Leak Down Test - Need Guidance

[oldbikedoc]

Bike is a 2000 YZ250 fresh bottom and top end (brand new cylinder,too!). Figured I'd do things right and bought a Motion Pro Leak Down tester. Bit funky because the exhaust side is not round (and the rubber plugs are!), but got things sorted.

Here's the rub, motor will not build pressure. Seems to be pushing the air out of the Powervalve breather...

Should that be pugged, too? Did I forget something when I rebuilt the powervalve that has created a leak?

Any advice would be most appreciated!

[AV8R]

Yes, you have to plug the powervalve breather. You may find that the engine will not hold pressure perfectly right after rebuild. It may need to be run a couple times.

[hallsy]

I haven't had mine apart so I don't know this for sure but you might have to find a way to prevent air loss around the powervalve linkage too. AV8R could you explain why you might have to run it a couple times?

Edited December 7, 2009 by hallsy

[AV8R]

On my bike the base gasket (which is paper) would leak until it runs. My guess is that the premix gets into the gasket and seals it up. The head o-rings leak somtimes until it's ran also.

[AV8R]

On my bike the base gasket (which is paper) would leak until it runs. My guess is that the premix gets into the gasket and seals it up. The head o-rings leak sometimes until it's ran also.

[oldbikedoc]

So, by the time I discover I have a leak, it'll be too late? Gotta be a way to test these things on the bench. Seems like a crapload of air coming out of the powervalve breather. I'm thinking I'll review the seals on that as well as plug it.

[shrubitup]

Not sure if this is new information but an epic thread on the topic:

https://www.thumpertalk.com/forum/showthread.php?t=506699

[AV8R]

So, by the time I discover I have a leak, it'll be too late? Gotta be a way to test these things on the bench. Seems like a crapload of air coming out of the powervalve breather. I'm thinking I'll review the seals on that as well as plug it.

You can test it on the bench or before you run it. It's just that you may note "some" leakage. This can be verified by using soapy water in a squirt bottle to visually see where it is leaking from. It will be very slow and will stop at some point prior to leaking down to zero psi.

As far as the powervalve vent goes, think about it for a second and you'll see that obviously you need to plug that vent. It is a direct line into the cylinder via the powervalve assembly.

[oldbikedoc]

Yep, read that thread, great info.

Have squirt bottle ready to rock. What's the hot ticket for plugging that hole?

Does seem like powervalve vent is direct line into cylinder. How does it NOT suck air when the bike is running?