I had bit of a mystery problem with one cylinder a while ago. Fortunately I managed to solve it. Here's the details, I hope it might help someone.
Sometime ago I started to have issues with cylinder #5. Problem was that occasionally this one cylinder started to misfire violently when accelerating and few times I solved it by replacing the spark plug which was completely covered in soot and and misfiring because of that. It took me a while to realize that problem started only after driving some 15 minutes very slowly and idling in stop lights. I also noticed that problem could be fixed also by driving in highway speeds. Misfiring got less frequent and at one point stopped completely and bike ran normally.
So the issue obviously was cylinder running very rich at low RPM's, but why?
I did a complete rebuild on carbs 2 years back and carbs were just recently synchronized so those 6 heavy drinkers weren't my suspects.
I happened to have one spare coil with spark plug wires so first I changed the wire. That didn't help so next I took of the coil. I was surprised how dirty all the coil connectors were, especially the ones that ground the coil. I'm pretty sure that the problem was with poor grounding and I've heard that all the weirdest of issues usually are due to poor grounding.
I wasn't in the mood to gamble with the original coil so I changed to the used spare I got. I could measure the original coil, what readings should I get from it?
Summa summarum
There's always a lesson and this one is: Check and clean the connectors.
