While this may not be V8 related, I still can't think of a better place to ask this question than LS1tech purely for the reason that some of the most educated and friendly people post here so here goes.

This is question concerns my DD, it's a 1976 BMW 2002 with a weber 32/36 carb on it - yesterday, after parking my car (it's running flawlessly at this point), I get back in it an hour later and start it only to have a myriad of problems start at the same time, and they're as follows.

1. The car will not hold an idle, even with the adjustment screw in all the way, it almost dies and then jumps up 800 rpm and just repeats until eventually after 30-40 cycles it misses and dies.
2. The car will not hold steady revs, even under partial throttle, the RPM fluctuates massively
3. No power unless under WOT
4. Missing at low RPM
5. Will not hold steady speed while cruising in any gear, only under full throttle or full deceleration does the engine NOT backfire
6. With the hood open - as I was cranking, the carburetor lit on fire at one point yesterday for about 2-3 seconds before extinguishing itself
7. Under any amount of partial throttle at a cruising speed, the car will backfire and hesitate
8. Everytime the car catches itself, it draws massive vacuum and pulls more fuel in through the secondary on the carb, this is causing a massive rich condition

-I suspected a vacuum leak, but all my connections are still good - the WD40 test revealed no leaks
-I pulled the plug wires one by one, and all of them gave a noticeable change in the engine performance, so all 4 are firing
-It seems like every 4 cycles the engine misses once on at least 2 cylinders, it's shaking violently in it's mounts
-Replaced the 4 plugs yesterday, cleaned rotor & dizzy cap
-Reseated all the plug wires
-Disassembled carburetor suspecting a float issue, neither of them are stuck open/closed
-I tightened the jets and backed each of them out 1.5 turns per spec, but no matter how tight/loose they were this didn't make a difference
-Tightened the idle primary/secondary adjustment screws and tried about 20 combinations of half turns and quarter turns, everything I could think of to get it to hold an idle, it refuses to

-I want to say it SOUNDS and LOOKS like a vacuum leak, but looking at the secondary on the carb that's getting flooded with massive amounts of fuel due to vacuum, that may be only contributing to making this problem worse, but with such a large amount of problems instantly appearing, I can't even begin to think where to start, any help would be greatly appreciated!

-Carburetor rebuilt 3 months ago, nothing like this has ever happened before
-Upgraded coil, plug wires, mechanical advance distributor, all less than 6 months old
-Rebuilt motor, ~20k on it
-This weber is NOT a stock carburetor for this car, it fits and is commonly used to replace the solex that came with them, but I don't have any other carburetor to replace with for testing purposes, and can't get any other replacement for at least 3-4 days and this is my daily and it's keeping me from getting to work and back, I need help with either diagnosing this or a point over in the right direction for me, if there is any more information I can provide just ask, I hope I gave you guys enough of a description that could ring a bell or reveal an idea for resolving these issues.