Compression Test

lmolson · Feb 2, 2009

Alright Silly question form a rookie here. How do I effectively do a compression test on my sled. I placed the gauge on a cylinder and pulled the rope. If I only let it turn once I get 96 psi. If I give it a full pull 3-4 turns I get 120 psi or so. How can this be? The gauge is not additive. What causes more compression on the later strokes?

Thank you in advance.

Lance

pernikm · Feb 2, 2009

Yes, it is additive. Hold throttle wide open and pull hard till it stabilizes. You have to fill the hose. 96 psi vs 120? Sounds like the volume of the compression gage is about 20% of the head volume...that does not sound far off. Does it? If it tests low, put a little two stroke oil in plug hole to help seal up the rings. If that does not help, check the reeds before deciding to tear down the motor.

Karl

guidoxpress · Feb 2, 2009

^^ what he said..

i hold the throttle open (KEY OFF)...pull till the gauge STOPS building pressure and check for leaks..and compare to other cylinder or previous readings..

sounds like your doing it right so far..

good luck!

lmolson · Feb 2, 2009

Thanks guys. pernikm - I think I have to tear apart the engine anyway to get to the reeds on my fan cooled 550. But what the heck it will make for a good summer project if the compression turns out too low. Is there a magic compression number that says better do a top end rebuild?

guidoxpress · Feb 2, 2009

it really depends on your attitude at fo rthe compression..lower altitude will have higher PSI..but my dealer told me when i had 100psi on a 2-stroke to start planning on a Top End..

but anything less then 90 for sure..your in for a top end..

FYI...
For example, a given engine will vary in compression about 3.5 % per 1000 feet in altitude. At sea level you might register 125 to 130 psi. The same engine if tested at 4000 feet may measure approximately 110 psi using a compensation factor of 14% (3.5 times 4=14%).

pernikm · Feb 2, 2009

The only number I know is 10% difference between jugs. If oil solves this problem, then you are leaking past the rings. Other than that 120 really doesn't sound that bad, for a fan...but you will have to ask what is normal for your altitude. Sea level compression will be higher.

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Compression Test

lmolson

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pernikm

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guidoxpress

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lmolson

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guidoxpress

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pernikm

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