Hi, I have a 1993 NSR 250SE MC21 with dogfight expansion chambers, derestricted, drilled out airbox. The bike has been starting and running fine since I bought it last October. But recently it bogs and stalls when I give it gas. Once underway with momentum, it runs fine...but it will stall when I try to get going, not accepting throttle. It feels like a cold bike that needs choke - - in fact, if I choke it about halfway, it runs better. Which leads me to believe it is too lean. But, how does a bike get leaner? Are there adjustments to make? Something may have vibrated out of adjustment?

Any tips much appreciated.
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1993 NSR250R SE MC21
2010 KTM 990 SMT
2004 KTM 300EXC
1996 Honda XR250

Was the airbox + pipe mods already done to the bike, or did you put them on?

Have you had the carbs off for inspection (clean jets) and also check all jet sizes, against standard, and taking into account the modifications?

Tuned pipes and free (er) flowing airbox will create a leaner mixture.

If the problem you describe is developing over time, it could actually be something more serious, such as a hardening / degrading crank seal allowing air to be sucked in, thus weakening the mixture. But do the easy checks first.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

I would pull the carbs, take them apart and clean them paying special attention to the slow jet. Since it ran well before maybe you just have some dirt clogging something up. Cleaning them is always a good thing and it's only going to cost you time not money.

RichG wrote:
Tuned pipes and free (er) flowing airbox will create a leaner mixture.

Not always!
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Andy.
NSR-WORLD.COM

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You have to complicate it don't you!

What you need is some sort of handle bar mounted lever that.....
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If I have to take the carbs off once more...

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Last edited by DifferentStrokes on Mon Sep 28, 2009 5:10 pm; edited 1 time in total

RichG wrote:Was the airbox + pipe mods already done to the bike, or did you put them on?

Have you had the carbs off for inspection (clean jets) and also check all jet sizes, against standard, and taking into account the modifications?

Tuned pipes and free (er) flowing airbox will create a leaner mixture.

If the problem you describe is developing over time, it could actually be something more serious, such as a hardening / degrading crank seal allowing air to be sucked in, thus weakening the mixture. But do the easy checks first.

The bike came with the pipes and mods. I have not taken the carbs off or touched jetting etc. I will do all the tuning as recommended and then see if further work is necessary. Thanks to all for the advice.
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1993 NSR250R SE MC21
2010 KTM 990 SMT
2004 KTM 300EXC
1996 Honda XR250

o o