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Compression test results ?

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I just performed a compression test on my 96 RMX250.

The test was performed with the engine cold and i got a reading of 190 psi.

However, I did not hold the throttle open while performing the test.

The question is, should I perform the test again holding the throttle open and if so would you expect the reading to be higher, lower or the same with the throttle open?

Thanks for the input..

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it will be higher with the throttle open.

I guess that's good news.

I will do it over again anyway to see what it actually is.

So right now I am at 190+ psi, is that a good number?

Not really sure what the range is.

Thanks for the help..:thumbsup:

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Compression tests are fairly meaningless in and of themselves unless there is something to compare them to. If you do one immediately following break in on a fresh motor, that test can be used to compare to one done later to see if the seal of the rings has dropped off. In multi-cylinder engines, they can be a useful diagnostic if they turn up one or more cylinders that are out of range with their peers.

But by themselves, compression numbers don't mean much. Cranking compression is affected by too many variables to be able to set a standard number, even for a single model of bike. Changes in port or valve timing, compression ratios, etc., all change the numbers you see. Most modern 450's won't produce full compression with the auto decomp systems they have, and without it, they can't even be cranked through compression.

Leak down tests, on the other hand, give an accurate picture of combustion sealing without a prior record to compare to, and can accurately pinpoint the source of excessive leakage. Because of the lack of a single fixed standard for the testers, they too, are even more useful when the latest test can be compared to a baseline, but they aren't as dependent on that as cranking compression tests are.

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What was the purpose of the test? 90psi is the minimum needed for combustion....so your well above that.

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What was the purpose of the test?

I have had this bike for about 3 years now and have never had the top end apart. I was just curious what compression readings I would get when tested. I know compression results do not tell the whole story about an engines condition but a good reading is better than bad one.

Even with 190+ psi I know I should take the top end apart and measure to determine if piston and rings need replacing.

I guess I was hoping the test results would allow me to prolong taking the engine apart for now since I was not having any complaints as to how the bike was running.

Just lazy on my part.

I know, break out the wrenches.

Thanks for the input.

Al

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If it was me with nearly 200psi(possibly more with throttle open)...I don't think I would worry about the top end at this point. Thats just my opinion and i'm sure that plenty will disagree. The rings are clearly doing what they are supposed to and I sure wouldn't think there could be much cylinder wall scoring with compression that high.

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^^ True, but there could be fatigue on the piston skirts. If it's an unknown, at least tear it down to have a look!

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If it's an unknown, at least tear it down to have a look!

That would be the right way to do it..

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