Hey guys.

Ok so I have a 21 that is relatively new, rebuilt about 1000km ago, and its just developed an issue over the last 2 rides.

I'll say first that I made a clerical error the other day and left the fuel tap on and the bike in the sun for a few hours. leading to much frustration from 2 wet plugs (same cylinder). and having to borrow a rather exxy iridium for the trip home (from a mates worked tz250).

Needless to say that since this occurred, the bike initially runs ok, but struggles at the powerband. 2 minutes further down the road and it wont run past 6k. it just bucks and misfires....

First thought was that the floats are now fubar, but turning the fuel off and running makes no difference (running oil injected). I have also tried a new battery. no change....

so. Im now leaving this to the learned masses. any ideas?

the struggling into higher revs makes me think fuel, but the misfiring makes me think electrical.

I had a similar problem on an NS400. Turned out to be crank seals. Did you replace the seals when you rebuilt it?
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If it ain't broke, fix it until it is.

Noncompressable fuel in the crankcase can blow the seals and on top of cylinder, it can blow the topgasket.

When you say "rebuilt", do you mean the top-end, or the whole motor?

Like Poul and Mark both say, I too would lean towards crank seals, but without more information it's not easy to be sure.
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Andy.
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Thanks for the responses.

it was rebuilt by the PO, but he only did 200km after it was finished. Yes. it had new crank bearings and seals. Ok so im guessing its not going to be a simple fix. Will go for a top end pull down in the next weekend or 2. Thanks guys.

but from my reading that I have done around here, if the seal(s) have gone, wouldnt the bike fail to idle correctly? This bike idles like a peach. I only start getting problems over 4k rpm.

Start the bike, and put a volt meter across the battery. You should get a steady reading of around 14.2V. Blip the throttle, and see what happens.

Any spikes or drops, and it's the reg/rec. Worth a try, if it's idling fine.
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Andy.
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Thanks Andy. I will try that when I get home. I have put a spare battery in (to no avail) but didnt check the voltages, so will look into that.

Hey guys. A happy update.

So, after seeking plugs from a supposedly well respected local 2 stroke shop. I was supplied with 2 BR9EG plugs which were fitted…

Needless to say, these seemed to be the main source of my problems, they have appeared to foul pretty quickly, (the electrode is super fine, much like iridiums) and after swapping to the BR9ECM's all of my backfiring and stumbling were gone….

Added to this, a visit to the local nsr guru, Neal, a quick power valve tune up, and 90% of my top end hesitations are now gone too…

Learning new stuff every day. So pleased with the outcome. we can see she is a little rich still, but nothing that can't be fixed after a few more rides.

EDIT: oh and Andy, the regrec is running good, 14.51 at idle and 14.38 at revs.

That's great news.

You can't knock Honda spec. They say BR9ECM, and it works... just like 99% of the rest of the things people try to "upgrade"!
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Andy.
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In my experience its always wise to try start at the basics and work through them... its often all that is required to fix a problem, as in your case. What could be a simpler and cheaper fix than new plugs!. This should always be the starting point for diagnosis... not "Crank seals" as often seems to be the case....

Glad you got a good result

That's always the "problem" with trying to diagnose something without actually being there! You are rarely given enough, let alone all, the information.

The way a question is worded often leads suggestions in a particular direction too.
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Andy.
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that sounds pretty much the same as what my mc21 did
i just parked it up last year and havent touched it since
ive just bought some plugs for it lol

Hopefully, it'll fire right up then!
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Andy.
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Andy wrote:Hopefully, it'll fire right up then!

i wish
my luck doesnt work like that lol

Haha! I feel your pain!
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Andy.
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