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70hp johnson compression

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  • 70hp johnson compression

    I have just bought a 1978 70hp. Johnson Outboard and was trying to get info. on the compression.. What is the specifications for a compression test.. I check all three cylinders and they were from 90 to 120lbs.. Is this good compression for this motor?The motor seems to run fine, but hasn't got alot of throttle.. After changeing the plugs I noticed that the top two are firing pretty good, they look like there fine.. The bottom one is clean as new.. I checked the firing and It's firing fine..I guess the carb maybe the problem.. What do you think?? I appreciate any help.. *** bless... DOC30


  • #2
    Re: 70hp johnson compression

    Howdy, Doc. Your compression is NOT-OK. Not only is 100# the minimum acceptable, 15# difference between high and low is minimum acceptable difference.Until that is fixed, other issues are academic.Good luck.Red sky at night. . . .JB
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    • #3
      Re: 70hp johnson compression

      Hey, glad to get a reply... Thanks J.B.//I decided to check the compression again after your reply.. I thought there for a minute I was going to try my first rebuild on a boat motor.. I wasn't real thorough with the first test I did.. On the second time around I had 120 lbs. on all three cylinders..There was no varance at all.. That was a big releive.. I decided to go ahead and check the carbs., and found no gas in that bottom carb. bowl.. I went ahead and took off the carb. and cleaned it out good, it was alittle gummy in the bottom..The needle and seat didn't look to bad but I decided to call a marina thats down the road and they had needle and seats sets.. I went ahead and replace all three carb. needle and seats. plus a good cleaning..Went ahead and replaced all the gas lines including the bulb. Put it all back together and went to try it out this afternoon.... She ran like a champ.. The only question I have left is my prop has some bad places in it, and I was wondering what do you recommend? Should I go back with the original or try something else..I really do appreciate your help... Thanks And *** Bless....DOC30

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      • #4
        Re: 70hp johnson compression

        Get your prop rehabbed at a local shop. Buy a cheap spare prop to keep on the boat. Congratulations on getting her going. That OMC triple has been one of my favorites since they were introduced.Red Sky at night. . . . . JB
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