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Is there any way to do a DYI leak down test to test compression on a YZ125? or do you need special set up to do it?

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I made my own leakdown tester a few months ago, pics shown and discussed in this thread:

https://thumpertalk.com/topic/1164419-making-my-leakdown-tester/

clamped in the intake boot, exhaust flange and YPVS breather hose tightly plugged

and used a bicycle tire pump to slowly pressurized the engine.

 

Rule of thumb is no pressure loss for minimum 15 minutes at 6psi,

my YZ125 has about 50-60 hours on it, it lost only 0.8psi during a complete hour pressurized at 7psi,

so minor that it could also be a loss from the tester set-up itself.

 

----

 

As for a compression tester, you could build you own from individual components

but since they are more common than an leakdown tester, buying one from an auto part store is problably cheaper.

 

Common procedure:

Get the engine warmed up to operating temp, remove spark plug / install tester,

While holding the engine kill button (to prevent potential electrical issues)

and also holding the throttle wide open, kick the engine over about 10 times and record the highest reading.

 

A late model 2006-2016 YZ125 gets about 180psi of compression when brand new and properly broken-in (I once got 187psi after a refresh)

by 160psi or less you should consider a refresh if you want to maintain best performance.

Edited by mlatour

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I made my own leakdown tester a few months ago, pics shown and discussed in this thread:

https://thumpertalk.com/topic/1164419-making-my-leakdown-tester/

clamped in the intake boot, exhaust flange and YPVS breather hose tightly plugged

and used a bicycle tire pump to slowly pressurized the engine.

 

Rule of thumb is no pressure loss for minimum 15 minutes at 6psi,

my YZ125 has about 50-60 hours on it, it lost only 0.8psi during a complete hour pressurized at 7psi,

so minor that it could also be a loss from the tester set-up itself.

 

----

 

As for a compression tester, you could build you own from individual components

but since they are more common than an leakdown tester, buying one from an auto part store is problably cheaper.

 

Common procedure:

Get the engine warmed up to operating temp, remove spark plug / install tester,

While holding the engine kill button (to prevent potential electrical issues)

and also holding the throttle wide open, kick the engine over about 10 times and record the highest reading.

 

A late model 2006-2016 YZ125 gets about 180psi of compression when brand new and properly broken-in (I once got 187psi after a refresh)

by 160psi or less you should consider a refresh if you want to maintain best performance.

Hey great info much appreciated! I actually did not know there were 2 types of test.

the leak down test is what is needed when when checking valve clearances right? Any leaking would indicate valves out of spec and shimming is needed right?

I need to check the compression on my sons YZ125, so I need to get a compression tester.

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I have never performed a leakdown test on a 4-stroke MX engine but yes it will give you

and idea how well the piston ring and valves are sealing the combustion chamber.

 

On a 2-stroke, a compression test will show the general condition of the top end (piston/ring to bore seal)

a leakdown test will show if there are any leaks of the cases mating surfaces, reed block gaskets or crank seals.

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Hey great info much appreciated! I actually did not know there were 2 types of test.

the leak down test is what is needed when when checking valve clearances right? Any leaking would indicate valves out of spec and shimming is needed right?

I need to check the compression on my sons YZ125, so I need to get a compression tester.

Use gauge feelers to check valve clearances on a 4 stroke, and from what I understand leak down test on 4 stroke and 2 stroke kind of different. If doing compression test on 125, Harbor Freight sells a kit. Leak down test shows bad seals, etc, you can make one as mentioned above. I did one on my 125 to discover I had an air leak between reed cage (vforce) which is supposedly common with those.

Edited by johnk408

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Just make sure to use very low pressure when doing vacuum or pressure leak tests on the combustion/crank chambers of a 2 stroke. I've heard of people blowing the seals out with as little as 10-12 psi. I know 6-8 psi sounds like not enough but it is what you need to test for.

A low pressure bicycle pump works well if it has a Guage on it then you don't need to mount a gauge on the leak tester. But the Guage on the leak tester works better imo.

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Just make sure to use very low pressure when doing vacuum or pressure leak tests on the combustion/crank chambers of a 2 stroke. I've heard of people blowing the seals out with as little as 10-12 psi. I know 6-8 psi sounds like not enough but it is what you need to test for.

A low pressure bicycle pump works well if it has a Guage on it then you don't need to mount a gauge on the leak tester. But the Guage on the leak tester works better imo.

 

Yep...6-8psi is good for leak down initiation.   10-12psi and you're pushing it.  I blew seals out right around 11psi I think, which wasn't too big of a deal because I had just put them in, but something you'd want to avoid.  I found that a bicycle pump was kind of cumbersome and used a air compressor....just a quick psshh and you're basically there.  You'll also want to seal the powervalve hose because leaks out of there will happen naturally at BDC.  I used a similar size bolt and and couple zip tyes.

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Use gauge feelers to check valve clearances on a 4 stroke, and from what I understand leak down test on 4 stroke and 2 stroke kind of different. If doing compression test on 125, Harbor Freight sells a kit. Leak down test shows bad seals, etc, you can make one as mentioned above. I did one on my 125 to discover I had an air leak between reed cage (vforce) which is supposedly common with those.

Ya I think I will get the HF compression test kit, looks pretty decent and compared to ones from any of the moto suppliers really good price.

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