Compression Tester

I've been told you can only use certain kinds of compreson testors for small engins. Is this true or have I been mislead.
Thank You!

There are older cheap ones that have a rubber tip, and you would hold it by hand in the spark plug hole while cranking the engine. They would do in a pinch but were not very accurate.

Another type has the thread in adapters and a rubber flex hose; they work pretty well. They are also the most common.

A third type has thread-in pipe adapters and are very accurate, but not always easy to use because the spark plug may be in a difficult area to access.

Compression testers basically have the same job and that is to obtain the cranking pressure readings inside of the engine cylinder.

Some people have a variety and usually one that they prefer over the others.

Here is one example of a compression test kit:


There are more expensive kits, but they all serve the same purpose.

Many of the auto parts stores provide tools at no cost. Might want to check stores in your area. Cheaper than buying one. I own and recommend the second one Doc told you about. I always do the test three or four times to verify my findings.
Just reread the OP’s post and I should add that I recommend looking into using a leakdown tester.

You guys are forgetting one important thing. That is schrader valve. Automotive types are not designed for small volume compression tests. Most schrader are too strong and second is the placement of this valve. It must be at the end closest to cylinder as if hose is used with valve at the gauge you first must fill the hose before the valve will even open thus giving false low pressure readings. The wrong schrader valve make this even worst. It becomes very apparent when you have a 50 cc or less engine.

Star is correct. Mine doesn’t have a scharder valve, and I forget things like that. Second reason why I use a leakdown tester more than a compression tester.

Thanks! so much for the info. My first go to to fix a non start-up trimmer was an aftermarket carberator. I have about 80% success with that. I figured it's time to check compession on the other 20%.
What about the carberator plug along with new gaskets. Is that worth the time to remove and clean out instead of carberator replacement.
Thanks Again!

I always try to repair before I replace, but I’ve got parts on hand. Grew up never throwing good parts away. My age is showing.

Mr Twister said:

Thanks! so much for the info. My first go to to fix a non start-up trimmer was an aftermarket carberator. I have about 80% success with that. I figured it's time to check compession on the other 20%.
What about the carberator plug along with new gaskets. Is that worth the time to remove and clean out instead of carberator replacement.
Thanks Again!

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Hello M.,

The tricky thing to always watch with 2-stroke engines and a compression test is the muffler "coking" up with unburnt fuel/oil. I have revived quite a few weed eaters that other shops condemned by removing the muffler and heating it up red hot to burn out the carbon waste.

As a basic rule: If exhaust cannot escape, fresh air and fuel mixture cannot be drawn in.

I have one similar to that and a leak down tester as well.

Here is some info about small engine compression testers and some more info at the arborist site with a search. You have to be careful when compression testing small cc engines and make sure your tester is truthful. I've had guys bring me chainsaws and other small cc engines that they had overhauled due to their comp tester indicating low compression and the compression on their tester was less after they done the re-build. Low compression was not the issue.

Link to a video about small engine compression testing 6/23


Here is a link about modding Schrader valves and homemade testers, actron tester, etc

Compression tester issues:


I got 3 of the Napa 700-9556 compression gauge Schrader valves today after the local NAPA ordered from their warehouse. The come 5 in a box and you can buy them each @ approx. $2.20 each. They worked A-ok. One was defective out of it's sealed package, was not spring loaded on back check due to a broken spring, noticed immediately before attempting install for test..

3 /2018
Napa #700 9556 is obsolete.

Here is a special low pressure schrader valve for compression gauges Milton and Lisle part number's and I seen them on flea bay as still available.

Milton 1251-2 @ about $5 each

Lisle 20100 @ about $7

Update info 12/20

Possible good compression gauge for chainsaws at $40, STANDIX 1023

Mentioned at this link 12/20

https://www.arboristsite.com/commun...mpression-gauge-and-tested-a-few-saws.348043/