Excessive blow bye

I have so much blow bye its running out of the air box drain and also filling the clear tube. It was sucking oil through the carb and making it run like crap. The engine was recently apart so I'm wondering If there is something put together wrong that would cause this or If the rings were bad could it cause this. When I took the valve cover off and cranked it over there would be a big puff of oil coming up from the timing chain area. Any help would be appreciated, Hawaii.

Sounds like rings to me.

it doesnt sound liek rings to me. Are you sure its motor oil getting sucked through or is it an over oiled air cleaner? Buy the vent removal kit. Does the compression feel fine? Did you replace and clock the rings correctly during the rebuild? Is your oil level going down?

It is definitely motor oil. There was a little bit of wear on the front and back of the cylinder when I had it apart so I lightly honed and put it back together. It never did this before taking it apart. If the rings where off could it have this much blow buy back into the cases. If the Piston and Rings were bad wouldn't it be smoking from oil coming into the combustion chamber. Last night when I looked at the plug it looked pretty good.I expected it to look black but was a nice dark tan. Is there any thing else that could cause this besides Piston and Rings? One other thing is I have the 270 kit which has the steel liner. Hawaii.

Just a long shot but check the large crankcase vent hose the routes to the air box,see if its twisted or kinked,1 time mine got twisted when i did the top end on my 450r,it ran great but had the same oil use problem.


Get the vent hose out of the airbox altoghther, and try it....also drain and refill to be sure you have the correct amount of oil..... 670ccs? Is that the engine you got from Crower?

i dont think its blow by. Are you burning motor oil, is the level going down? At high rpms the oil splashes up and into the valve vent. The heavier oil is supposed to fall to the plug while the air is supposes to just go back and forth. here is the pic. Blow by doesnt push the oil up into this cavity. High revs off the timing chain and valves throw the oil onto the top of the valve cover and normal pressuer equaizations suck the air back and forth. unless you are losing power or burning a shit load of oil i dont think you have a problem. Both these images are from rick ramseys site


When Thumper says their piston skirt is wider than stock, which reduces rocking of the piston in the bore, this is what they mean. The front and rear skirts are wider, left-to-right, than the stock skirts. The Thumper piston is the same height as the stock piston. Notice the blow-by on the old piston? It's actually not as bad as I thought it would be; there's virtually none below the rings.


I lost almost half my oil and yes I had the right amount to start with. Somebody told me about a one way check valve for the oil but they weren't real sure. Does anybody know about this and if it could be put in back wards. Something is causing alot of oil to be forced out of the breather on the valve cover. Yes it is the Crower motor but this problem was naught caused by them. The motor was recently taken apart due to another broken case. Hawaii.

Just got done checking the compression and its shot. Only reads about 60psi. What are people doing when changing piston and rings with the steel liner. I'm thinking maybe I should go back to stock since I only got about 10-15 hours out of it.Hawaii

did you check you valves?? did you replace the rings with the last rebuild? Isnt 60 psi normal? I dont know

I guessed I jumped the gun and forgot about the auto decompression. I just hooked up my air compressor and put about 120psi into the cylinder and couldn't hear any air leaking out and it seemed to hold the pressure. Now I'm back to square one. :excuseme:

Sounds like rings to me.

Fred didn't you have a similar issue.


As mentioned check that your vent hose is not blocked or twisted.

There is a case reed listed as #7 in the parts fiche... Not sure if this is an issue.

If you have the stock cylinder and piston, throw it on and try that.

Check your tranny side to see how much (or how little) oil is there. Sounds to me like textbook right crank seal gone.

What happens is the seal goes and the motor sucks the tranny oil over into the motor side to being over filled. Then it blows it out of the vent tube into the air box. Give it a look.

60 psi is normal; spec is 57psi.


I dropped the bike off this morning and will report back what they find. I never had time to check the tranny oil level but that sounds like it could be the problem. The crank seals were changed from what I was told when the cases were split. Thanks for all the help, Hawaii.

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