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.

I've just got back from a 100mile trip and its been running perfectly so I'm now hoping I don't have to take the carbs off again before the winter. I used one strand of picture hanging wire for the orifice - similar to a wire brush strand. Can't begin to imagine what you mean by a small round thing for syncing. Oh, I see , maybe a drill bit. I get the impression the sync might be just slightly off still , but it's not bad enough to warrant taking the carbs off again right now . Now that she's running again it's given me the urge to do a bit more on her. I've changed the rear brake pads and I'll be doing an oil and filter change over the weekend. I might even buy a set of pressure gauges - then I could do a dynamic sync. I'll also be looking into coating the inside of the tank in due course as there's a bit of a rust issue in there. I'm relying on a newly installed fuel filter to keep any rust out of the carbs.

I have a '94 and when doing an oil & filter change, I do it w/o removing plastic.
I slip a filter wrench like this one in between the slot in the plastic and position it on the filter.
Working from the right side of the bike, I turn the handle bars to the right and attach a midsize extension to the filter wrench and loosen the filter.
Tipping the filter up, it can be removed out through the space between the plastic and the engine and the down pipes.

Hi TBT - Thanks for the tip. I vaguely remember last time I changed the oil filter (more than 12 years but only 5000 miles ago) that I found a way to wriggle the filter out without removing the plastic. Whilst I'm on here, and now that I've been on the road a bit I'm keen to "tune out" one minor ****le I've noticed. When in neutral (or with the clutch pulled in) and I blip the throttle, the response is a split second slow. It's not the end of the world but must be a symptom of something. Have you come across this and what might be the cause. I'm thinking it's carb related (again) and either sync (yes I know you've already told me lol) or mixture??? I might not get around to sorting it immediately as I'm enjoying being back on the road, but if I can find some indicator as to what might be the cause I could be tempted to sort it (after I've changed the oil&filter). Also (sorry to drain on) I found an old-ish thread a few days ago where somebody (it might well have been you) mentioned shimming the needles in the carb up one to improve mid-range performance. I can't find the thread again now, but if you know anything about it then I'd like to hear about it too. You can see from the way I'm rattling on that now the bike's going again it's rejuvenated my enthusiasm for something that's been stuck in the garage over here in France for 10 years due primarily to power limitation laws which have now been lifted! She may be 27 years old but you'd never guess it from her condition.

From what I have learned, here in this forum, the power limitation is done by choking down the ID of the rubber boots between the carbs and engine. If you replace them with regular boots and go with 129 main and 42 pilot jets you will be in for a big surprise.

I have a K&N air filter and run with 135 mains and 54 pilots. She runs great. I went up with the jets due to the ethanol but run non-ethanol premium with very good results.
Also, I have done the stick coil mod and that smoothed things out even better.