I bought my MC21 in 2001 with 12k km. The mechanic said it was in good shape. Basically I just kept it filled with 2 stroke oil, checked the chain and brakes, and put about 3k km on it over the next 4 years.

Due to a move (and the missing license), the bike then sat unused, covered outside for about 3 years. After sorting the license, I started the bike up last summer (after 3-4 kicks), and put about another 1k km on it.

Decided that it was time to do some maintenance and found the plastic gears in the rc valve servo motor were stripped. Then opened the airbox and found that there was not much left of the original filter, with some pieces of it in the carbs.

So, time to pull and clean the carbs. After removing the reed cages, I looked down in at the crank, and found that the furthest to the right side is shiny, while towards the left, it appears brown. See the picture here:

http://xn--qckst9n.com/

I have no idea if the top or bottom end has been touched before,
but am planning to pull the cylinders and do a top end rebuild. I searched some posts about crank condition and am now concerned I might need to do the bottom end too. I will pull the flywheel cover and see if there is play in the left bearing. As the bike was starting within a handful of kicks, and seemed to be running "fine", I want to avoid the bottom end for now if possible. Looking at the photo, does anyone have any ideas about the condition of the crank?

Most of them look like that. The colours are nothin unusual.

The seals on the other hand may be called into question if it's sat that long.
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doug3 wrote:As the bike was starting within a handful of kicks, and seemed to be running "fine",

It's not broke, stop fixing it!
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Nothing wrong with a bit of preventative maintenance!

I'd get the barrels off and check the condition of the plating personally, then renew the pistons, rings and small end bearings. Fitting new gaskets as you go along. Daresay it would be worth cleaning the rc valves; if the servo has stripped gears they could be seized in position?

Check the big ends for play and signs of blueing, feel the mains for smoothness and have a good look at the small end of the conrod for any abnormal wear. If it all checks out I doubt there would be any reason for removing the crankshaft from the engine.

Just because its running right its worth having a look over every so often, just assume that it was never serviced in the time before you owned it and it was hammered from cold! Always assume the worst, less dissapointment when you come across a problem then!

Paul

I see what you're saying but pulling the barrels off, replacing the pistons, rings, little ends and every gasket along the way is not "preventative maintenance", it's a top end rebuild and £170 on bits into the bargain.

Whatever you're trying to prevent with this maintenance is irrelevent, you've replaced 90% of the bits that could possibly have been damaged and probably spent far more than you'd have done to get it running again if some component had failed.
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Sorry I disagree, what if he finds something that could cause damage to the cylinder plating if left untreated? Thats a bloody expensive job especially if there is welding involved as well as replating! 90% of the bits? More like 40% when you take into account 2 barrels and 2 heads surely? They certainly dont make up 10% of the top end of the engine!

Changing a piston is part of the servicing, preventative maintenance. IE changing of worn parts to prevent further damage to more expensive items.

I always have stuck to this, whether while racing or road riding. rather change a £90 piston than pay £90 for a piston then £150 for cylinder repairs...

Or maybe nsr's are so magical they never wear out?

Oh, they wear out and there are service limits to tell you when they're considered to be worn out.
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Service limits as in measurements or mileages? I'd be interested to know what Honda recommend for servicing.

Even if he didnt replace the pistons, working it out maybe 16,000km is a bit soon to change the pistons themselves but I'd certainly put rings in while its apart, a top end strip would surely be worth doing to inspect the components. You never know what you might catch in time...

OK. Thanks for your discussions. Assuring in some sense. I was looking at photos of used cranks for sale and found that they mostly had the same appearance as mine, so I became less concerned about the coloring.

As the servo motor had stripped out, I did find that one of the rc valves was binding a little when moved by hand. It did not seem too serious, and after moving it back and forth several times with a little bit of lateral pressure on the shaft, it became quite smooth.

However, I must suspect that it froze up enough to cause the cable to stick, thus stripping off the plastic teeth of the gear in the servo motor. So... I will at least pull the cylinders to find out what the condition of rc valves, pistons, cylinders, etc., actually is.

And I received my new MC21 Service Manual in the post today, so I am ready to get down to it!

http://xn--qckst9n.com/

2Smoke wrote:Service limits as in measurements or mileages? I'd be interested to know what Honda recommend for servicing.

Measurements, there's no mention of replacing major parts by milage in the NSR250 workshop manual.
There are however figures for everything, cylinder size, out of round, warpage, piston size, ring gap, pin and pin bore size, ring to ring grove clearance, crank clearance and run out.
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fontyyy wrote:

Measurements, there's no mention of replacing major parts by milage in the NSR250 workshop manual.
There are however figures for everything, cylinder size, out of round, warpage, piston size, ring gap, pin and pin bore size, ring to ring grove clearance, crank clearance and run out.

How odd, so there is no maintenance schedule for checking piston size, bore shape, crank tolerances. Just the figures?

I see you have a race bike, what sort of maintenance schedule do you stick to out of interest? I was sticking to top end every meeting (check) and full strip every 4 meetings (replacing some parts; rings and some seals usually).

doug3 wrote:
OK. Thanks for your discussions. Assuring in some sense. I was looking at photos of used cranks for sale and found that they mostly had the same appearance as mine, so I became less concerned about the coloring.

As the servo motor had stripped out, I did find that one of the rc valves was binding a little when moved by hand. It did not seem too serious, and after moving it back and forth several times with a little bit of lateral pressure on the shaft, it became quite smooth.

However, I must suspect that it froze up enough to cause the cable to stick, thus stripping off the plastic teeth of the gear in the servo motor. So... I will at least pull the cylinders to find out what the condition of rc valves, pistons, cylinders, etc., actually is.

And I received my new MC21 Service Manual in the post today, so I am ready to get down to it!

Glad the crank is ok, bit expensive to replace although with your location you'll probably find a replacement easier than us in the uk!

Its a good idea to have a look in the engine, just so your happy with whats going on in there

Service manual looks useful too! Wish I had one for mine...

Cheers
Paul

Service manual looks useful too! Wish I had one for mine...

Cheers
Paul

New Japanese language MC21 manuals are ~$40. Used go from ~$20-$30. If you want me to send you one, give me a pm.

I think I'd go for an English '28 one over a Japanese '21 one, no matter what bike I was servicing, a good 75% of it applies to the '21 as well.

It's all very well seeing something is 0.065-0.095mm with a limit of 0.12 but unless you can read Japanese well enough to be able to translate "piston ring to ring groove clearance" it's not much help.
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fontyyy wrote:I think I'd go for an English '28 one over a Japanese '21 one, no matter what bike I was servicing, a good 75% of it applies to the '21 as well.

It's all very well seeing something is 0.065-0.095mm with a limit of 0.12 but unless you can read Japanese well enough to be able to translate "piston ring to ring groove clearance" it's not much help.

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