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I would think that compression is by-passing the rings is causing excessive crankcase pressure and that is what is blowing oil thru the crankcase vent tube -----if the vent tube was disconnected from the air box and run into a clear water bottle and hung on side the frame while taking a ride might show if that is where it is coming from or direct you in another direction
If its a 2001 have u checked the spark screen on the muffler - if its plugged
 

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If its a 2001 have u checked the spark screen on the muffler - if its plugged
Applejack , this thread was from July 2018 ------- it is kind of a shame , a lot times people never take the time to come back and post what the fix was
 

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Hey everyone! New to this forum hope to get some good advice from some knowledgeable people! So I have a 2001 Rancher that keeps blowing oil out crankcase breather I bought it from a guy who had 4 experienced mechanics try to fix it but it puzzled them all so now me being a mechanic I dove right in first I checked the compression which it was not good at 60 psi but the motor has a fresh rebuild? I was curious why and I came to find the valves were partially opened at TDC so I readjusted the valve clearance to spec (.006) and now it has 100+ psi so I thought that had to be the reason it was building pressure in the crankcase blowing what gases are supposed to stay in the cylinder out into the valve cover and down the pushrod passage into the crankcase? Much to my surprise tonight I took it out after buttoning it back up and wouldn't ya know it the same problem just pumped damn near a quart which is half the oil capacity into the airbox and she had one hell of a cloud of blue smoke behind her haha. Just curious if anyone else has run into this issue and if there is something I'm over looking here? Also btw I took the cylinder/ jug off and inspected the cylinder and piston/rings which was all basically brand new with out a scratch, another thing I noticed when taking the rings on and off..............the piston had no alignment pins for the rings? They are not supposed to be in line they should be staggered 120 degrees which if isn't done correctly could effect compression and would cause some blow by? If anyone else has encountered this issue any tips to save me a little time would be greatly appreciated!
 

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I did find a problem there’s 2 seals and 2 dowel pins on oil pump there’s a transfer hole to vent side try replacing those 2 seals on the dowel pins
 
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