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Ford 9N, 2N & 8N Discussion Board

Re: dissappointing compression test

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Posted by TheOldHokie on December 12, 2012 at 09:13:18 from (

In Reply to: dissappointing compression test posted by JMOR on December 12, 2012 at 08:17:48:

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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