Valve blow by??? How can I tell???

My buddys bike was sputtering and hesitating a little and we thaught it was a simple jetting issue. Took it to our local race mech. and he said that the valves have blow by and the motor or the valves at least, need to be rebuilt. Is there any tests I can do to confirm this is the problem. Thanks. He said he did a leak down test to determine that there was a leak probley in the intake valve. Blowing back into the carb. Help guys... Thanks..

compression test?

also, you may be able to see some carbon blow by when looking into the throat of the intake. Run a clean white cloth in there and see if you had dark carbon soot that may indicate some blow back into the intake tract.

Leak down test, same as the mechanic said he is the best way to determine where compression is being lost! You can hear and feel where the air is going.

Thanks, I was told that you can't do a compression check on this engine only a leak down test. Is this true and even if I could do one, how the hell do i crank it over enough times to see what the compression is without breaking my ankle? Would taking the timing back to the wr setting help. Or if it hasent been done, edvance the ex. cam to the YZ timing. I dont know if either have been done yet. I havent had the valve cover off yet. Any ideas please.......

A compression test is difficult, but doable. It won't tell you what is leaking, though (rings or valves). Only a leak down test can do that.

how exactly do you do the leak down test??? Any special tools needed?

It will use heaps of oil!

how exactly do you do the leak down test??? Any special tools needed?

You need a cylinder leakage tester and shop air to perform this test. The tester consists of 2 gauges and a pressure regulator. When you put regulated pressure in the cylinder (usually 80psi) the second gauge will tell you how much of the air is being held by the cylinder. The engine needs to be set at top dead center and the tester will screw into the spark plug hole. When the air leaks out, you can feel and/or hear it to tell where it is coming from. Less than 10% leakage is good. Anything over 20% requires repairs. Here is an example of a tester

Pull the cylinder head, and remove the valves and see for yourself. Its not hard to do, I can tear down a YZF in under an hour easily. If you don't feel comfortable doing it yourself, then you just going to have to trust the mechanic and let him fix it for you.

Dont overlook a tight valve

Dont overlook a tight valve

just what I was going to say.... :thumbsup:

whatd the clearance suppost to be. Is there an exact rutine to checking them properly? Thanks again guys...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now