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I have just rebuilt my 1998 wr400 to replace a worn selector fork, which seems to have solved the problem I had with the bike jumping out of 5th gear.

Now on the not-so-good side.

I now seem to have very high pressure in the crankcase, enough to spray oil out of the channel that links the generator (rotor) area with the gearbox, and coats the rotor and stator with oil. Its as if the oil isn't being picked up properly, or maybe if the oil runs through one of the gearbox shafts - do I have to align the oil feed hole in this with something? This was the one thing that I wasn't certain about when I rebuilt the gearbox.... would this cause the problem?

I can't find any crankcase breathers that could be blocked so I assume the bottom end vents up the channel for the timing chain, and then out the top camshaft cover and the thick black pipe on the top (which is clear).

Any ideas most welcome, thank you.

Cheers, Red.

Did you use genuine Yamaha gaskets?
Either you have excess blowby or a passage somewhere is blocked.
Leak down test first, then its back to removing covers until you find whats blocked

Thanks for your reply.

I have used genuine Yamaha gaskets.

I can't understand why i would have excessive blowby - this was not happening before I split the crankcase, the piston is OK and the rings are within tolerance.

There are no passages blocked - I have since stripped down the engine again , found nothing wrong - so re-built it and still have the same problem.

Cheers, Red

Are you sure the pressure is too high?

A single develops a lot a pressure when the cylinder comes down, and then lowers the pressure as the cylinder goes up again.

If the oil is not blowing out of the crankcase breather, I would expect the pressure in the crankcase to vary +/- the displacement of the cylinder once per revolution. Even m y lawn mower engine does this. If I leave the crankcase open, it really moves a lot of air back and forth.

Unless you are sure it wasn't this way originally, I think it sounds normal.

My CB360 sprays oil on the generator and stator when running with the cover off, and it is a twin. Just the air moving back and forth from one side to another creates quite a storm inside.

Thanks Richard

I'm gonna run with that.

Fingers crossed.

Cheers, Red