Switched to 55 idle jets, no popping, but "stumbling"
 

I had 45 size idle jets on my webers and they slightly spat up. The car ran just fine otherwise. Now I switched to 55 size jets, and I had to adjust the carbs again. The problem is that the now the mixture screws make almost no effect whatsoever.

The car pulls just fine, but there is quite a lot backfiring through the muffler (its NOT an exhaust leak) on deceleration and it kinda..hesitates with low revs...running of the engine does not sound so balanced (its like some cylinders won't do jack nor the other).

Any ideas?

What emulsion tubes are you running? This is one of the causes of unusual transition running (just off idle, to mid throttle range).

I am running with F26 tubes and 27mm venturies. The engine is 2.4T. Air correction jets..hmm..185 (??)

Sounds like possibly the step from 45 to 55 idle jets was too much. Do you have jet drills?

We used to drill out a small ammount at a time, to get to the right point. On webers the idle jetting is WAY more important than most people think, because when you are driving normally, most of the time the throttle is in the "just off idle" point in the carb's operation. If it's not jetted just right, all kinds of surging and popping will take place.

I think the F26 tubes are correct, sorry I can't be more help. I think I have some old PMO carb stuff around here, I'll try to dig it out. Or possibly someone can beat me to it.
Meanwhile, can you describe the rest of your engine? Displacement, CR, ignition,

Also, another thought.... how old are the carbs? Have you checked for air leaks around the throttle shafts? You can do this with a propane bottle, or some carb cleaner, etc...

the 55 jets should be fine. the idle screws need to be adjusted with the idle at 1000 rpm. you won't notice much change at 600. do one at a time, and screw it in slowly a bit at a time, until the engine just begins to slow down. then back it out just enough to get the fastest, smoothest idle without going too far out. then on to the next one. several times around is normal. tune in on the sound of the engine, in a reasonably quiet place, and close your eyes to help you concentrate. then you can hear the slightest change. the timing also needs to be in a decently advanced place to have good response. 35° total usually works well.