hi, I just picked up a basket case mc21 yesterday.
I bought i as a non running bike. i managed to get it fired up, issue is, it seems to rev ok in neutral, but when i get it in 1st gear, the revs are moving up very slow and it doesnt seem to want to rev past 5000.
its got a R motor with the wet cluch. compression is about 120psi on each of the cylinders.
The bike is on premix, not sure what the mixture is since i just got it last night.
I believe the bike should be stock, since its still has stock pipes.

Have you checked/replaced the spark plugs?

How old is the fuel?

Battery health?

i grounded the spark plug against the frame to check for spark so i assume its ok.
i charged the battery last night but i think it was a bit weak when i test rode it around the street.
gas is 8 months old, ill go get some premix and new gas in the morning. and give it another try.

is my compression ok?

oh the bike has br8es plugs instead of br9

Those plugs would be fine. My guess is it is only running on one cylinder. Check those coil connectors.
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MC21 Honda NSR250. TYGA 300 conversion.

Picked up some br9ev plugs , emptied the gas, bought some motul 800 premix, and cleaned the carbs today. I checked the air filter and it was drenched in oil. I tried to give it a rinse but it started to fall apart. Will try to get a new filter too. Going away this weekend so won't be able to try it until next week.

If you're going to get new plugs, why not get the correct plug? The NGK plug is a BR9ECM. Of course, the plugs you bought will be fine, except you might experience problems with the plug fitting in the front cylinder, or even the tire bottoming on your plug cap since the -CM denotes a short, compact plug.
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MC21 Honda NSR250. TYGA 300 conversion.

The br9ecm plugs are not stocked by any shops here . I have had an rgv250 for the past 12 years and had a mc21 in the past and have never ridden them hard enough for the front wheel to hit the plug cap. I have always used standard length plugs up front with no issues .

If you mean that the engine doesn't rev in 1st gear while the engine is on the rear stand, then it sounds like there is something wrong with the GPS system. Do the wire splice suggested on this forum as this will bypass the gps.

If the engine doesn't rev under load, then it sounds like the carbs need some attention.
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2t-racelab.com

I mean under load.

I changed the plugs , got new gas, cleaned the carbs, clean the air filter. I put it on a rear stand and it revs fine, put it through the gears on the stand and it revs clean to redline. Then when I take it out around the block again it won't rev out past 5000rpm and I can't seem to get it past 20 or 30km per hour.

Hmm, that seems pretty odd!
Assuming that your carburation is standard (jets, needles, float level) and working well,
and assuming your PGM is OK,...
... all I can think of (and it is a total wild guess, long-shot) is that maybe the variable resistor in the Throttle Position Sensor on the carb is giving trouble or is wired up backwards?
I do not know what the PGM does in terms of ignition timing in relation to throttle position, but I imagine that it probably retards the spark a bit if it thinks the throttle is near closed?
I assume you are only using small amounts of throttle when the bike is on the stand, as opposed to large amounts of throttle when out on the road???

I'll keep thinking about it anyway.

Also, check the front brake isn't on!! Just kidding.

took the battery from the RGV, and adjusted the powervalves and it seems alot better. still a bit slow down on the lower rpms but at least it revs to like 11k now.