Just pulled the plugs from my newly acquired 91 MC21 that was running sluggish on the bottom end and through part mid-range. When riding, power comes on strong at around 5000 rpm, but below that feels lackluster. The plugs (bike had NGK BR9ECMs) were in pretty fouled shape and since I was unable to find a new set I wire-wheeled them clean and reinstalled. Compression is good at around 100psi in both cylinders, but when I started her back up it appeared as though the rear cylinder wasn't firing consistently at idle and through mid-range. Spark was there (it looked a little weak while cranking by hand, in the dark for visibility, with plug attached and grounded) and fuel appears to be flowing reasonably well to both cylinders. I swapped plugs between cylinders and it didn't make a difference, rear cylinder was obviously colder coming out the pipe (also blowing more smoke) and would pop when the throttle was cracked from idle, while front cylinder had good, hot, clean exhaust with either plug.

Any suggestions on why the rear cylinder isn't running consistent at idle/low rpms? My ideas center around these issues: plugs bad, inconsistent/weak spark, bad plug lead, weak coil, carb not flowing at capacity through low speed circuit. Bike has factory pipes and around 19k miles. Not sure if it has had wire splice, jet mods or is deristricted. It runs strong when into and past mid-range (5k to redlline). Any help on areas to check is appreciated.

Also, can I run the same plug in the Iridium version?





'96 MC28 SE
'92 MC21 SP
'91 MC21
'89 MC18SP Terra Racing Replica
'85 NS 250R
'85 NS 250R Rothmans Replica
'86 NS 400R
'83 MVX 250F
'82 Motocompo
and a whole lot of other smokers!
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http://nsr250.net/forums/viewtopic.php?t=6130&postdays=0&postorder=asc&start=15

read the last post, that might help

Something else you may want to consider as a possible fault - crankcase seals. Your problem is on the rear cylinder so if it was a seal issue then look at the crankcase seal behind the Flywheel / Stator.

Before I had major cold starting problems, I noticed that the rear cylinder was running lighter, with regards to pulses out of the pipe. I did a carb vacuum check and found the rear pot carb was pulling about 30% of the front pot carb.

When riding the bike, it was foggy as hell under around 5000 rpm and just wanted to bog all the time. But it still ran very well over 8000 rpm every time.

I did a crankcase compression test and found the cases would hold zero compression or vacuum. You've done a cylinder compression test which only shows the rings are OK. You could try spraying 'carb cleaner' or 'easy start' behind the flywheel on tickover and watch for revs rising, that would tell you the seal is sucking in. But remember you really need to test the seals in both directions to be absoluetly sure.

Your problem could well be something else completely, but I wouldn't rule this out from your fault diagnoses. I stripped the carbs several times thinking it was a fuelling issue before I tested the cases.

This thread shows my problem:
http://www.nsr250.net/forums/viewtopic.php?t=7023
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30