Its been a while, but everything has been going so well. And now as usual I'm here when I need help.
During a great enthusiastic ride on Sunday over the Helmsley TT the bike running crisp and well.
Refuelled at Malton with Excellium (best I could find) within 10 miles bike occasionally had a power delay on opening throttle around 9k then after a change down we were off (mostly).
Did this quite a few times and seemed to be worse after a few moment on a closed throttle (for example coming upon a line of cars prior to overtake)
Called in at Sherburn and then noticed very rough running and it stopped, refusing to Idle and would not start. Had to drag the wife out with my van and rewinding my day put everything down to some crap in the fuelling.
Anyway carbs out and tank emptied plugs checked and found nothing, all seems well.
It will kick over and reluctantly start with the top cylinder running well. Getting PHUTT type backfires from the front cylinder and it will not idle. Front cylinder runs fine above 5k but seems to be nothing below but occasional backfires.
Done search for backfires on this site and now think I may have an electrical issue that developed on my ride.
I'm ashamed to say I haven't layed a spanner on it for 2 years, just ridden it.
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if it had a tail - it would wag

My guess is that if it's not fuel, then it's plugs.
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MC21 Honda NSR250. TYGA 300 conversion.

Probably seized it! Get the pipes off and have a look up the exhaust port.
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Andy.
NSR-WORLD.COM

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Thanks Andy. Job for the weekend then.
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if it had a tail - it would wag

It's not a cert', but I've seen it before. Blue flashes out a pipe or a "phutt phutt" is not a good sign!
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Andy.
NSR-WORLD.COM

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My MC16 did almost exactly this when the CDI's failed. Worth swapping over if you have an MC16 as the problem should move to the other cylinder.

Thanks, Its a 21. I'll try that swap if its possible on mine. Fingers crossed for a simple answer. I am guilty of poor maintenance and haven't checked the coils or cables for.... must be 2 1/2 years
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if it had a tail - it would wag

a clogged pilot jet (from bad fuel?) will cause the cylinder not to fire below 5,000 rpm.
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MC21R9N track
MC18R2J track
CBR954
CR250

I would hazard a guess and say you now have a 150.
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If I have to take the carbs off once more...

I would check for seizure first and if no sign of that, check coil connection for signs of arcing (pitting, charring). Actually, I'd check coils first because it's easier and I'm lazy. but I had the puh puh out one exhaust and backfires and it was bad coil connection, so maybe you'll get lucky too.
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-Jim
'93 MC21

Yes it nipped up!
I.ve misled everyone though by describing a faulty front cylinder when it was infact the upper one. Everything else was explained correctly though.
Took your advice Andy and looking through the exhaust port and I see a piston with a bit missing from the edge of the crown, bugger.
When I got a Phutt type backfire I also saw a small puff of smoke from between the bottom of the barrel where it meets the crankcase as you would from a blown gasket.
In my ignorance is this likely to stop at a replacement piston and barrel or should I be looking further.
Advice , help , parts sourcing advice all welcome
Thanks, Mal.
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if it had a tail - it would wag

I don't have much 300 stuff any more, its all broken, but I do have some head gaskets if they are any use to you Mal.
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If I have to take the carbs off once more...

You told me earlier you had at least 3 perfect barrels, Steve!
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Andy.
NSR-WORLD.COM

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Yeah, but the perfect ones are 250's though. All my 300 ones are seized!
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If I have to take the carbs off once more...