# What Is True Compression On a Honda Engine With a Decompressor?



## orangputeh (Nov 24, 2016)

Is there a formula or a % that you add to your compression test reading on a Honda engine with a decompressor ? The Honda shop manual states that the normal compression is achieved after it reaches 600 rpms where it disengages the decompressor.

Most of the Honda's I test are around the 80 psi range . I know this is not the true compression since these engines have a decompressor .

So what should I add to reading? Is there a perhaps % increase to add? I watched a donyboy73 video on it and he said what he does is test a fairly new engine and then file that number and then compare it to the similar used engines since.

Or he slow pulls the recoil just to "feel" the compression. I want a more accurate reading.

Since most of my readings are in the 80-85 range and the engines run real well I am assuming this is a good reading. When I get a reading of 60-70 I won't use that engine and it is regulated as a parts machine.

anyone here have some knowledge on this?


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## ST1100A (Feb 7, 2015)

Honda never released the "True" numbers because it could vary depending on the valve clearance.
Honda did have in a technical bulletin issued to Authorized Dealers that the exhaust valve clearance should be loosened to the point that the "Decompressor" will become de-activated, then you will get a more accurate number with a minimum amount being above 100 psi. You should get more around 110-140 psi area.
Most people will not be able to "Pull" start the engine fast enough, above 600 rpm's to have the automatic de-compression de-activate itself, especially on a larger engine. A very small engine, you might come close to that speed, and the electric start will not get it spinning that fast.
Most diagnoses was done with a "Leak-down" test to better diagnose internal wear. The "Compression test" was the fast way of checking and also to be able to tell if the automatic decompression release was working.


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## CalgaryPT (Dec 7, 2016)

Good question, and good points. I too watched the donyboy73 vid. My expensive digital compression tester with a "HOLD" function on it is useless on my Hondas. Having said this I've got 76 psi on a GC160 and it's still running great 10+ years later. But I think the test was off and I just don't trust testers on engines with decompression. I wish there were a fast and cheap way to test them properly.


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## orangputeh (Nov 24, 2016)

ST1100A said:


> Honda never released the "True" numbers because it could vary depending on the valve clearance.
> Honda did have in a technical bulletin issued to Authorized Dealers that the exhaust valve clearance should be loosened to the point that the "Decompressor" will become de-activated, then you will get a more accurate number with a minimum amount being above 100 psi. You should get more around 110-140 psi area.
> Most people will not be able to "Pull" start the engine fast enough, above 600 rpm's to have the automatic de-compression de-activate itself, especially on a larger engine. A very small engine, you might come close to that speed, and the electric start will not get it spinning that fast.
> Most diagnoses was done with a "Leak-down" test to better diagnose internal wear. The "Compression test" was the fast way of checking and also to be able to tell if the automatic decompression release was working.


doing some more homework on this the laeak downtest is a better way so I will do that fromnow on. The compression islisted in the Honda shop manuals and I will have tolook it up.

thanks.


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## orangputeh (Nov 24, 2016)

CalgaryPT said:


> Good question, and good points. I too watched the donyboy73 vid. My expensive digital compression tester with a "HOLD" function on it is useless on my Hondas. Having said this I've got 76 psi on a GC160 and it's still running great 10+ years later. But I think the test was off and I just don't trust testers on engines with decompression. I wish there were a fast and cheap way to test them properly.


most engines i check are in the 80 range and those engines run real strong. like mentioned above I am going to do a leak downtest from now on to get a better pictureof engine health.

thanks.


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## ST1100A (Feb 7, 2015)

orangputeh said:


> most engines i check are in the 80 range and those engines run real strong. like mentioned above I am going to do a leak downtest from now on to get a better pictureof engine health.
> 
> thanks.


80 is good with the decompression working. If you disable the decompression you will get a higher reading. Just loosen up the exhaust valve clearance and check it again to get a different reading.
They are not high compression engines so the reading should be at least 100 psi, it will probable be closer to 120-160 range, it can vary.


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## orangputeh (Nov 24, 2016)

ST1100A said:


> 80 is good with the decompression working. If you disable the decompression you will get a higher reading. Just loosen up the exhaust valve clearance and check it again to get a different reading.
> They are not high compression engines so the reading should be at least 100 psi, it will probable be closer to 120-160 range, it can vary.


That's what I was thinking that 80 was good.just wish there was a formula for converting 80 to approximate true compression .but anyway with a leak down test I'll have a better idea on health of engine.

Thank You.


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## paulm12 (May 22, 2015)

I bought a leak-down tester a few years ago, and trust the readings much more than compression tests. Also, you can do some diagnosis with the leak-down test as well. 

tx


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## groomerz (Feb 7, 2015)

wont the leak down test show a leaky exhaust valve because exhaust valve is slightly of it’s seat from the compression release 


Sent from my iPhone using Tapatalk


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## paulm12 (May 22, 2015)

groomerz said:


> wont the leak down test show a leaky exhaust valve because exhaust valve is slightly of it’s seat from the compression release
> 
> 
> Sent from my iPhone using Tapatalk


groomerz: no issue, you just have to get to TDC where the compression release is no longer bumping the exhaust valve open.

tx


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