I search old threads without conclusive results. The last two rides I have been loosing quit a bit of oil out the breather vent, not just a few drops. I added a filter but didn't help. I decided to do a Compression test while cold with the Compression release disabled and only got about 98 lbs. seem low to me. I suspect a ring or piston problem but thought before tearing into it I would see if others have ideas.

Any advise would be great!
Thanks

 

if you only got 98lbs. with compression release disabled then you got a problem.
you get 120lbs. without disabling decompressor.
one would normally have to consider whether or not its valves losing compression, or the piston/rings, but in your case since your losing all the oil you'd have to assume piston.

a little oil out the breather is normal, and if you ride alot of hills then even more is normal, as steep downhills put more oil in the way of the hose barb on the stator cover to get coughed up.

 

I mainly ride Dunes, A lot of ups and downs. I checked the Shims while I had the cover off, one Intake was very tight and one was loose. I swaped shims and now there are with in .002 of each other, both exhaust looked good.

Could a burnt valve or giuld cause a positive pressure? If I hold a finger over the vent tube, I can feel the pressure build.

 

You may have a holy piston. Ive seen this happen before to raptors, Have you been running lean?

 

This is going to be a piston or ring problem, or both. [img]i/expressions/face-icon-small-disgusted.gif[/img]

If you want, you could confirm this with a leak down, but you pretty much did that with your thumb. Time for a tear-down. A guide or valve can't do this.

 

did you hold throttle wide open and crank a few times until gauge peaked?

 

i read on the BMW page something about countershaft seal leak causeing oil to leak out the vent, this ever happend to anyone?

 

OK, I'm convinced it's either a holy Pistion or very bad ring, either way the top end is getting opened. Can anyone tell me if I have to pull the motor to get the piston out? looks tight but if I don't have to it would save some time. That top motor mount may be an issue, perhaps dropping the front mounts willallow the motor to tilt forward enough, Never had the DS apart yet, I've done a top end on 400 EX before in the bike was no big deal getting the Head and Cylinder out.
Anyone have a good piston laying around if it comes down to that? Please PM if so.

Thanks for the info.

 

Did you notice any loss of power? If not, I would double check the compression measurement. You should have got over 200psi with stock piston and stock cams. If you're only getting 98psi, it must run like crap???

 

Ya, Scooby, it's been hard to start, and lacking power. I thought at 1st it may have just been the Shims, one was really tight on the intake side but the last two rides, well, it's lost all it's FUN factor. Getting beat by smart a$$ kids on 400's, now that sucks..... My last DS ran around 150lbs compression stock.

You've had a few apart, I 'm going to start on this tonight or tomorrow, can I do the Top end in the Bike?