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Ford 9N, 2N & 8N Discussion Forum
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dissappointing compression test

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carlntexas

12-11-2012 09:53:30




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I did a compression test on my 49 8N converted to 12v That I've owned for the last 29yrs. Tested with throttle wide open and breather tube disconnected here are is what I got.

dry wet

#4-75 80

#3-80 95

#2-70 80

#1-85 95

I was expecting better numbers because it starts up easy without using choke, has good power, uses very little oil ( probably leaks more out of governor arms than it burns ). I'm not planning on doing anything to it just wanted to get some of you 8N gurus thoughts on these numbers. I just bought this $20 compression tester on Ebay and maybe it's not accurate.

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Royse

12-11-2012 19:55:23




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
Compression test on my Jubilee was all over the place, but no
where near where it should have been.

Turned out, after testing two other engines, I had a bad gauge.

Tractor's still running along happily.

If it ain't broke, don't fix it 'till it is! :)



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russb wa

12-11-2012 17:33:26




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
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.

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carlntexas

12-11-2012 19:07:40




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 Re: dissappointing compression test in reply to russb wa, 12-11-2012 17:33:26  
Russ I think what your telling me is my cheap tester could be part of the problem. This is what it looks like,no name, most likely for China but professional.



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JMOR

12-11-2012 18:00:01




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 Re: dissappointing compression test in reply to russb wa, 12-11-2012 17:33:26  
Quoting Removed, click Modern View to seeright on russ. A quality compression test setup will have that check right at the threads



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TheOldHokie

12-12-2012 07:45:24




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 Re: dissappointing compression test in reply to JMOR, 12-11-2012 18:00:01  
Quoting Removed, click Modern View to see

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

TOH

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JMOR

12-12-2012 08:17:48




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 Re: dissappointing compression test in reply to TheOldHokie, 12-12-2012 07:45:24  
Quoting Removed, click Modern View to seeYou 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.



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TheOldHokie

12-12-2012 09:13:18




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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 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

TOH
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.
TOH
This post was edited by TheOldHokie at 09:15:04 12/12/12.

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JMOR

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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TheOldHokie

12-12-2012 10:11:00




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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 09:27:05  
Quoting Removed, click Modern View to see

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.....
TOH



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JMOR

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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TheOldHokie

12-12-2012 10:46:00




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 Re: dissappointing compression test in reply to JMOR, 12-12-2012 10:22:53  
Quoting Removed, click Modern View to see

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.

TOH



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Hobo,NC

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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Colin King

12-11-2012 16:13:30




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
I think that if you're happy with how she's running and the work she can do, don't worry about it. Unless you're looking for a winter project, let her work until something serious requires attention.

Good luck!

Colin, MN



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ASEguy

12-11-2012 15:14:44




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
I also do five puffs as Dell noted. First puff needs to be at least 1/2 of final puff otherwise suspect rings. Also compression tests need to be done on a warm engine that can achieve proper cranking RPM so... good starter and fully charged battery are necessary. If you are ever concerned about where the leakage is going you'll need to check cylinder leakage with guages. Gerard



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carlntexas

12-11-2012 16:14:09




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 Re: dissappointing compression test in reply to ASEguy, 12-11-2012 15:14:44  
Gerard everything I know about compression testing i learned on the internet, mostly on this site. I did drive it and get it to operating temperature before testing, because that's what I read to do. Tractor is 12v and battery had full charge it was spinning fast.



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ASEguy

12-12-2012 01:10:03




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 16:14:09  
Carl, Yours sounds like relatively even wear for an older engine. If it's doing the job she's good to go. There are three compression tests that can be performed: dry cranking, wet cranking and running. If dry and wet cranking are inconclusive then cylinder leakage tests will isolate the leakage. Running compression tests the valve train for issues like bent pushrods, worn cam lobes "etc".



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soundguy

12-12-2012 06:20:17




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 Re: dissappointing compression test in reply to ASEguy, 12-12-2012 01:10:03  
push rods, neat...



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Dell (WA)

12-11-2012 13:09:55




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
Carl.......didja do at least 5-puffs per cylinder??? Them numbers look like 1-puffs. Me? I letter crank until the numbers don't come up ennymore before I writ'em down. The uniformity of compression re-enforces the suspicion of first-time shadetree wannabees mechanics. .........Dell



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carlntexas

12-11-2012 15:12:25




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 Re: dissappointing compression test in reply to Dell (WA), 12-11-2012 13:09:55  
Quoting Removed, click Modern View to see

Dell your right on the wannabee mechanic, first compression test ever, but wrong on the number of puffs. I puffed till the needle quit moving I wanted those numbers to come up. I even went back to the two low numbers and rechecked because of the uniformity. :) Carl the shadetree wannabee compression tester.



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soundguy

12-11-2012 12:51:18




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
i'd have to agree with TOH.

it's just out of specs.. and does look like valves not rings.

I wouldn't even invest in a 20$ head gasket and hand lap on the valves . if it is starting fine and working fine?

me? I'd leave it till it NEEDS work.



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TheOldHokie

12-11-2012 10:26:32




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  

carlntexas said: (quoted from post at 13:53:30 12/11/12) I did a compression test on my 49 8N converted to 12v That I've owned for the last 29yrs. Tested with throttle wide open and breather tube disconnected here are is what I got.

dry wet

#4-75 80

#3-80 95

#2-70 80

#1-85 95

I was expecting better numbers because it starts up easy without using choke, has good power, uses very little oil ( probably leaks more out of governor arms than it burns ). I'm not planning on doing anything to it just wanted to get some of you 8N gurus thoughts on these numbers. I just bought this $20 compression tester on Ebay and maybe it's not accurate.


Looks like valve not ring leakage to me - with the exception of #3 you only see 5-10 PSI improvement on the wet test and it's pretty even across the board. If its runnig OK and getting your job done I'd put the tester in the tool chest and fine something else to spend my time on.

TOH
This post was edited by TheOldHokie at 10:28:01 12/11/12.

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old

12-11-2012 10:05:41




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 09:53:30  
E-Bay and compression gauge why not buy local as in from an auto parts store?? Shoot some will rent you one and the real cost is you buy it and then you bring it back and get your $$ back. That said it could be as simple as sticking rings or carbon build up. I would put say a 1/4 cup of ATF in each cylinder and let it sit a day then run it and after getting it warm do another compression check and I will almost bet them numbers come up a good bit

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carlntexas

12-11-2012 10:32:27




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 Re: dissappointing compression test in reply to old, 12-11-2012 10:05:41  

old said: (quoted from post at 13:05:41 12/11/12) E-Bay and compression gauge why not buy local as in from an auto parts store?? Shoot some will rent you one and the real cost is you buy it and then you bring it back and get your $$ back.
That said it could be as simple as sticking rings or carbon build up. I would put say a 1/4 cup of ATF in each cylinder and let it sit a day then run it and after getting it warm do another compression check and I will almost bet them numbers come up a good bit
After seeing all the posts on here about soaking in ATF that was my thought it's been soaking for 3 days now. I don't need to use it for a while so I'm gonna go out and add some more ( when it warms up a little it was 24 degrees this morning ). When I go to start it up should I leave the plugs out and turn it over to blow ATF out or just start it and let it smoke.
This post was edited by carlntexas at 10:39:26 12/11/12.

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old

12-11-2012 11:07:29




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 10:32:27  
You will need to pull the plugs out and spin it over to clear out any excess ATF left in the cylinders if you have any that is. Some of the ATF will fall down threw the rings and then into the oil pan which will not hurt any thing since ATF is a simple oil that is very high detergent and that detergent is why it works well for many things and has even been proven to work better then things like PB blaster and Kroil

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TheOldHokie

12-11-2012 11:04:39




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 Re: dissappointing compression test in reply to carlntexas, 12-11-2012 10:32:27  
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The purpose of doing a "wet" test after you get a low "dry" result is to identify the source of compression leakage. The reason you add the oil is it acts as a sealant around the rings. If the compression improves significantly after you add the oil - 40 PSI or more- it indicates the rings are leaking. If it only comes up a few PSI as is your case it indicates the rings are sealing fine and the major leakage is somewhere else. I think you are wasting your time with the ATF.

TOH

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carlntexas

12-11-2012 12:47:25




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 Re: dissappointing compression test in reply to TheOldHokie, 12-11-2012 11:04:39  
Quoting Removed, click Modern View to see



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old

12-11-2012 11:11:12




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 Re: dissappointing compression test in reply to TheOldHokie, 12-11-2012 11:04:39  
ATF will in fact help if it is a valve problem due to the fact valves stick and get carbon build up and ATF will remove some if not all the carbon if that is the problem.



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