How can you tell definitely that the center crank seal has gone? What are the classic symptoms?

My MC21 is hard to start and seems very lean at idle. It has stock slow jets, each carb has a K&N filter so it's basically running open carbs. It has the HRC black box and stock pipes.

Once it's warm it will idle steadily at about 1250 RPM but it takes some revs to get moving. Once going, it runs well in the mid-range and top end.

The bike sat idle for a coupld of years but once I cleaned the carbs I got it running right away.

Thanks in advance for the advice.

Skunkbait

one way is to do a leak down and check to see if your getting cross leak between half's (If I remember correctly, the center seal should allow you to do a leak down on individual cylinders on the NSR250).
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Charles Gallant

The common, simple test is to remove both barrels and fill the top cylinder crankcase with fuel:oil mix. If it starts to run out of the bottom then the seal's not doing it's job!

You really want to leave it over night to see the results. If you can see transfer of fluid within a couple of hours then you have problems!

It's not as comprehensive or accurate as a leak test, but it can be done without any special tools.
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Andy.
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Skunkbait,
You need to consider the outer crank seals as well as the centre, namely the flywheel side. My bike's symptoms (starting, getting it off the line) sound very much like yours. Have a read of my experience, hope it is of help.
http://www.nsr250.net/forums/viewtopic.php?t=7023&postdays=0&postorder=asc&start=0

Maybe worth looking for a leak-down test kit, they are useful in diagnosing such problems......http://www.accu-products.com/accu-products/leakdown-test-kit/prod_3019.html
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

i have recently rebult mine.

mine was also a tw*t to start and it just did not like the throttle being touched at all until it had been running for a while, once going it was ok not the performance i was expecting.

Mine turned out to be the centre seal, the outer o-ring was ok but the bearing and presumably the seal also were shot.

Just as a comparrsion, when i bought the bike it took 10+ kicks to get it going, then only bumping would get it going, then i got fed up and after investigating decided a rebuild was in order, it needed the top end doing too.

I started it for the first time last night and it fired straight away on the first prod of the kick start, I've owned a lot of twostrokes and this has been the first with a crank seal failure, i'm no expert on nsr's but the only other bike I know that suffer from crankseal failures are the vj23 rgv, that also have an integrated seal in the bearing rather than a labyrinth seal.

What your saying about your does seem to point the same way

RichG wrote:Skunkbait,
You need to consider the outer crank seals as well as the centre, namely the flywheel side. My bike's symptoms (starting, getting it off the line) sound very much like yours. Have a read of my experience, hope it is of help.
http://www.nsr250.net/forums/viewtopic.php?t=7023&postdays=0&postorder=asc&start=0

Maybe worth looking for a leak-down test kit, they are useful in diagnosing such problems......http://www.accu-products.com/accu-products/leakdown-test-kit/prod_3019.html

True,

of course in order for a leak down to tell you anything, you need to make sure nothing else is leaking

THis will require you to do a pretty good tear down (removing the generator and right side case cover to get to the outer seals, check the intake, base gaskets, head gaskets, etc..). If nothing else is leaking and you are loosing air from both cylinders, then it's probably the center seal.

I am thinking you could hook up a gauge to each side, set on to say 9 psi and the other to say 5 psi and if they start to equalize then you know you have a bad seal in the middle.

Andy's method will work as well, but not sure if there would be enough pressure to tell you of a seal that is just starting to go bad.
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Charles Gallant

Whenever I do a primary leak check on a twostroke, I do a vacuum check as well as pressure. I have had crank seals that would seal positively (100%) on the pressure check, but would leak terribly when a vacuum was applied. (-5lbs) Just something to keep in mind.

Yes, very correct Jeffco. The seal integrity should be tested in both directions.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

I appreciate the advice. I'll try Andy's method first and see what happens.

I see Tyga has the bottom end rebuild kits that includes the crank. Has anyone ordered one of those before? Does the crank have rods and bearings alread on it?

Also, for the top end rebuild kits they offer, there's an A, B, and C kit but no explanation as to how they differ. Does anyone know? Maybe I just missed it.

I've ordered from Tyga before (have their stainless pipes on my RGV250) so trust them but I want to make sure I get the right stuff for the money I plan to shell out.

Thanks.

Skunkbait

Skunkbait wrote:I appreciate the advice. I'll try Andy's method first and see what happens.

I see Tyga has the bottom end rebuild kits that includes the crank. Has anyone ordered one of those before? Does the crank have rods and bearings alread on it?

The Honda cranks are sold as a complete unit (bearings, rods, and the center seal.

Skunkbait wrote:
Also, for the top end rebuild kits they offer, there's an A, B, and C kit but no explanation as to how they differ. Does anyone know? Maybe I just missed it.

This has been discussed here before. The pistons need to match to the appropriate cylinder. so you want to look for the marking on your cylinder to determine what piston you need. The cross reference is in the parts manual, as well as the service manual.

Skunkbait wrote:
I've ordered from Tyga before (have their stainless pipes on my RGV250) so trust them but I want to make sure I get the right stuff for the money I plan to shell out.

Thanks.

Skunkbait

Send an email to them and make 100 sure before completing the order. They are pretty good about getting back to customers and know the NSR250 pretty well.
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Charles Gallant

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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30