Small Engine Compression Checker

whizkidkyus · Mar 31, 2014

Hi ,

Can an automotive type compression checker be used on small four or two cycle engines as long as the spark plug fitting will fit the spark plug hole ? Or do they require a different tester ? Thanks for any and all help .

Whizkid

t1 · Mar 31, 2014

yes they will work, BUT most small engines have a compression release built into them to make them easier to start. Even electric start engines have it. This makes a compression tester of little use on these engines. The release is built into the exhaust valve lobe of the cam on most. A few Briggs and Stratton engines had it on the intake lobe. I"ve found with most, that no matter how good the engine, you won"t get much better than 60 to 70 pounds of compression because of the release mechanism.

turning the engine backwards will de-activate the release mechanism, but its usually difficult on most engines to turn them backwards fast enough to get a good compression reading.

A FAR more useful tool is a Cylinder Leakdown tester. you"ll need an air compressor to use with it. But, it"ll tell you if you have a good tight engine, and if not, you can tell where the problem is by listening and feeling for where the air is leaking out. One of these tools can be used on automotive and larger engines as well.

504 · Mar 31, 2014

Stick your finger over the spark plug hole, if you can hold your finger over the hole easily you have low compression. The old single cyl B/S check was to spin the flywheel backwards, it should bounce hard forward if it is alright.(keep you fingers out of the fins)

DelMN · Apr 1, 2014

You can use an automotive comp tester on a small engine. Spin the engine over by using a drill on the flywheel nut and spin backwards this will deactivate the compression release. The flywheel nut should not come off when doing this , if it does, just put it back on.

T.R.K. · Apr 2, 2014

I have used an automotive compression gauge for years. Like some of the others said, you have to work within the limitations of the tool.

Most of the late model 4 cycle engines do have a compression release so the readings are fairly low. A good engine will sometimes be 60 psi or less. In some cases a high reading actually indicates valves out of adjustment.

For some reason, it seems like when there is a carburetor/fuel issue with a 4 cycle twin cylinder, the failing cylinder will almost always read a bit higher.

valve problems will usually cause a lower reading. That is when you switch to the leak down tester.

For 2 stroke, I usually reach for the compression gauge first, especially if it is a cheaper unit from one of the big box stores. Keep pulling the recoil starter until the gauge won't go up any higher.
If the engine won't do between 125 and 150 psi compression it is usually not practical to do any further repair to it.

Small Engine Compression Checker

whizkidkyus

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t1

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504

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DelMN

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T.R.K.

Well-known Member

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