Home Forum Ask A Member Low Compression

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  • #290738

    Would anyone care to offer some thoughts on a couple of  (low Compression) motors i have please ?

    1955 Evinrude 7.5 Fleetwin Whisper Power        both cyls.  @ 50 / 55

    1956 Johnson 10 hp. QD-17    both cyls. @ 50 / 55

    Using a cheap Amazon compression tester.

    They both seem to run fine but………..?              Any thoughts ,  Thank you !

     

    #290739
    The Boat House
    Participant

      If they work don’t fix them.
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      #290740

      OK , TBH   Just wondering how low is low.   They seem to work fine.  Thanks

      #290741
      joecb
      Participant

        US Member

        If they start easily, you are golden! Hard starting and poor idle can be a sigh of troublesome low compression.

        Joe

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        #290742
        The Boat House
        Participant

           

          I don’t think you can get a motor to start if the compression is below 60.lbs. I’m positive you cant at 50 lbs. I would suspect your gauge is off. There are plenty of motors out there that don’t start and don’t run well.  Get one of them to fix and  just run the others. Tubs
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          #290750
          reivertom
          Participant

            US Member

            Cheap testers have made many people unhappy for no reason. If they run OK and idle, quit worrying. If they haven’t been run in a long time, there’s a chance they will get better compression after being run and getting up to temperature a few times.

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            #290753
            necks
            Participant

              US Member

              Rkymtn, Were these motors sitting for a long period of time?. If so, that will make for low compression. At least the compression is close on each cylinder, that’s a good sign. If you can start that engine, let it run for awhile, so the rings reset themselves, just make sure the engine doesn’t overheat, or take out for a boat ride. When done testing motor, check compression, should be better than 50psi. Let it sit for awhile try starting when motor  cools off. Compression should be as high as it’s going to be. If it stays at 50 or below, then it’s time to check rings or overhaul. I agree with Tubs, compression should be at least 60psi. If you are going to buy some gauges, or tools, pay the extra dollars and buy name brand stuff, you only buy them once, just my 2 cents worth. Gene.

              #290757

              Thank you all very much.   They both seem to start easly and idle fine.

              Im not going to worry about them right now.  Ive just been barrel testing so far but will put them on the water and get some more time on them asap.

              I appreciate the info.  These are the first two i have rebuilt and have learned alot with your help.   Thanks D

              #290761
              seakaye12
              Participant

                US Member

                Suggest at your next meet that your Chapter have a “compression gauge comparison”.  All you need is one motor for consistency….along with a prescribed number of pulls.  Everyone hook up your testers and see how they compare.

                Wile “poundage” is certainly an important figure….consistency between cylinders has greater significance.

                #290773
                jeff-register
                Participant

                  US Member - 2 Years

                  I had a 3.5hp Scott that had no compression. Took it apart & the rings were glueed to the pistons from bad oil. Soaked them c;ran, reassembled & it ran fine.

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