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Discussion Starter #1
Out of curiosity, I measured the compression of my 2 10ML55's. I read that due to the decompression mechanism, the compression reading is not the actual compression. However, it is a good point of reference when comparing 2 same model engines. Measurements were taken with the throttle wide open, and engines cold.

#1 Measured around 80 psi



#2 Measured around 100 psi



Thoughts, comments?

-Ken
 

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Carbon build-up on valves?

Cylinder wear?

Imperfect ring seal?

Normal wear?


I don't mean to to sound enigmatic, but compression reduction is just a symptom of something else. It could be a number of things, that may or may not be related to the decompression mechanism.
 

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Discussion Starter #3
Good point. I will start with some seafoam and recheck. Even without the gauge, I could feel the difference in compression when I pull the recoil slowly. Operationally, the machine with the lower compression starts and runs fine. I have not had a chance to run the higher compression machine in the snow.
 

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Looks like these don't have electric start.

I would warm them up and pour some oil in the cylinder just to see what happens. If compression rises with the oil you probably need to rering. If no change something like head gasket or valves. I'm sure you already know this whole routine if you already have the gauge.
 

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Discussion Starter #5
Thanks for the input, scripper. I've been reading up on it, but have never done it before. how much oil should I pour in? Correct, no electric start currently. looks like #1 had one at one point.
 

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These readings are both very acceptable for Tecumseh engines. Tecumseh never published standard compression readings for their engines. I have witnessed good running engines with compression from as low as 60 psi to as high as 110 psi. Readings above 100 can sometimes indicate a compression release mechanism that is not functioning properly. Older engines with ring and cylinder wear will offer higher readings when warmed up. A leak down test will give you credible results for finding subtle leaks. It will tell you where a leak is coming from, too. MH
 

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Agreed. I don't think you should try to rework either motor I was just thinking that if you already had the compression tester set up it would be no trouble to go one step further just to see.

I had a briggs with a bad head gasket that was running fine with #70. It would bog at the end of the driveway but other than that it ran and started pretty good. After the head gasket was replaced the compression jumped up to #130 or so. A cap full of oil would not have raised the readings on that motor and I would have known that the rings were not the issue (I already knew the head was leaking).

Sometimes the exhaust valve gets fouled with carbon and a little seafoam can break it up and raise compression. This is another reason why I like to know if the rings are the main culprit. Simple tests like this are free and easy, even if you have no intention of doing anything after finding the results.
 

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Discussion Starter #9
Excellent suggestions and info, guys. Correct, my intention at this stage is mostly out of curiosity and exploring non-invasive methods such as adding oil in the compression test, seafoam, and comparing compression results between cold and hot engines.

It's running pretty good, but feels weak to me. Not sure if I am setting my expectations too high for a 53 year old 5.5hp engine, to be honest. My experience has been limited to 8hp and 12hp units.
 
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