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

11 replies to this topic
  • steve lemley

Posted January 19, 2006 - 02:17 PM


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..

  • ARin

Posted January 19, 2006 - 02:44 PM


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.

  • RichBaker

Posted January 19, 2006 - 03:14 PM


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.

  • steve lemley

Posted January 19, 2006 - 03:16 PM


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.......

  • CycleWriter

Posted January 19, 2006 - 04:31 PM


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.

  • steve lemley

Posted January 19, 2006 - 06:59 PM


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

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  • Matty05

Posted January 19, 2006 - 07:02 PM


It will use heaps of oil!

  • sah81

Posted January 19, 2006 - 07:10 PM


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

  • 642MX

Posted January 19, 2006 - 08:50 PM


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.

  • Gadsen

Posted January 19, 2006 - 09:04 PM


Dont overlook a tight valve

  • firezwr426

Posted January 20, 2006 - 05:45 AM


Dont overlook a tight valve

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

  • steve lemley

Posted January 20, 2006 - 07:26 AM


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


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