The mechanic who works on my bike (trained with Yamaha), as well as a top yamaha mechanic in South Africa are stumped.
If I ride at a constant speed in mid-rev range, example 100km/h (4000rpm),then stop or idle, I have oil coming thru the breather pipe into the airbox which then drains out the overflow.
The harder and longer the ride, the more oil runs out. It is not a drip, but actually runs in a little rivulet for a few seconds.

Numerous checks with the amount of oil were made. (2.6 liter)
The first job was to change the piston/rings as pressure blowthru was suspected.
No improvement to the oil leaking.

The second job was to replace the oil pump (primary and secondary) as it was suspected that the oil wasnt being pumped back up to the resevoir fast enough. This did improve the oil circulation (visibly seen thru the top when the cap was removed).
No improvement to the oil leaking.

The third job. A sieve at the bottom of the crankcase was cleaned as it was apparently blocked from when I cooked my clutch, preventing oil being pumped up to the reservoir.
Still no improvement.

Finally, we replaced an oil seal which had been inserted incorrectly and minimally damaged. This seal was behind the clutch case cover.
No improvement.

This running out of oil is not just the odd drop or two either.
Any ideas?

I forgot to add. Its a '98 XT600e (blue and white model)

__________________
my other home wilddogs
i support the think bike campaign

Just maybe there is a problem with the crankcase breather ???

Have you tried running the bike with the vent hose disconected from the airbox, maybe pull it out to the side so you can see it while riding. should give you an indication of whats coming out of engine, eliminating the vaccum of the intake air.

__________________
Winston
85 xt600Z
02 YZ250

... just also replace the valve stem stem seals. They wear out before the piston rings and they also let combustion gases through into the crankcase.

The phenomenon you have here is hot combustion gases 'absorbing some oil and leaving the engine the only way thre is, via the venting system. In the airbox, the gases cool off, oil is deposited inside the box and runs out.

Auke

__________________
Auke
http://aukemarieke.blogspot.com

Today, we did some more tests.
As i mentioned before, the oil comes out rather fast when in high revs.
So we had it running till the oil started coming again, then loosened the oil cap and there was a noticeable difference.
It seems there is a pressure buildup in the oil sump (upper one) which prevents the breather to allow oil to first pass up to this sump. so instead, it seeks the "easier" way out - via the vent to the airbox.

Does my explanation make sense?

Is there some kind of one-way valve to prevent a pressure build up in the sump which could be faulty?

thanks for all help so far....

__________________
my other home wilddogs
i support the think bike campaign

When you say upper sump are you refering to the oil tank? there is a small vent hose that connects the tank to the top of the cyl head to equlize the pressure, could this be plugged?

__________________
Winston
85 xt600Z
02 YZ250

there is a breather pipe from the crank-case (stator side) leading up to the sump which is in the frame. then another breather pipe from the central part of the crankcase, leading to the airbox.
blowing thru either of the pipes (direction of the motor) there is some restriction. If one then unscrews the oil cap, the restriction falls away.

__________________
my other home wilddogs
i support the think bike campaign

I Think That The Air Filter Is Clogged...