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Does anyone know what the compression psi should be for a 1978 Johnson 25 hp?  Ive been having some issues and did a compression test over the weekend.  It read 90psi in both cylinders.  Is that good/bad?  I tried to search around for some info about this from johnson and have also searched other forums but have found nothing solid.  Thanks in advance!

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If both cylinders are within 5%, there's probably nothing wrong with the motor.  There are a couple of possible reasons for the 90psi, one is the gauge.  I've seen more than compression gauge have the wrong or bad Schrader valve cause low readings.   Also, not sure when OMC started dropping the compression to compensate for the removal of lead as an octane booster and the junky gas.  I know in the early 70's 130psi was normal, and the mid to late 80's 100 psi was not uncommon in most motors but I've actually never dealt with a late 70's.

Just in case there are those that try, DO NOT use anything other than the proper Schrader valve in a compression gauge.  They use a one ounce max Schrader valve, which normally the ones I've seen have a white band.  The typical tire Schrader valve is around 30 psi and have blue or black bands and if you try to use the wrong one, it WILL make the compression gauge read low, sometimes very low.  This was one of life's many lessons I learned the hard way many years ago.

The tool trucks usely have the valves for a compression gauge


I should have mentioned also, if you do a leak down test, that removes the possible errors a compression gauge can have and is a lot more accurate way to analyze possible compression problems.





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