It apparently broke the 75rwhp towards the end of last year, but I didn't witness the run in person, however I don't have any reason not to believe it happened. I think that run was with new heads and on AVGAS. The bike was built and maintained by StephenRC45, who works at GT Performance Engineering in Plymouth, but I understand it has now been sold.
Unquestionably, the MC21 is the best NSR250R chassis. The MC18 is twitchy, but still good, but the MC21 is definitely superior.
Without going into all the intricacies, [very] basically, the later the year, the better the package. However, whilst the most powerful, the MC21 is also the heaviest. Not to say the earlier bikes are bad in any way; the 1987 MC16 F3 is rated at 65ps out of the box on 28mm carbs and analogue electronics, but if you can get a 90/91, that's the one to go for.
HRC TT-F3 quoted figures:
1987 MC16 65ps
1988 MC18 68ps
1989 MC18 70ps
1990 MC21 72ps
Lightest I've seen an MC21 is 112kg, if memory serves me correct. Matt@TYGA says he got his down to 110kg back in the day. Generally, I think slimming one down to 114-115kg is doing pretty well.
Getting down to the nitty gritty... in my personal opinion, and real-world experience, you're wasting your time with RS250R parts. The NSR250R is very similar to the NF5, especially in TT-F3 form, but it's far from simple to use RS parts. You're far better off using the correct HRC parts, or modified NSR parts. Not saying you can't get RS parts to work, but you're going to really need to know what you're doing, and be prepared to spend a lot of time and money on setup. Many RS parts are, without doubt, better than their NSR equivalent [machined RC Valves, for example], but getting all the NSR & RS parts to work in harmony is an absolute minefield, and the end result is not worthy of the effort.
You can use the 38mm RS250R carbs, but you will sacrifice all the bottom-end and mid-range that the NSR is renowned for, for maybe 500-1000rpm over-rev. StephenRC45's 74rwhp MC21 ran the 32mm HRC carbs, would pull full-throttle from below 4000rpm, and out-grunt an early NX5 out of the corners at Donington. (Obviously the NX5 was noticeably faster.)
https://www.facebook.com/502467319852378/videos/132733854804906
I believe Dan Williams still has his TT-F3 spec MC21 for sale that he won F250 on a few years back.
_________________
Andy.
NSR-WORLD.COM
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