# Two Stroke Compression



## RKDOC (Jun 20, 2006)

I have always used 90psi as a base line for weather a two stroke engine is good or not. Today I tester a Craftsman string trimmer (poulan made) the trimmer has less than an hour on it. It barely made 90psi on the compression gauge. Do I need to change my compression base line to a lower number? Are the new engines not as high of compression? The trimmer runs great. (customer couldn't figure out the trimmer head) Thanks for your insight.


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## geogrubb (Jul 28, 2006)

RKDOC said:


> I have always used 90psi as a base line for weather a two stroke engine is good or not. Today I tester a Craftsman string trimmer (poulan made) the trimmer has less than an hour on it. It barely made 90psi on the compression gauge. Do I need to change my compression base line to a lower number? Are the new engines not as high of compression? The trimmer runs great. (customer couldn't figure out the trimmer head) Thanks for your insight.


IMHO you need a new gauge. Have a good one. Geo


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## usmcgrunt (Sep 16, 2007)

I agree with Geo - BUT - I purchased a remanufactured boxed Poulan BVM200 a couple years ago and couldn't get it to start.Checked compression and found 60psi.Guess the rebuilding tech was having a bad day.Since the unit had a factory warranty,took it back and got a replacement free which had 120psi.Make sure the owner is using correctly mixed two cycle gas\oil.It wouldn't take long to score a cylinder on straight gas.


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## paulr44 (Oct 14, 2008)

I use 120 as baseline for smaller engines, such as 25cc. The larger, the higher compression, with 70cc being about 170 lbs. with a 150 min. reading.

If the guage you're using doesn't have a schrader valve in the fitting that screws into the plug boss (such as is with some extensions), the hose effectively becomes part of the combustion chamber volume which will give you a false (lower) reading.


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