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dissappointing compression test

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carlntexas
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PostPosted: Tue Dec 11, 2012 9:53 am    Post subject: dissappointing compression test Reply to specific post Reply with quote

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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old
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PostPosted: Tue Dec 11, 2012 10:05 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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TheOldHokie
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PostPosted: Tue Dec 11, 2012 10:26 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

carlntexas wrote:
(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
 


Last edited by TheOldHokie on Tue Dec 11, 2012 10:28 am; edited 1 time in total
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carlntexas
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PostPosted: Tue Dec 11, 2012 10:32 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

old wrote:
(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.
 


Last edited by carlntexas on Tue Dec 11, 2012 10:39 am; edited 1 time in total
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TheOldHokie
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PostPosted: Tue Dec 11, 2012 11:04 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

carlntexas wrote:
(quoted from post at 14:32:27 12/11/12)
old wrote:
(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.


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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old
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PostPosted: Tue Dec 11, 2012 11:07 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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old
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PostPosted: Tue Dec 11, 2012 11:11 am    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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carlntexas
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PostPosted: Tue Dec 11, 2012 12:47 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

TheOldHokie wrote:
(quoted from post at 14:04:39 12/11/12)
carlntexas wrote:
(quoted from post at 14:32:27 12/11/12)
old wrote:
(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.


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.
I'm Sure your right it's a valve problem,but ATF is cheap and I got lots of time. I'll just soak it a while can't hurt anything thanks for the help.

Carl
TOH

 
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souNdguy
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PostPosted: Tue Dec 11, 2012 12:51 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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Dell (WA)
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PostPosted: Tue Dec 11, 2012 1:09 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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
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PostPosted: Tue Dec 11, 2012 3:12 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

Dell (WA) wrote:
(quoted from post at 16:09:55 12/11/12) 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

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. Smile Carl the shadetree wannabee compression tester.
 
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ASEguy
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PostPosted: Tue Dec 11, 2012 3:14 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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Colin King
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PostPosted: Tue Dec 11, 2012 4:13 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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carlntexas
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PostPosted: Tue Dec 11, 2012 4:14 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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russb wa
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PostPosted: Tue Dec 11, 2012 5:33 pm    Post subject: Re: dissappointing compression test Reply to specific post Reply with quote

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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