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Ebola

Is blow by on a piston acceptable?

9 posts in this topic

When checking the valve clearances, it's fairly easy for me to turn the motor over with the stator nut and a wrench. The weight of my arm is enough to turn the motor past the compression stroke. I don't need to pull in the decompression lever to move the motor. I also hear what sound like air blowing by coming from the cam chain galley. From hearing other people state that they can stand on their kickstart lever and it won't move, it leads me to believe I need to replace the piston and rings. The bike is not smoking, but isn't very easy to start either. Warm days help. Thoughts on next step?

 

I've never rebuilt the top end. I'm at least the third owner and the bike is 15 years old.

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When checking the valve clearances, it's fairly easy for me to turn the motor over with the stator nut and a wrench. The weight of my arm is enough to turn the motor past the compression stroke. I don't need to pull in the decompression lever to move the motor. I also hear what sound like air blowing by coming from the cam chain galley. From hearing other people state that they can stand on their kickstart lever and it won't move, it leads me to believe I need to replace the piston and rings. The bike is not smoking, but isn't very easy to start either. Warm days help. Thoughts on next step?

 

I've never rebuilt the top end. I'm at least the third owner and the bike is 15 years old.

 

You have answered you own questions.

Hopefully you did not wait to long and destroy the bore...

You are lucky you cam chain did not skip or break, too....

You need a Service Manual so you can follow the service intervals.

Edited by Krannie

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I have a service manual, and I replaced the timing chain when I got the bike. Okay, top end rebuild it is, if it's not too toasty. :-)

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It is absolutely normal to hear air going past the rings when you turn the engine by hand and have it opened up like that.  Anyone who tells you otherwise is rather delusional.  The rings, even brand new ones fresh back from a perfect break-in period, have end gaps, in the first place, and there is nothing to prevent air from running right through these. 

 

Another thing is that even without these, rings are not designed or intended to seal 100% effectively at a relative standstill, for one thing, and compression pressure is the least of what they are tasked with sealing.  Combustion pressures are vastly higher.  RIngs should be capable of sealing more than 97% of the volume above them, but they only have to do so for 14 milliseconds at a time (the approximate duration of the compression and power strokes) at the low speed of 3000 RPM. 

 

Also, remember that you have auto decompression, which limits your compression stroke at cranking speeds to about 20 degrees of rotation.  Wedge a matchbook cover under the flyweight on your exhaust cam with the weight pinned outward and see how hard it is to turn over then.

 

If you want an accurate appraisal of the ring condition, have a shop do a proper leak down test.  That will tell you if it leaks excessively, and where it leaks from.

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I actually don't have auto decompression. Grayracer, in your opinion, given the age of the bike and the fact that it's not smoking, would you pull the head and piston and check things out or would you ride it as is?

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I actually don't have auto decompression. Grayracer, in your opinion, given the age of the bike and the fact that it's not smoking, would you pull the head and piston and check things out or would you ride it as is?

 

Missed this.  To answer, have a leak down test done, you'll know more then.

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