Low compression and valves

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Low compression and valves

Postby Ronny Bailey » Wed Mar 18, 2009 3:16 am

Reading Larry's post, I noticed a lot of folks think his low compression (40#) is probably related to the valves.
I understand that if they don't seal, you can't get good compression.

If this is a valve problem:
The fact that all four cylinders have the low readings must mean that each has at least one valve with a bad seal.
Is this unusual? It seems to me the likelihood that all four would have valve problems would be kind of remote.
I read that the engine hasn't run for about 20 years and some valves were stuck.

I was just curious about this and thought I'd ask.
After the valves on mine were loose, I chipped away a lot of carbon off the valves and seats.
Used an old vacuum cleaner while doing it so nothing would fall into the engine.
Haven't done a vacuum test on it, but it sounded good at the LA Cubfest when it was running.
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Re: Low compression and valves

Postby DanR » Wed Mar 18, 2009 5:20 am

Ronny you are right about a 'bad seal' on each valve. I would rather think it is a carbon problem. Carbon is not selective. It can cause valves to stick or nor seat and hold rings tight. Just put a pint or so of kerosene in the oil and run it under load for a few hours. You will be surprised how much dissolved carbon will drain out . You can do this until the oil looks more like oil and not hurt anything.
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Re: Low compression and valves

Postby Lurker Carl » Wed Mar 18, 2009 6:35 am

A rich fuel mixture left unchecked over years of operation, as will worn rings, will create the carbon deposits on the exhaust valves that result in poor compression. So will disregarding regular maintanence. A too hot combustion temperature will erode the valves and seats. Worn valve guides can cause the valves to rest off center on the seat. Rust could be a culprit with an unused engine.

I'm not a big fan of diluting the crankcase oil with any solvents that are meant to "clean out" particles within the engine. The Cub engine can not efficiently remove these particles from the oil because only part of the oil flowing from the pump is routed through the filter. The rest of the crud-laden oil makes it's way to the various bearings throughout the engine. Engine deposits are best left alone or physically removed when the parts are dismantled.
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Re: Low compression and valves

Postby Former Member » Wed Mar 18, 2009 9:19 am

For cleaning up the carbon without adding solvent to the oil or anything mechanical, My Dad and I like to do this process when we get a new, used motor.

We use Lucas fuel treatment. Use your favorite brand.

With the engine running at high idle, slowly squirt the lucas directly into the throat of the carb. It will start to load up and blow white smoke and probably spit some carbon out the exhaust. Stop squirting before it dies and let it clear out.


Do it again. The third time, squirt enough in the kill it. Choke it right down till it dies. Turn it off and let it sit overnight, so that it has time to desolve what is left of the carbon in the cylinder, rings, and both intake and exhaust valves.

When you restart it, it will smoke the white smoke, but will clear up and should hep your compression problem.
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Re: Low compression and valves

Postby Lurker Carl » Wed Mar 18, 2009 10:13 am

dshawz wrote:Do it again. The third time, squirt enough in the kill it. Choke it right down till it dies.


Be very careful when adding liquids into the air intake of any engine, it can create a hydraulic lock up. It's a proven method for instantly destroying an engine.

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Re: Low compression and valves

Postby ToddW » Wed Mar 18, 2009 10:31 am

I've read posts here in the forum about running the engine briefly with 1/2 pint kerosene added to motor oil to clean the crud out. Wouldn't this be a good idea to try to improve compression and remove deposits
Last edited by ToddW on Sun Mar 22, 2009 9:28 am, edited 1 time in total.
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Re: Low compression and valves

Postby DanR » Wed Mar 18, 2009 10:37 am

Don't run the engine with kerosene only! A pint or so with the oil will do the trick. A few minutes will not work either. A few hours is needed for the kerosene to do it's work.
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Re: Low compression and valves

Postby ricky racer » Wed Mar 18, 2009 11:17 am

A simple way to see where compression is lost is to induce compressed air to each cylinder and see where it leaks from. You can use the threaded end of a compression tester on many brands or make an adapater from an old spark plug that you can attach an air hose to and induce regulated compressed air into each cylinder to see where it's leaking. If you hear air coming out the exhaust, it's leaking past the exhaust valve. If you hear it leaking out the carb, it's leaking past the intake valve. If you hear it leaking out the crank case filler pipe, it's leaking past the rings. Once you know where it's leaking, you'll know what you have to do to "Get-R-Done"!
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Re: Low compression and valves

Postby Eugene » Wed Mar 18, 2009 7:25 pm

I noticed a lot of folks think his low compression (40#) is probably related to the valves. I understand that if they don't seal, you can't get good compression.

The fact that all four cylinders have the low readings must mean that each has at least one valve with a bad seal. Is this unusual? It seems to me the likelihood that all four would have valve problems would be kind of remote. I read that the engine hasn't run for about 20 years and some valves were stuck.
On one cylinder both valves are open for about 15 degrees of crankshaft rotation. Depending on where a 4 cylinder engine stops in the combustion cycle it is possible to have a valve open in at least 3 if not all 4 cylinders. An engine sitting for 20 year - gonna have problems.

More than likely had problems before it was parked.

Grammer correction.
Last edited by Eugene on Wed Mar 18, 2009 8:28 pm, edited 1 time in total.
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Re: Low compression and valves

Postby ricky racer » Wed Mar 18, 2009 7:42 pm

Eugene brings up a good point. In my earlier post I should have stated that each cylinder needs to be checked on it's compression stroke.
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Re: Low compression and valves

Postby Ronny Bailey » Wed Mar 18, 2009 9:51 pm

Good info! Here's another question:
I've noticed that it's usually recommended that you remove all the spark plugs when using a compression checker.
Why is that?
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Re: Low compression and valves

Postby Eugene » Wed Mar 18, 2009 10:12 pm

Ronny Bailey wrote:Here's another question: I've noticed that it's usually recommended that you remove all the spark plugs when using a compression checker. Why is that?
Example. On a 4 cylinder engine, the 3 cylinders not being tested will compress the air/fuel in the cylinder - slowing the number of rpms the engine is being cranked over by the starter. This may or may not effect the reading on the cylinder being tested. High compression engines, this will definitely make a difference. C-60 engine - not so much.

Actually, I done the compression test both ways. I'm looking more for even readings, in the satisfactory range. On a well worn engine it won't make any difference in the readings.
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Re: Low compression and valves

Postby Gary Orr » Wed Mar 18, 2009 10:34 pm

I have to agree with Lurker you don`t have a full flow oil filter & all that sludge will go through the bearings. Plus you can bend a rod if you put a licquid in the combustion chamber.
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Re: Low compression and valves

Postby Corky's Cub » Wed Mar 18, 2009 11:30 pm

Eugene wrote:
Ronny Bailey wrote:Here's another question: I've noticed that it's usually recommended that you remove all the spark plugs when using a compression checker. Why is that?
Example. On a 4 cylinder engine, the 3 cylinders not being tested will compress the air/fuel in the cylinder - slowing the number of rpms the engine is being cranked over by the starter. This may or may not effect the reading on the cylinder being tested. High compression engines, this will definitely make a difference. C-60 engine - not so much.

Actually, I done the compression test both ways. I'm looking more for even readings, in the satisfactory range. On a well worn engine it won't make any difference in the readings.


Eugene's got it...and there's another problem that might not be so pronounced on the cub...on bigger engines, if you constantly run un-burned fuel through the engine by turning it over, it can cause premature wear on the valve guides and rings and make the engine run smoky way before it's time.

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Re: Low compression and valves

Postby Winfield Dave » Thu Sep 09, 2010 6:09 am

Earlier this week I was reading some posts here about using a vacuum gauge to help diagnose engine issues.
Maybe this would be of some help?...It seemed to identify different problems by the gauge reactions.
I haven't done it....just sharing.
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