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Re: dissappointing compression test

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Author  [Modern View]

12-12-2012 09:13:18

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JMOR said: (quoted from post at 12:17:48 12/12/12)
TheOldHokie said: (quoted from post at 11:45:24 12/12/12)
JMOR said: (quoted from post at 22:00:01 12/11/12)
russb wa said: (quoted from post at 21:33:26 12/11/12) Hi Carl,
I had a 250cc two cylinder bike that tested low.
After all tests and figuring. I came to the
conclusion that the extension and hose used to get
to the plug hole (extremely deep plug recess) were
of sufficient volume to affect the test. The small
displacement with large equipment gave a low
reading. Might be different if there was a check
valve right close to the plug end of the tester.
right on russ. A quality compression test setup will have that check right at the threads

I don't think it makes much difference JMOR - there's very little volume in a 1/4" x 24" long hose. You can easily pump up 150 PSI gauge pressure on a 45cc pull start chain saw even with the check valve at the gauge end of the connection:

Chainsaw Compression Test

You may be right on the volume, but in that video, I never saw where the check valve was located, only that the release valve was at the gauge end. My gauge has a check in the threaded part at spark plug end and release at gauge end, just as the video gauge might? have.

I'm always nervous before I post anything that even hints of being counter to your judgement or opinion and that thought did cross my pea brain but not until AFTER I hit send :roll: I went back and looked at the video again but couldn't be sure one way or the other either. I have a decent quality Sun compression tester that I have used with no problems on my chainsaws so I checked and it does not have a check valve in the hose or plug end fitting. The ID of the hose on it is pretty small - 1/8" at the max - and about 18" long. Doing the math I get .025 cubic inches = ~4 cc for the volume in the hose. So one compression stroke on my 55cc Husky is going to produce an 13X pressure rise (PV=k, 140+ PSI) inside the hose - right or wrong??? In any event it's a more interesting queston than a lot we get here and my brain needed a little exercise.
This post was edited by TheOldHokie at 09:15:04 12/12/12.

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12-12-2012 09:27:05

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 Re: dissappointing compression test in reply to TheOldHokie, 12-12-2012 09:13:18  
I concede that 4cc isn't going to make much difference, especially where the TDC chamber volume is 60 or 70CC in many engines & well beyond that in your N tractor.

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12-12-2012 10:11:00

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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 09:27:05  
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I wasn't looking for concession - just a corroboration or correction I can trust :D Obviously the closer you can get to measuring the actual engine volume the better. Npw on to far less interesting tasks - fixing the coolant leak in TOOH's Cat. At least that has an outside chance of buying a little domestic tranquility.....

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12-12-2012 10:22:53

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 Re: dissappointing compression test in reply to TheOldHokie, 12-12-2012 10:11:00  
Quoting Removed, click Modern View to seeOn that coolant leak, first are you sure that you have a coolant leak? Have you checked your tire pressure? What is the position of the shifter when you observe this? I recommend a different viscosity coolant.

If all else fails, adding ATF to any opening that will accept it seems to be the primary fix....have you tried that? How is condenser & relief valve? Primed the oil pump lately?

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12-12-2012 10:46:00

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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 10:22:53  
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Actually I have only taken TOOH's word for the coolant leak analysis. Perhaps it's really A/C condensate in December and she is actually pouring coolant into a drain tube.


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12-12-2012 09:52:16

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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 09:27:05  
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I have checked my gauge again the one on my compressor,,, they both read the same... Its simple and fast and good enuff for me....

Compression test is just a link in the chain of diagnostics (as you know but very few Guru's here understand)....

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