First off I should say I am very new to NSR's and bikes for that matter. I recently bought a MC21 and have a problem with it and thought that this would be the best place to start.

When I have the choke on it runs fine, but when I turn the choke off it is very difficult and rough running. If the choke is off and I try and start riding in first gear it really wants to stop running. I really need to give it about 7,000 rpm's to get a semi smooth start. Also when rolling and any gear with the rpm's at about 5,000 or lower it studers/chokes and dramatically loses power. If I switch the choke back on it works fine again.

I am only slightly mechanically inclined but a friend of mine has a shop here in Fukuoka, and he isn't sure what the problem is either. He admits he doesn't have a lot of experience with NSR's though. Any help or advice would greatly be appreciated.

If it runs better with the choke (it gets more gas)this tells you that it doesnt get enough fuel without choke.
If the jets hasn't been touched recently, I would clean out the carburator and maybe the tank and petcock (valve)

It can also be an airleak somewhere, but then there are many possibilities
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Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom

If it's standard (or even if it's not) start by cleaning everything, checking everything.

These bikes are really easy to work on, by far the easiest of all the last generation 250's. With (or without) your workshop friend;
Get the fuel tank off (two bolts, a fuel pipe, turn the fuel tap off!)
Get the airbox lid off (6? screws), fit a new airfilter if it's filthy.
Get the airbox off (two screw clips at the base).
Get the carbs off (two screw clips at the base, make a note of where the wires, cables and pipes go)
Get the bottoms off the carbs, get the jets out, CAREFULLY make sure they're clear.
Clean it all up, put it back togeather.

Now get the inspection panel off (right hand side "middle" fairing panel) and check the RC valve timing (details here, bottom of the page).

Put it back togeather make sure you've got clean fresh fuel, new plugs and then take it for a decent run.

If your happy you've done that lot and it's still not right let us know.

I know this sounds like a lot of work, but it's not, I destroyed my NSR within hours of getting it and spent £1300 having it fixed, an absolutely stupid move (read the saga here), an incident like this now would do no more than make me text Andy as it's so annoying it's funny and then reach for the spanners.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Thanks for the advice guys. I have had the carbs taken apart and cleaned (twice actually) so I know it's not that. But I will get my friend to check the RC Valve timing.

Thanks for the links too. The repair manual is really great and useful. I know I'll be using it often. I haven't read your story yet fontyyy, but I will. Looks like a really nice bike you have now though.

Is it possible this thing has the wrong stator and is getting the wrong ignition timing? Has it ever run correctly?
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james
Its not how fast you go...Its how you go fast.
MC21 Rothmans
RS250
RC31
CRF250X
748R
Porsche (many)
http://members.rennlist.com/j911brick/rennlist3_002.htm

Like I said I am new to NSR's, as I have only had it for a month, and got it from an out of town shop. So for as long as I know it has been like this. Sorry I now this doesn't help.

Please forgive my ignorance, but what is a stator?

Stator/flywheel, its called a couple different things. Basically it sits on the left side of the motor and has magnets in it for the charging system. It also has metal nubs on it and that is what determines the timing. Different bikes use different timing and have different flywheels because of this. If they swapped it for one off a different NSR you will have problems.
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james
Its not how fast you go...Its how you go fast.
MC21 Rothmans
RS250
RC31
CRF250X
748R
Porsche (many)
http://members.rennlist.com/j911brick/rennlist3_002.htm

Thanks, how can I tell if it is the correct one. My bike is an MC21 SE.

I forget. You might try the technical section or do a search. This issue has been brought up several times; but I'm just guessing.
_________________
james
Its not how fast you go...Its how you go fast.
MC21 Rothmans
RS250
RC31
CRF250X
748R
Porsche (many)
http://members.rennlist.com/j911brick/rennlist3_002.htm

Hey I am willing to try anything.
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Les

MC21SE
CB400SF

Have you tried a compression check and a leak down check?

What colour are the plugs when running without the choke on?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

I didn't see the plugs when we changed them but my mechanic said they looked good and slightly black. He also said they weren't very old looking so maybe the old shop had changed them recently.

not sure if he did a compression test but will do soon.

Thanks.
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Les

MC21SE
CB400SF

Maybe your perception of the on and off position of the choke is wrong?