# Low Compression..How to diagnose



## Spit (Nov 3, 2006)

I picked up a used 6HP Craftsman mower (with a Briggs engine)the other day. When I checked the compression with my gauge I barely got over 30 lbs. I pulled the head off and the intake valve was coated with oily/carbon deposits/ residue. The exhaust valve was white. The head itself as well as the piston had a fair amount of burnt oil deposits on them. There was thin strip off carbon deposits around the cylinder mostly at the bottom .I did not see any scoring of the cylinder. 

I got a Briggs valve compressor tool for Xmas and had some compound and a lapping tool so I lapped the valves but I figured that I must have more problems with the rings/cylinder/piston but lo and behold it started with a couple of pulls and seemed to run okay. It was 9:30 pm and I couldn't let it run very long in my residential neighborhood ..but I did not see any smoke to speak of.. 

So It made we wonder...how do you all evaluate low compression and know if it's valves or rings etc. Any procedures that you follow would be good to know. What should the valves look like...the head, cylinder etc on a good or a bad engine look like etc. Any thoughts appreciated

Spit


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## Sir Thomas (Dec 7, 2013)

I am working on a Tecumseh Lawn Mower engine. When I pulled the head I notice heavy buildup of carbon, a tad bit under 1/8" and mostly towards the bottom. I'm getting a little less than 60 psi. In my mind this is indicating ring problems. I do not see any scores on the cylindar. The oil is as black as tar. Looking at the diagram, there are three rings on this motor. I'm wondering if the rings are all seated too tight in their landing.
I haven't opened it up and pulled the piston yet. I can't get the blade mount off in order to get into the cam/crank area so I can unbolt the rod and pull the piston to check it's condition.


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## Spit (Nov 3, 2006)

I was thinking of stuck rings...its just a bigger job than I wanted to get into, opening up the engine at the sump and detaching the rod from the crank in order to remove the piston from the cylinder. Have you ever found an easy way to loosen rings with spray cleaners and compressed air etc.? something that doesn't require so much disasasmbly? 

Can we normally suppose that if there is no sign of scoring, and or metal in the oil that the cylinder is okay?

Spit


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## RKDOC (Jun 20, 2006)

The best way to test a 4 cycle engine is with a leakdown tester. It will tell you if it is valves or rings. with ACR's on engines a compression test will show low when the cylinder is fine. You can get a leakdown tester from many places including harbor freight.


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## Sir Thomas (Dec 7, 2013)

Spit said:


> I was thinking of stuck rings...its just a bigger job than I wanted to get into, opening up the engine at the sump and detaching the rod from the crank in order to remove the piston from the cylinder. Have you ever found an easy way to loosen rings with spray cleaners and compressed air etc.? something that doesn't require so much disasasmbly?
> 
> Can we normally suppose that if there is no sign of scoring, and or metal in the oil that the cylinder is okay?
> 
> Spit


Sometimes it helps if you have the engine turned with the spark plug hole up, turn it till it's almost TDC then soak it with some Seafoam Cleaner or some other carbon remover. Let it soak for a while, if some trickles down, put more in then in a day or two take a wood dowel and put it through the hole and tap on it.
You may have to change the oil after all this.


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## 30yearTech (Sep 14, 2006)

4 cycle engines can use oil for many reasons and it does not have to be stuck rings. In fact, I rarely see stuck rings on most 4 stroke engines, mostly see that on 2 cycle engines. As RKDOC stated,(and others have alluded to) a leak down test is the best way to check for problems with rings, and valves as well.
A dirty or faulty oil breather or a leak in the crankcase can cause excessive oil consumption on a 4 cycle. If the engine has been run without an air filter, or for a long time or with a dirty one, and there is indication of dirt ingestion, then there is a good chance the rings have wear, but probably not stuck. If you pull the piston, you can install the rings into the cylinder and check the end gap against reject.


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## Rentahusband (Aug 25, 2008)

RKDOC said:


> The best way to test a 4 cycle engine is with a leakdown tester. It will tell you if it is valves or rings. with ACR's on engines a compression test will show low when the cylinder is fine. You can get a leakdown tester from many places including harbor freight.


Agree with the leakdown test for checking if compression is low due to valves, rings or head gasket. I made my own leakdown tester from a you tube video. It has pointed me to leaky valves and head gaskets when I find low compression on 4 cycle engines. Even after lapping valves on a 5hp tec (snowblower) I still had low comp and poor performance. I wound up taking off some of the ex valve stem so the valve did not stay open as long on the during the compression release. Has plenty of power now.


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