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What should 1987 stock OMC 15A compression be..?

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  • What should 1987 stock OMC 15A compression be..?


    Looking at a stock 1987 Johnson 15A for friend and wondering if 75psi to 80psi is way too low compression..?




  • #2

    Re: What should 1987 stock OMC 15A compression be..?

    Everyone's gauges read a little different, but most of the time my fresh rebuilt engines are between 125-128 psi on my gauge.

    Jeff Scheffler

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    • #3

      Re: What should 1987 stock OMC 15A compression be..?

      Looking at a stock 1987 Johnson 15A for friend and wondering if 75psi to 80psi is way too low compression..?
      Any thing less than 100 psi is not good.

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      • #4

        Re: What should 1987 stock OMC 15A compression be..?

        Yes it is low. Squirt some outboard oil in each cylinder, pull it over to distribute oil open carb butterfly and retest compression. If it goes up a lot rings stuck or rings and cylinders worn a lot. Using same gage you will at least have relative results. If CD ignition ground plugs while testing.
        "Keep Move'n" life is catching up!
        No man's life, liberty or property are safe while the legislature is in session.

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        • #5

          Re: What should 1987 stock OMC 15A compression be..?

          Thanks guys... I'll get my own compression readings to verity my friend's compression readings. Also need to replace charge coil (open circuit = infinite resistance on ohms test and zero peak voltage reading while cranking). New coil has been ordered.



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          • #6

            Re: What should 1987 stock OMC 15A compression be..?

            Thanks again guys... I'm reporting back as I said I would do last week. With my own (well used) compression gauge, the engine came in at 124psi and 125psi. The charge coil was shorted out and replaced with a new one. Still no spark and after replacing CD powerpak with a brand new aftermarket unit it still had no spark. After a couple hours with a meter diagnosing the entire ignition system, I pulled a powerpak off my 1988 40hp engine and it fired right up! Will NEVER buy aftermarket ignition parts but it's what was handed to me. Cost me 1/2 day of head scratching, but got this Johnson 15A engine running and will be helping rig it for a 10yr old new driver to run the Top-O-Michigan race this summer.

            One dead OEM charge coil, one dead OEM PowerPak, one D.O.A. aftermarket PowerPak... One well used 40hp Powerpak and engine up and running.

            All is well.
            Last edited by 25xs; 06-05-2018, 06:13 PM.



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            • ricochet112
              ricochet112 commented
              Editing a comment
              Depends on the brand of aftermarket. The only quality aftermarket is CDI, Rapair. Any other brand I wouldn't trust.

              I will never install an OEM Mercury stator on any job I do, only CDI...

              Are you using just a VOA to test or with a DVA? Testing only Ohms is a noninclusive test.

          • #7

            Re: What should 1987 stock OMC 15A compression be..?

            CDI aint great either!! Id take a factory Merc stator any day over CDI! Ive had 3 POS CDI omc powerpacks fail in 2yrs...
            Daren

            ​DSH/750ccmh/850ccmh

            Team Darneille


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            • #8

              Re: What should 1987 stock OMC 15A compression be..?

              For what it's worth... CDI Electronics (used to be CDI/Rapair) is now the OEM for most older Mercury and OMC ignition components now. Have ordered them from OMC System Match and they come is a CDI box with OMC sticker covering the label and a CDI instruction sheet. I've always received great quality and even greater phone support from their Tech Services staff as well.

              RIchochet: It was a "Titan Marine" aftermarket box with absolutely no markings on the 2-cyl Powerpak at all... Really cheesy looking (and was D.O.A. to boot!). I replaced it with a factory 1988 Powerpak off my 1988 Johnson 40hp engine, dirt, grease and all, and the Johnson 15A runs fine now.

              Edit: Also, I do have a DVA adapter on my Fluke meter...
              Last edited by 25xs; 06-06-2018, 07:04 PM.



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