CB250RS engine trouble

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CB250RS engine trouble

I bought an '81 CB250RS yesterday and rode it back to Manchester from Morecome without any issue.

This morning when I was going to work it siezed.

After the AA had dropped me off back home I set about taking a look.

It seems that the oil seal behind the gearbox sproket had been forced out, losing all the oil. I just managed to pull in the clutch in time.

Is this a common problem with this bike?

I am going to order a new seal but who's to say it won't happen again?

If it's not the gear sprocket seal it'll likely be something else on a 30 year old bike. I thrashed one mercilessly for 18months / 18,000 miles back in 81/82 trying to keep up with my mates on LC250s and it held together without any problem, apart from the odd drive chain and bike / tree-stump / diesel interaction.

TBH it might well be fucked which is a shame 'cos they're cracking little bikes. Those engines are renowned for doing their top ends (cams run directly in the head rather than in replaceable bearings) but when run without oil they do their bottom end very quickly instead.If the cam bearings are goosed the head can be repaired with bushes. I've never heard of anyone having a similar gearbox seal problem with either that or an XL250 (same engine). I could've sworn the seal's retained by a seal retaining plate. Is it missing? Check the engine breather's OK - it's conceivable that excess crank-case pressure could've pushed an old seal out.  If it's survived then the one leak to not worry about is a weep from the ends of the rocker shafts. The O-rings used to seal them always fail. The weep never gets bad, and never uses enough to cause problems. Replacing the seals is a sod, and they only go again anyway.

Hey Tappy, thanks for info. TBH I had thought about fabricating some kind of retaining plate or strips to keep the oil seal in place, also the breather pipe.When you take out the seal there is an oil feed drilling at the 6 o clock position. I replaced the old seal and tapped it in using a large socket. I put fresh oil in and started the engine, 3 seconds later it was forced out pouring oil over the floor. I pulled off the breather and blew down the pipe - it was clear.There is definitely no sort of retainer on the seal that I took off. Behind the seal is some kind of oil thrower which also seals on the shaft and fits inside the outer seal. I have ordered the new seal from CSNL an apparently, it has been replaced by something else. I will have to wait and see what they send. Mike

Are there any witness marks on the back of the seal after it's been pushed out? Am just wondering if it's being pushed out mechanically or hydraulically?

The seal is retained by a lip around its circumference. , you have to remove the engine and split the crankcases to replace it, it cannot be replaced externally , it just does not work because of the oil pressure just behind the seal (its the main bearing) some people cut the lip off and try to use silicone to seal it this does not work either , .and its scary when it doesn't because it usually dumps all the engine oil straight onto the back wheel ....not good if your doing about 50mph at the time .   , its a bit of a pain to replace really, but can be done it just takes about 5 or  6 hours depending on the rear engine bolts (which are usually siezed and need to be cut or hammered out) it is a common problem on RS250 (and most bikes really) that have been stood or not used for a few years . once replaced if the bike is used regular then it will not blow out again .it has only blown because the bike has been stood and the seal has perished .   

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