should the carboard gasket go on top of the spacer or below it?  Interestingly, the original spacer was cracked which is why I thought that was the problem when I removed the carb. 

The thicker gasket is correct, if it completely covers all of the open areas of the base plate. You have to really watch, because some of them do not fit every application, and it may leave just a bit uncovered around the front, and sides, and create a vac. leak.

The base plates can, and will warp, if tightened too much, and may even snap the ears on the rear, while warping the top in the front. The base gasket should have little hard plastic inserts around the bolt hole to keep from over torquing, but...
Also, you should always make sure the gasket doesn't interfere with the throttle blades. I've seen many that would actually keep the secondaries stuck open a little.

If your EGR valve is flat on the bottom, you can put a temporary block off plate in there for testing. Just a thin sheet of tin would work for that.

So if the carb base plate is warped, that pretty much means new carb, doesn't it? 

IF the baseplate is warped/cracked, it can be changed. No need to replace the entire carb for that, if a suitable replacement base can be found.

The EGR gasket is paper-based, similar to intake gaskets, but it does have a metallic/lead-like layer on the outside, and some do have a thin stainless steel core. Normally, if that gasket is leaking, you should be able to hear what amounts to an exhaust leak around the valve, but it could be a small amount of old gasket would create a small vac. leak without being loud enough to hear.
I wouldn't really recommend using starting fluid to check for vac. leaks, tho...use some spry type brake cleaner, (or carb cleaner if you have an aluminuminum intake) as starter fluid could cause a really nasty explosion if the car happens to backfire...

I had a leaking EGR on my L-48 and made up a little block off plate with some steel sheet. Solved the problem and I guess I never bothered to repiar the EGR valve since it worked well with it blocked off.  Oh well....

I finally got time to do some more troubleshooting on this problem. I had it running today and continued to look for vacuum leaks using the carb cleaner spray method. I found a spot that dramatically increased idle speed if I sprayed - surprisingly to me it was right around the back of the RH exhaust manifold (at the interface between manifold and head for #6 and #8). Is it possible for a leak at the exhaust manifold like this to cause a lean misfire, i.e. sucking in outside air on the intake stroke? No amount of spraying anywhere else changed the idle, and I sprayed a lot at the base of the carb and at the interface between intake and heads.

So a little more background here: A few years ago, I replaced the stock exhaust with 2-1/2" dual Magnaflows. While doing this, I broke an exhaust pipe stud on the manifold in question. I proceeded to remove the manifold, managed to get the stud out and replaced it and reinstalled the manifold. Ever since then, the car has always had a slight backfire on decceleration, but I attributed that to the fact that I eliminated the cat and associated backpressure. I've also been able to see a slight bit of smoke from that corner of the engine - I blamed that on a leaking valve cover gasket, but now I'm thinking I may have been wrong on both assumptions. So if this is indeed the problem (or if it's even possible to cause the symptoms) what should I do? I remember taking that manifold off and reinstalling was kind of a nightmare, but I'm concerned that it may be cracked there.   I'm going to try retorquing bolts first, but should I really take that manifold off and check it? What about using a gasket - stock doesn't have gaskets and I didn't put one in when I reinstalled. Should I have it planed? Also, does anyone have a better locking method than those French lock things?

(sig pic)Adams' Apple2007-10-14 08:18:55

i have never seen and exhaust manifold or any other metal to metal surfaces without a gasket in between them. is it possible that whoever had the car before you left them out. on my 75 when i got it there was one and a half on passenger side. thought i bought a nightmare. i replaced it and the only thing i did was loosen the header from the pipe so i could turn and manuever it. bolted up flat against the head and no problem. whoever worked on the car before was too lazy to do what i did.