I have taken my 21 out today and the spark plug failed after about 15mins of running.I came home and put a new plug in and all was well until the same thing happened again.I have ifactory RS coils.I have now parked it up and would appreciate any directions on the cause of the fault.I have also done the front cylinder head conversion if this helps.

I have started the bike again and it runs but the front cylinder is backfiring and chuffing like a steam train plese help!!!!

Have you looked at your reed valves? Pull the carbs, and look down the inlets. Verify that the reed valves are not broken or anything.

Thanks,
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Charles Gallant

Thanks.I have took the head off and found that the piston crown has bits missing.Oh well time to start all over again!!!

Can you e-mail me some pictures please Frank, and let me know what jet setup you were using.

I did a bottom cylinder today, and the symptoms sound similar.

Cheers,
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Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

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Welcome to the owners club of "the bikes that go nip"

Like andy says it would be good to see some pics. Did yours take out the exhaust port side too?
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If I have to take the carbs off once more...

SAY WHAT!? ANDY'S BIKE WITH 1.3 MILLION KMS WENT BOOM?!

Yes it did.I have to chunks missing.I have got the ache because I 4 dats off and a dead NSR.I will email some pics this afternoon.I do not know what the carb settings are as the dyno shop set it up.However they must of set it up wrong for it to go pop after 100kms!!Next time it is set up I will make sure I know what is going in there.

Very sorry to hear that, sounds just like andys, its caused by a lean set up or too higher compression ratio being used. Both times andys has done it has been with the HRC head convertion on the front and i see youre running that.
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If I have to take the carbs off once more...

I was running a shortie plug in the front as well part no.NGK R6120-11.Do you think this was too hot for it?

Andy was using that too! but i dont think thats the prob to be honest.
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If I have to take the carbs off once more...

It may not be the plugs fault, but the plug may be one reason for the seizure. If you are reading the wrong portion of the plug insulator during plug chops(plug TIP, as opposed to inner insulator BASE) the cooler plug will give you a false darker reading that "looks" like the bike is jetted correctly when in fact it is running LEAN. In order to properly check your plug, you either need to run the plug down on a lathe to see this portion, or get one of those handy plug magnifier/illuminator tools.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS

I have just taken the cylinder off.I have inspected it and it has three cracks in it so I presume it is cream crackered.Now I have the dilemma is it a new cylinder or a 300 kit?The only peroblem is I have a piston kit winging it's way to me now.Does anyone have a 'B' piston front cylinder they want to sell?If not any advice on alternatives would be greatly received.

Just get it replated. Langcorts will turn it around in almost a week. you coudl be back on the road for next weekend and seized by the following monday... ahh yes the joys of.. oh you know
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If I have to take the carbs off once more...

Stephen,will they repair the cracks?The cracks run into the wall of the barrel and in the web of the exhaust port.If so who are langcourts?