1 cylinder downpipe goes "cold" when I come off choke
 

I've been doing some carb cleaning over the past few weeks. Now, when running on choke all the exhaust downpipes get hot suggesting all 4 cylinders are running "normally", but as soon as I come off choke when the engine's at normal operating temperature the downpipe from one of the cylinders starts to go cold indicating the cylinder is no longer running normally. Anyone come across this before?

Your carbs are out of sync.

I've done a bench sync after cleaning. I understand it's not as good as a dynamic sync but I doubt any slight difference is enough to cause the problem. Of course, if I end up taking the carbs off again I'll recheck it.

When the enrichners are engaged, you are dumping raw fuel in. If you don't have sufficient vacuum you can't draw fuel from the bowel. At idle, the butterflies are barely open and need to be very close to each other from a vacuum point of view. If you open the throttle you would create enough vacuum to pull fuel up and the cold pipe would get hot.

Or, if the the pipe gets hot when when the throttle is opened and you can prove the sync is spot on, the slow speed jet may be plugged.

TimBucToo, thanks for your latest comment. In fact, I've taken the carbs off again and found a slightly unusual situation. Firstly, I don't have a chemical or ultrasonic "bath" - just aerosol carb cleaner, and I don't have a compressor either - just a pair of puckered lips and a bicycle pump! So you can see my carb cleaning kit falls short of ideal. Anyway, I noticed after "cleaning" again and looking through a jeweller's loup there appeared to be what looked like a miniscule "flap" of gum/varnish that kept intermittently folding over the tiny orifice in the pilot jet - actually 2 of them. No matter how I tried with carb cleaner and copper wire (and puckered lips lol) I couldn't get rid of it so thought I'd just put it back on the bike and run it with some more seafoam, then soak, run, soak, run......I've been doing that for the past 48 hours and it seems to be staying clear (fingers crossed) and running OK through to operating temperature in the garage. I'm hoping the seafoam is slowly dissolving the problem away. I'll be trying it on the road later this afternoon and hoping I can get a few miles in without constantly wondering when the problem's going to reappear. If I find the problem comes back, I think I'm going to be obliged to buy a new set of idle jets (not the end of the world). At least that way I'll be certain that it's not those which are the problem but I'd prefer to be able to at least get through the season without taking the wretched carbs off again. I'd rather be sweating on my bike than sweating over it! I'll let you know how I get on on the road.

Just read your comment about the lack of a compressor. A super easy fix is to buy a can of compressed air like is used to clean electronic equipment. It'll provide ample pressure and has enough in a can for an entire carb clean and then some.

Hamlin6, I looked at a couple of compressed air aerosols but they seemed rather feeble. Maybe I overestimated what's needed so if there happens to be a "next time" (hopefully not for a long time) I'll take your advice.

Have you tried popping the jet out and hitting it with spray cleaner from the back side. It may be a bigger piece that is lodged in the back side that you can see from the front side.

TimBucToo, yes, sprayed carb cleaner from both sides several times and all over the garage as well! At least it got rid of the mosquitoes for a few minutes (lol).

If you don't have something small enough to clean the hole, snip off a wire from a wire brush and try to get in there and clean the hole out.
Also when checking/setting the sync, a small round object is better than a feeler gauge.