alvarem

Member
Mar 22, 2009
3
0
I have an 02 YZ250 that is leaking oil. It is coming from around a rubber seal that is to the top left of the water pump. I am not sure what this rubber seal is. It is kinda thick and gives the appearance that it is just stuck to the outside of the case next to the pump. It is Yamaha part number 5CU-11995-00-00 and is listed as a housing seal on the cylinder head diagram. Anyone have any experience with this seal and what it takes to replace it?
 

AV8R

Member
Jan 20, 2006
45
0
I think the seal you are talking about is the powervalve cover dust seal. That seal is not designed to hold back any oil is only to prevent dirt and mud from getting to the powervalve linkage.
If you are leaking oil around it then the oil is actuallly coming from the powervalve shaft seal at the top or the actuator shaft seal where it goes into the transmission (bottom). Both of these seals can be replaced, but not without some disassembly of the associated parts.
 

alvarem

Member
Mar 22, 2009
3
0
I am still a little confused. My tranny oil is fresh and even when I check it through the weep hole it is still pretty clear. The oil that is leaking from this gasket around the power shaft seal is very dark and looks almost like premix oil?
 

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
It is premix oil. Half burnt premix oil.

It is common for a 2 stroke to have the black oil residue dripping out of various spots of the exhaust system. The obvious spot is out the exhaust at the silencer but they can also leak at the joint between the silencer/pipe and pipe to cylinder.

The power valve gets drenched in this goo and can leak out the seals on either side of the cylinder. Like AV8R said, what you describe sounds like the dust cover over the linkage to the power valve.

I wouldn't tear the engine apart to fix that as it won't really hurt anything but the next time you are doing a top end you should consider changing the seals on the power valve.

Rod
 
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