Andy wrote:The MC21 (and MC18) carbs are actually very good, and very difficult to better. The D shape venturi acts more like a 34mm [maybe more!] carb at 50%+ throttle opening, and like a small round-bore 32mm carb at low RPM and throttle openings, giving the best of both worlds.

Putting larger carbs on an NSR250 almost invariably destroys the bottom-end and mid-range. Unless you've done substantial supporting work to the crankcases, barrels, heads, and exhausts to reap any potential benefit it's just not worth it. Even then, you are still more than likely to only gain some over-rev and still lose bottom-end. Another thing to take into account, is you lose any provision for the stock air correction, making setting both fussy and more difficult. (Remember, the TT-F3 setup is designed around the removal of the air correction, but the standard PGM-III is not.)

Something we've always wanted to do, but sadly never got around to, was fit 32mm HRC TT-F3 carbs to an NF5 on the dyno. I reckon the results would surprise a few people!

As an aside, the 300 conversion does seem to respond better to larger carbs, but the problem I see most often is people thinking the answer is always to go too big. Personally I think a 36mm would be perfect on both a very hot 250 and a 300, but I've seen people using (trying to use!) 39mm and even 40mm. Much to many owner's chagrin, we've never seen an NSR (250 or 300) with big carbs out perform an equivalent one on the '88-'93 32mm carbs... and we've dyno'd dozens and dozens of NSRs.

Not saying you couldn't make it work for you, or that you shouldn't try, but in my experience you're better off spending your money elsewhere... unless it's really important to you, or you're loaded and have nothing better to do!