YZ270F Oil Coming Out of Crankcase Breather Tube

Stevo683
Posts
83
Joined
12/20/2018
Location
Westerville, OH US
4/6/2021 6:20pm
Hey guys. I recently rebuilt and added a 270 kit to my 250F. Put about 5 hours of ride time on it. Loving it so far. I noticed oil is running out of the crankcase vent tube this past weekend. It has a ton of compression, runs great . What gives?? Any help would be great..
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Stevo683
Posts
83
Joined
12/20/2018
Location
Westerville, OH US
4/6/2021 6:33pm
Oh yea I measured and put the correct amount of oil in the crankcase. Now it’s showing low on the sight glass 🤷‍♂️
FGR01
Posts
5091
Joined
10/1/2006
Location
AZ US
Fantasy
1334th
4/6/2021 9:08pm
First guess, something is messed up with your rings and you're getting compression blowby that is over pressurizing your crankcase.
2
Leeham
Posts
1085
Joined
10/29/2018
Location
Rochester, WA US
Fantasy
3059th
4/8/2021 1:10pm
Never go off the sight glass. Its never accurate. I did it once on my 19 Honda 250, and I was short ~250 mL of oil off from what the manual calls for.

Unsure what the crankcase breather tube means, it could be slightly over pressured (not draining all the oil out of the bike) and empties out some.
DuroTools
Posts
758
Joined
9/9/2015
Location
Middletown, CT US
4/10/2021 10:17am
I had same problem with my kx250f big bore, I tried rerouting the vent hose and even relocating it to the top of the valve cover like yamahas do and that helped but didn't solve it. I also tried a larger piston size and new rings and that didn't help. Just couldn't figure it out, I ended up putting in a HC piston instead and that made it better but still not perfect with no mist like stock. I also did alot of research and many others have had this issue with many different bike brands. My Fathers 2004 yz250f had a big bore and it was a ton of fun and ran perfect though

The Shop

DuroTools
Posts
758
Joined
9/9/2015
Location
Middletown, CT US
4/10/2021 10:22am
One idea I had but never did was to use a oil catch can and filter device that would catch the vapors and I would have to empty it after every ride. If its light mist then that is probably doable but if heavy then probably not

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