Hi All,
Just got a 91 MC21 and would like to know weather to use 91 unleaded or 95 ultra, I live in the land down under. It has being hard to start but fired up this evening after two kicks. The preivous owner didnt use it much or hopefully with more regular use it will be more reliable .

Thanks in advance

Matt

It depends on how much ethanol you have in your fuel

https://classicsworld.co.uk/guides/e10-fuel-a-classic-owners-guide/
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Why do things simply when you can complicate them

Hello "bluethermal", and welcome to NSR-WORLD.COM

The [integrated] centre crank seal on the 89-96 models is a known NSR250R weak point, and even more so with fuels containing ethanol. If you can get pump fuel without ethanol, we advise you to use it, whatever the cost! Failing that, use the fuel with the least ethanol content you can find.

Not only does the centre seal appear to deteriorate significantly fast when using ethanol content fuels, the ethanol can also be highly incompatible with many of the fuel system components from the 80s & 90s bikes.

We have often found that when setting up bikes for modern fuels, we've needed to increase the pilot jet size slightly. This can improve both starting and slow speed running.
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Andy.
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Thanks for the advise. Is the centre seal a non replaceable part?

Officially, the centre seal isn't replaceable, as (a) it's integral to the centre main bearing, and (b) is not officially sold by Honda, so the cranks are considered a consumable item.

Unofficially crankshafts can now [usually] be rebuilt, as the special Koyo bearings have been made available by a few suppliers. However, new cranks are being produced by Honda again, and a rebuild from a reputable builder isn't really much cheaper than buying new. Back in the day a rebuild was a good option, as standard cranks could be quite badly out-of-true, and a good [trued] rebuilt crank could make a noticeable difference in a tuned motor. The new Honda tooling means new cranks being sold today are considerably better than those from 30 years ago, and are built to noticeably tighter tolerances.

Bear in mind that a rebuild will re-use a number of components such as conrods and needle bearings, that may already have [tens of] thousands of KM on them. OEM rollers and cages are available, but are prohibitively expensive, and by the time you add them to the cost of main bearings and the rebuild, the costs far exceed that of a new crank from Honda. Some aftermarket items are available, however they're cross-referenced and not NSR250R specific. We've never used them, and therefore cannot recommend them.

So, I reiterate, do your best to use fuels with the lowest possible ethanol content, as it wrecks '89-'96* centre seals! I would strongly recommend against using the 91, although the NSR will run on it. You won't notice any difference in power between 95 and 97/98 (if you can get it?), however the higher octane rating will be more resistant to knock/detonation, which can occur on the front/lower cylinder on MC21 & MC28 models, and will often be lower ethanol content as a bonus.

* 1987 & 1988 models don't suffer anywhere near as badly, and reproduction '88 centre seals [that are a different size to '87] are available from T2Racing in Japan. The point about re-using components still stands with those cranks though, but there's no "buy new" option from Honda for them, so rebuilding is the only course of action open to owners.
_________________
Andy.
NSR-WORLD.COM

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Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

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