# Testing Compression on a Two-Cycle Engine



## spschulte (Oct 28, 2005)

Hi, I have a Craftsman blower model 358.794960 that starts and runs fine. Mainly out of curiosity I tried to test its compression using a threaded compression gauge. 

I believe the normal compression should be at least 90 or 100 PSI to even start the blower. However, my compression gauge is giving me a reading of 30-35 PSI. 

Does this blower (or all two cycle engines for that matter) have a compression release mechanism that is causing the low reading?

Thanks


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## billsmowers (Mar 21, 2008)

How Many Times Did You Pull The Cord? You Need To Pull Atleast 5 Times I Bought A Proper Tester For Motorbikes As This Is Better For Small Engines By The Way I Have A Mcculoch Blower Giving 130 Psi But Still Will Not Run .first Job Tomarrow Find The Air Leak Vacuum / Presure Test The Crank Case

Bill


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## justin3 (Apr 10, 2007)

You should have at least 90psi of compression for a 2-stroke to run. Those craftsman cheapo units are known for scored cylinders and stuck rings. Take the muffler off and look to see what condition the cylinder is in. If the Piston is scored you will need a whole new cylinder, piston and rings to get the job done.


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## geogrubb (Jul 28, 2006)

A gauge is only going to be a gauge, the results will depend on how small you make the combustion chamber when you screw in the gauge and whether or not there are any leaks from scoring or stuck/bad rings. If it runs, it's good. Have a good one. Geo


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## bec98x (Jan 5, 2007)

usually i look for at least 125. yes I have tools that run rough at 80 but i will never get them to run properly. also as the engine heats up it will loose compression.

here is how I test.

cold engine, throttle wide open and pull about 5-6 times. if you have a longer hose on the tester you may have to pull more times to build the volume to push the pressure up.


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## hankster (Jan 1, 1998)

For some reason some compression testers do not give proper readings when testing 2-cycle engines. I haven't investigated why this is but I've seen this a number of times. MTD and Poulan based blowers need at least 90 PSI compression to run properly. Chain saws generally need about 125 PSI.


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## Dale Burgher (Nov 23, 2009)

*polan chain saw*

The saw was working fine and then it just quit from a full open run, while sawing wood. I have good spark, gas is ok carb is ok . you can use starting fluid in carb . It will not even pop. piston does not look all that bad.
it feels like it has enough compression. I used oil in plug hole to up the compression. It did not make difference. It is like no gas mix. is getting in to top of pistion. ????. thanks Dale


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