i just got my 1990 MC21 running but it does not run on the lower (front) cylinder until rpm exceeds 5000. above 5000 it goes well when under load, but at no load it stutters. it has fresh fluids, plugs and battery. it is all original with 6600 km. All the lights and guages work fine. Temperature guage reads just below 70 degrees c when running, going up to 80 if left idling. i've put 100 km on it thinking it is carburator related and might clean itself up. Troubleshooting guide for poor performance at idle/low speed gives the following protocol:

check ignition timing
check carb air screw
check for leaking intake mainfold
test cylinder compression (top cylinder is 155 psi, front is 140 psi)
clogged carburator

Any words of wisdom before i dive in? Seems to me i should just start with carburator cleaning. I'd like to avoid it as i have no experience cleaning carbs.

Dive in mate . Owning a stroker the best thing to do is your own repairs the best way to learn is to do any probs lots of people on here will help.
I`d clean carbs , Check wire connectors , Batterey.
Look here will help with cleaning the carbs http://www.nsr-world.com/tuning/250-tuning/carburation%20index/

I have a very similar problem, in that the right cylinder does not run at idle. Anything above is fine for me though.

After checking all the obvious things - connections to the coil, jetting etc I decided to have them sonically cleaned, which should have removed any gumming up of the tiny passages within the carb body.

I have a pretty good idea what I'm doing, so I stripped the carbs completely before having the work done as it's cheaper! However, the firm I use would have stripped and rebuilt had I simply delivered the complete carbs to them, and charged me an hour's labour.

The best way to do a job like this is take pictures as you go, so that you have some reference for putting it back together. If you do get stuck, there's plenty of us have photo's already to consult and post. Best of luck!
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball



Last edited by Dave Ett on Sat May 22, 2010 9:12 am; edited 1 time in total

Also have a look at the sync between the two carbs Dave.

If this is out then the once cylinder wont run at idle.

thanks guys. will start the process and keep you advised.
now to find the uninterupted time required.
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MC21R9N track
MC18R2J track
CBR954
CR250

Isn't there a chance that bad crank seals will cause a situation like this?

Every chance!
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Andy.
NSR-WORLD.COM

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if i'm reading the bottom end rebuild instruction properly it appears these seals can be replaced without removal or disassembly of the engine cases. is this correct?
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MC21R9N track
MC18R2J track
CBR954
CR250

The middle one could be tricky
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Just for info, having rebuilt my carbs the bike fired first kick. I let it run cold, then tried again and it fired second kick. Seems the clean has resolved my problems, and for the £25 it cost, maybe you should consider it before you start ripping cases apart?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

right. got it. carbs first. electrical connections whilst i'm in there. found a local dealer that will sonic clean and help me evaluate if parts are worn and need replacement. hope to remove carbs this long weekend. holiday here in the states on Monday. will let you know how it goes.
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MC21R9N track
MC18R2J track
CBR954
CR250

3 months later and the carbs are sonic cleaned and now it is firing on both cylinders at idle. i replaced the reeds with Boyeson and may have some jetting to play with. and one cylinder has more exhaust pressure than the other but i am ecstatic the bottom end is not the culprit. will play with carb synchronization and freshen up the top end this winter but in the mean time hope to put in some miles now that cool fall weather approaches.

thanks guys.

and fontyyy, i now understand the center crank seal.
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MC21R9N track
MC18R2J track
CBR954
CR250

the top end is fresh enough if you have that kind of cylinder compression

my r2j runs fine with around 120 psi on both.

my nsr 125r ran fine with around 120psi before i sold it.

which brings up a good question: how many kicks should it take to get a compression reading? i kicked quite a bit, until the guage would not go up any more. is this right?
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MC21R9N track
MC18R2J track
CBR954
CR250

i normally kick around 7 times with the throttle open and the engine at operating temp.