Yz426 compression test? Fried rings?

5 replies to this topic
  • Tevonini

Posted April 15, 2013 - 06:33 PM


Hello I raced a local hare scramble last weekend and half way through my first lap I blew an I ring on a water neck, I ran half the lap with no water on accident, I dropped out of the race at the end of the first lap cause I came into the pits with no water but it was running fine, I turned it off and walked it to the truck loaded it up and today I my OEM o ring came in do I installed it and now she won't start, could I have fried my rings? It was running fine she usually starts 1st 2nd kick I just kicked for 15 min and nothing not as much as a back fire... How would I do a compression test with an auto decomp cam? Is it possible? Please help, this top end only has 8 hours of ride time :(

  • grayracer513

Posted April 15, 2013 - 07:41 PM


... How would I do a compression test with an auto decomp cam? Is it possible?

A conventional compression test is pretty useless with auto decompression. What you need is a leak down test. This will not only tell you whether your compression is low, it will tell you where the leak is. Most reasonably well equipped shops have testers, and won't charge much to run the test if you bring them the bike with the tank off.

If it got really hot, and you've lost compression, you'll want to look at the head closely, particularly the exhaust valves and the area around the seats. I recently did one that got so hot that the surface of the aluminum between the exhaust seats reached the melt point and the seats got slightly ovaled. Saved it, but barely.

  • Tevonini

Posted April 15, 2013 - 08:07 PM


What you need is a leak down test.

What is a leak Down test? How is it proformed?

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  • grayracer513

Posted April 15, 2013 - 09:36 PM


With the crank held in position, compressed air is fed to the cylinder through a special gauge setup that allows the user to determine how much (because some is normal) the combustion chamber leaks and if it's excessive, where the problem is.

  • marv02

Posted April 20, 2013 - 07:35 AM


The other way to see what's bad make sure both valves are closed I used my compression tester hose hooked up to my air compressor I think I put around 100 Psi if you hear air coming out of the case vent tube it your rings if comes out the exhaust it your Exhaust valves if air comes out the intake boot it the Intake valves.

I did this on my Small Block Chev I sure it work on the bikes.

  • grayracer513

Posted April 20, 2013 - 07:18 PM


The rings will always leak because they have gaps at the ends.

The question the leak down test answers is whether it's leaking too much.

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