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Compression Test question

This is a discussion on Compression Test question within the Yamaha YFZ450/450R (2004-Present) forums, part of the Performance /Technical category; i got my bike all apart and want to do a compression test on it. i know the compression ratio is 11.9:1 but thats not ...


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Old 02-08-2005, 01:19 AM   #1
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Default Compression Test question

i got my bike all apart and want to do a compression test on it. i know the compression ratio is 11.9:1 but thats not what im looking for. when you stick a compression tester, whats the PSI supposed to be? ive looked all over the place and can't seem to come up with it anywhere. anybody know the specs for this? im guessing its gotta be at least 150 PSI, if not a lot more. i've yet to stick my tester on there... id like to know what it is first beforehand. thanks!
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Old 02-08-2005, 09:15 AM   #2
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The problem with the YFZ is that it has an auto decompression release to lessen the strain on the starter. This device gives you inaccurate readings when using a Pressure tester because the engine rotation is slow. When the engine speeds up the auto decompression is disabled.

What you need to do is either

1. Disable the auto-decompression
or
2. Perform a leak down test.

I would like to know the details of both if someone has them. I have only heard people mention these two things not talk about them in detail. Any one else have some insight on this issue?
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Old 02-08-2005, 05:19 PM   #3
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i done it on my old honda and it had 100 psi.me brothers 450r had around 120 i believe.so i wouldn't hink it would be anymore thatn that
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Old 02-08-2005, 08:05 PM   #4
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Quote: Originally Posted by blwettstein
i done it on my old honda and it had 100 psi.me brothers 450r had around 120 i believe.so i wouldn't hink it would be anymore thatn that
...thats a honda...and we all know that blue is faster...
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Old 02-09-2005, 09:17 AM   #5
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I dont know about the hondas but I do know that you cannot get an accurate reading with a compression tester on a YFZ without disabling the decompression release or performing a leakdown.
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Old 02-09-2005, 09:05 PM   #6
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On my bike I ran a compression test before the cam mod and got 140 psi and then after I did the cam mod and the decompressor is retimed with the cam the cranking compression dropped to 90 psi. If you have a yz dirtbike cam with the taller decompression pin the numbers will be lower.
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Old 02-10-2005, 11:25 AM   #7
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True, but you get so much leakage from the decompression pin you are not getting a true reading of the compression from the rings. If your rings are shot you are still going to get about the same reading on a compression tester because the leakage from the decompression release is the path of least resistance.

Only way to truely tell is to perform a leakdown. This is why the shop manual or the owners manual does not state a number for a compression tester. Ask any reputable shop and they will tell you the same thing.

Honestly, if you disable the decompression pin you would probably be reading around 190psi im guessing.

Here is a halfway article on this
http://www.ppbb.com/scgi-bin/boards/...ames;read=6923
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Denton Steering Stabilizer
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Pro Design Kill Switch
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Shredders on douglas beadlocks
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Old 02-10-2005, 04:55 PM   #8
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thats what i was using is a snap-on leak detector
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