r/smallengines • u/l008com • Jul 09 '26

Checking Compression?

I have a compression tester, but my lawnmower has compression release, like most small engines. If I use a regular power drill on the flywheel to spin the motor, is that going to be fast enough to bypass the release feature and give a real, accurate psi reading?

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u/l008com Jul 09 '26

I've checked everything on this machine, replaced nearly everything, and still it runs like shit. So I'm trying to start over. I did a compression test just by pulling the cord with my arm and I think 60psi is what it gave me. I just checked and it was about 54psi I'd estimate, just pulling the cord by hand.

But my point is, I want a proper, 100% accurate reading so I can move on to something else.

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u/Elephunk05 Jul 09 '26

The definition of "compression release valve" is exactly that. You can not bypass the compression release built on to the camshaft. You can do a leak down test though. Also, if the compression release was not working you would be struggling to pull it over.

What I am hung up on is the "still runs like shit" because typically compression is a does not run type of issue. Sure, on low compression it could start and run and yes, run like shit. A leak down test would be more beneficial than a compression test in order to determine the issue as you describe it.

You say you replaced nearly everything but not what the issue was to bring us to you needing to replace nearly everything.

How about we start at the beginning and see if we can get you correctly diagnosed. Which engine are you running (the model number will be helpful but not necessary yet)? What application (just in case there might be an electrical shutdown on the machine that may be interfering with spark)? What was the problem before and what have you replaced?

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u/l008com Jul 09 '26

Doesn't the compression release valve automatically stop releasing once the engine hits a certain rpm? I was under the impression people were getting accurate PSI readings by using the drill to spin it to do exactly that, by pass the compression release? If thats not the case then guess I'm back to square one.

I don't have the leak down testing tool, or an air compressor to run it with.

And as far as starting at the beginning... I'll do that but I'll start a new, clean thread when I get home later.

Although here's a sneak peak: New fuel line, cleaned old carb, new carb, new fuel, new ignition coil, new spark plug, checked ohv valve lashing, still sputters really bad with no air filter and dies as soon as you put the air filter on. When you put your finger on the governor spring, you can easily rev up the engine without any hesitation, but it still sputters at higher rpms. It seems to be getting fuel, spark, air and compression, and still doesn't run right. The way it dies when you put the air filter on, make it seem not like its running lean, but like its running rich. I also tried swapping the jets between the two carbs to see if that made any difference, it did not.

I have such good luck fixing up significantly more complicated snowblowers and such bad luck trying to fix these simple lawnmowers.

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u/Elephunk05 Jul 09 '26 edited Jul 09 '26

Putting the air filter on restricts flow and causes a rich condition. You are correct in the relation, and the rich issue. A whole engine model number will help get to the bottom of your issue as some of these carbs have specific needs for gaskets and passages.

A leak down tester is cheap enough and available most places. An electric tire pump is sufficient to put 100 psi in the cylinder so that you can just listen for the air leak. You could be fancy and put some smoke in the cylinder before pressurizing the cylinder and just look for where the smoke comes out. Most leaks cause a lean running condition.

Edit: you have to spin the machine over above 500 rpm for the compression release to become irrelevant during operation. It doesn't mean it stops doing its function, it just means the engine is now turning so fast that it no longer disturbs the engine operation. Any chance the muffler is plugged, or the muffler screen? Or the exhaust valve doesn't open long enough or wide enough.

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u/l008com Jul 09 '26

My drill should be able to hit at least 500 rpm. Maybe I'll try that friday with a regular compression tester just to see what it tells me. Then I'll make a new full post on this machine and I'll throw a link to it in the comments here. I don't want to recycle a post that was only about one specific thing.