Cylinder 3 stumbling at off-idle

Ok I'd sorted out most of the weird running issues on my bike, but I still have one cylinder stumbling with the throttle just barely open. I think it's cylinder 3 because I can't remember the numbering, basically it's the one with the vacuum line to the petcock.

I've done some searching around the forum and didn't seem to find a lot. When I put the color tune on it you can see it missing with the throttle just barely open and firing just fine otherwise. So far I've come up with these ideas:

1. Carbs slightly out of sync and that one isn't opening up with the rest
2. Petcock issue and it's getting fuel through the vacuum line
3. No back-pressure due to a hole in that cylinder's header tube (which I keep patching)

These feasible? Anything I could be missing? Carbs aren't my thing, all my training is for fuel injected stuff.

 

Although I doubt this is the case, it's easy to test for your 2nd scenario. The petcock vacuum line can be attached to any of the carb nipples, so just move it to an adjacent carb and see if the problem clears up.

Probably, though, you're dealing with #1. Get yourself a Morgan Carbtune Pro, and you can sync all four of your carbs at once in a matter of minutes.

I'd also check for air leaks in your throttle shaft seals and airbox boots, and I'd check the voltages and resistances on the ignition coil that powers that plug. If it's getting a weak spark, it might do what you're describing.

This all assumes that your plugs are new and properly gapped.

 

I seem to have the same problem. What feels like a slight miss but when you open up the throttle all is well. The plug in cylinder 3 (third from the left) is smutting up as well.

I thought it might be the plug so switched it but the same cylinder smuts up.
Still a work in progress with me. Any new developments with you?