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coop63

Leakdown test findings

6 posts in this topic

Recently performed a leakdown test on my 04 yz450, and the results were, well confusing.

First i removed the timing chain at TDC and installed the adapter hose to the spark plug hole. Pumped up the air regulator to 100psi, then connected to the

cylinder adapter. Honestly, I have very little if any leak down. Maybe 1-3% at best. Using an automotive stethoscope, i listened at the exhaust , the carb intake, the exhaust head pipe,

and could not detect any leakage. Obviously, the valves are sealing, as well as the piston rings. No bubbles in the coolant either.

Question, the bike will go from being full of oil to almost empty after 20 miles of single track. I have absolutely no leakage externally, nor does the bike smoke.

Correct me but as i see it the only other way oil could be lost would be due to faulty valve seals, or valve guides. The intake valves clearances are all about 0.00mm. and need adjusting. The exhaust valve clearances are in spec.

But again, the combustion chamber will hold 100 psi all day long.

So where is the oil going?

This did not occur when i first got the bike ... say 20 riding hours ago.

Edited by coop63

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So, then, you've learned that the engine is sealing compression. This probably means that the bore is in good condition and there's no reason to suspect the oil rings. Probably. But that doesn't vindicate the valve guide seals at all, so your oil is either leaking externally, blowing out the breather (less likely with the LDT results you got), being stolen by gremlins and resold, or going past the valve guides. I'd try replacing the seals next. There aren't that many gremlins in the used oil business.

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So, then, you've learned that the engine is sealing compression. This probably means that the bore is in good condition and there's no reason to suspect the oil rings. Probably. But that doesn't vindicate the valve guide seals at all, so your oil is either leaking externally, blowing out the breather (less likely with the LDT results you got), being stolen by gremlins and resold, or going past the valve guides. I'd try replacing the seals next. There aren't that many gremlins in the used oil business.

Gotcha, will try to replace the seals without removing the head, then if that doesn't work the head comes off and new valves and guides will be in order. Eventually i will win! Thanks for your help!!!!

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There should be no reason to replace the valves or the guides. The valves are clearly sealing as is, although inspection may show them to be on the brink of wearing out, and the guides rarely need replacement. It's the seals that are under suspicion.

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I hear ya. Now is it the best approach to remove and replace the seals with the valves in place using compressed air to keep the valves from dropping,

or to take the head off and approach it that way. I have seen many tools to take out the keepers with the valves in place, but the middle intake valve looks like a

booger to get to with some of these tools. I want to lessen the work, but most importantly do it right. Thanks.

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If you want to do it with the head on,you'll need to devise a spring compressor that will work in that situation.

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