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  • Compression

    Can anyone tell me what the compression values should be on a 1972 Johnson 65 hp

  • #2
    A good one would read 140+ on my gauge.

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    • #3
      Thanks racerone. The motor hasn't been cranked in about 4 years. Just thought I would tinker with it a little to see if it would crank. I checked the compression and it read 70- 80 psi on all cylinders. Is this bad low and what would this mean by having low compression?

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      • #4
        There's not much complicated about the compression stroke of a two stroke motor. There's no valves involved, so that eliminates that being a factor.

        Piston rings and cylinder diameter/condition are what matter the most involving compression stroke.

        The other and less direct/obvious side effect of low compression due to cylinder wear is piston ring blow-by which affects how well the engine pulls the mixed gas in and pushes the exhaust fumes out.

        It's probably safe to say about 90% or more of the situations with low even compression across all cylinders are the result of cylinder walls out of tolerance, which means the engine is going to run like crap if it runs at all.

        Did you ever do a wet compression test with fogging oil and let it sit before cranking it for the first time?

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        • #5
          Originally posted by oneluckydog View Post
          Thanks racerone. The motor hasn't been cranked in about 4 years. Just thought I would tinker with it a little to see if it would crank. I checked the compression and it read 70- 80 psi on all cylinders. Is this bad low and what would this mean by having low compression?
          It might be just dry rings, depends on how it was prepared for storage. Try another compression gauge as the one you used might not be accurate. Did you do the test with all the plugs out and cranked the engine until the gauge stopped climbing?

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          • #6
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            That's some useful info thank you very much. No I didn't do a test with fogging oil. I didn't even try to crank it before I did the test. I did the test with a new tester that I just bought and did it with plugs in except for the one I was testing. I did crank it until the gauge stopped climbing

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