I guess this question has been asked a thousand times but I couldn't find the answer.....

I bought a MC21 SP last Autumn and spent winter sorting it cosmetically, now I'm trying to sort the engine. The bike pulls well up to about 8000rpm but then coughs and splutters. I did the orange/blue wire mod but the gear position work hasn't been done.

Sitting here at the PC I don't know whether to go for carbs, ignition, power valves or whatever so any pointers would be welcome.


On the last two stroke I owned (RD400) it was a case of clean the plugs and off you go but this is a little more complex!

Thanks in advance.

Chris.

To my limited experienced mind, this sounds like a carb problem.

May be worth pulling the carbs down and checking MJ sizes and having a clean up

If nothing changes, you can cross that option off the list.

It could also be a powervlavle problem, but that would just choke the engine, not really cause it to cough and splutter.
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Winning isn't everything, but loosing isn't anything

As 80XAR says, check the jetting first. You never know what someone has done (or tried to do) to it before you took possession. Standard carb settings can be found in the Workshop/Technical section.

When you are sure the carbs are set correctly, ensure the RC Valves are set up accurately. You could disconnect the RC Valve cables and pin them open with a "knock pin". It'll run like crap at the bottom, but should take off about 7~8000rpm. It won't necessarily rev right out, as the alignment holes aren't always that accurate, but if it pulls through 8k nicely, it's probably identified the problem.

This is all once you have checked/cleaned the air filter and fitted new plugs first, of course!

One last thing, if the battery isn't 100%, it can cause very similar symptoms on the MC21.
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Andy.
NSR-WORLD.COM

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something i was told just recently when my bike was playing up was to check the lower coil for high rev spark

what the person did was removed the cap of the bottom coil, held it with plastic pliers near the sparkplug, started the bike and revved it out over 7k and would see if the coil would still spark at those rpms

he said it was something to do with the bike having a high rev spark or something i cant remember... but something on those lines.. no harm in trying it though