Long time, no post. For me anyway. In the process of pulling my mc21 out of a 2 year hiatus. Been over the whole bike, get it running, and the front / right cylinder is not happy. It cuts in and out. Mainly out with the odd cut in. When it does cut in, there is a fair amount of smoke, so I'm assuming its getting fuel. I have a heap of testing to do tomorrow, but I had changed the reg/rec previously.
Question is, can the reg/rec effect each coil independently ? I am fearing when I swap the coils, nothing will change, ruling out the coils.
Knowing this info before I start testing other things will help.
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Winning isn't everything, but loosing isn't anything

It's had been a long time!

No, the condition of the reg/rec won't effect only one cylinder. It "simply" converts AC (generated by the flywheel) to DC, smooths the flow, and regulates the voltage to the electrical system to a more manageable 14.2V.

You want to be looking at the coils, plugs, plug caps, and slow running circuit in the carbs. If you still have no luck, and it all checks out, you could be looking deeper at the centre crank seal, or sticking piston rings.

First thing I would check would be the harness terminals on the coil. They tend to corrode/blacken, causing poor conductivity, especially if someone's been a bit over-enthusiastic in the past and rubbed the plating off the spade connectors with sandpaper!
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Andy.
NSR-WORLD.COM

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If a cylinder misfires when cold, but then fires normally after a few seconds or even a minute of running and then runs fine is likely the centre seal?
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MC21R9N track
MC18R2J track
CBR954
CR250

I really do not know.
But how's this theory:
Assume the battery is not holding its charge all that well.
But it has enough charge to fire one cylinder (yeah that sounds weird to begin with I know)
After a little while of running the engine and charging the battery, both cylinders fire OK.

So maybe ?? it is all to do with the battery??
Just a wild random guess.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Cheers for the info guys. Good to know about the reg. I thought that was the case, but wanted to confirm it.
Battery is brand spankers. I know it can cause dramas when not healthy, so ticked that box before doing anything.
Rings and general piston health looked ok best I could see through the exh port. Still good hoan pattern, and the rings looked the same on both pistons.
One thing I noticed when pulling it down was the left cylinder crank case was half full of fuel.
I took the bowls off the carbs and cleaned all the jets (they had started to corrode). Set the floats. When I put fuel to them ready to run, the left carb started leaking from drain/ overflow. I run it anyway, and the left cylinder is the one that runs fine.
Ill report back more once I've tested some more things.
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Winning isn't everything, but loosing isn't anything

So, at this stage, it's looking like the front/right coil is failing. Swap coils, and the cylinders reverse. Different rpm does not seem to make a difference when it cuts in and out.
Battery voltage when running is 14.75.
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Winning isn't everything, but loosing isn't anything

I had a similar issue with my bike. It was starting reluctantly and not idling and spitting unburnt fuel and oil out of the front cylinder. When I rode it, it ran fine. In the end it was the coil. Luckily all I had to do was clean up the connectors but I think it must have been an intermittent short or whatever.

There are a couple of topics on here about checking the coils, do a search and see if they help narrow things down...

Yeah. Been though those. The spade terminals on the the front cylinder wiring loom are a tad ordinary, but they run the rear coil no prob. Ill replace them reguardless though.
Ill check the primary and secondary resistance on both of them next time i am working on it. Found an old thread on here about a bloke that used some MSD coils that worked really well. My brother is an agent for MSD, so im getting a price on those. Would just replace both if they are not too dear. They are getting old after all. Wont hurt to have a spare that i know works either.
Everything seems ok so far.
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Winning isn't everything, but loosing isn't anything

So found a thread with a rather informative demo by dave showing how to check the coils. Both of mine read very similar.
The good coil reads 0.6 / 8.35 (@ 20k)
The bad coil reads 0.6 / 7.54 (@ 20k)
So I'm guessing if one was fooked, one of those would read way out.
Now, one of the terminals on loom to the front cyl, is 3/4 stuffed, but runs no prob with the good coil. I though that might of been the problem, but it appears not.
The terminals on the front coil are also pretty tired looking, and the terminals on the back cyl coil are very good, so next on the check list is cleaning the terminals on the dodgy coil and seeing if it improves.
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Winning isn't everything, but loosing isn't anything

having been through this years ago, I would suggest in addition to cleaning the terminals, make sure the connection is good and snug - that turned out to be a key in my case. Loose connection produced arcing. Gentle pinch with needle nose pliers did the trick. Good luck with it!
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-Jim
'93 MC21