Hi-di-ho fellow NSR nuts.


I'm back onto the nsr project, still trying to diagnose whats stopping this grouse bike from running clean on both cylinders. The top bang hole fires fine, but I can place my hand over the others exhaust while its running and it dont make a difference at all.

I bought a compression tester and both cylinders are reading about 85/ 90psi.. Is this ok? can someone let me know what is spec?

Yes it has new plugs....

Your readings seem too low. See:

http://nsr250.net/forums/viewtopic.php?t=9011
http://nsr250.net/forums/viewtopic.php?t=1630
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I was reading in another two stroke forum that compression tests aren't the best way to determine wear, you are supposed to measure ring gap, that will give you an empirical answer too.

wow, 90psi seems WAY low.. Maybe its time for a little head work.

Cheers, Alan.

All it would need is a top end rebuild, so just pistons and rings. I don't think it will solve your top cylinder not firing though. Have you done a leak test?

120-125psi is a more normal reading. Depending on the skirt wear the piston might well go again and will only require rings.

Piston to bore clearance can be anywhere from 0.030 - 0.045mm. For a road bike the tighter side is better. You'll need a good bore gauge.

Check the ring end gap of the rings but they will most likely be worn out.
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If I have to take the carbs off once more...

Thanks for that, I have all the tools to do the top end and I'll get onto a kit ASAP.


I honestly believe that there is a good possibility that the carbies are the problem with the lack of bang. I'll rip them off again and give them a good going over.

Make sure you clean tour jets and emulsion tubes nice and clean. You got an ultrasonic cleaner?

I had the same issue with my mc18,guess what..The front ignition coil was dead