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  1. #1

    Default Correct procedure for compression test

    I want to do a compression test on a 1981 Johnson 70 hp, (never done a compression test on anything) The Clymer manual I have says, do it with the engine warm, well that's the first problem, the engine won't start, so does it really make a difference whether its warm or cold? It also says disconnect the "power pack-to armature plate connector to disable the ignition system" Is this also necessary considering the motor doesn't start anyway? Thanks

  2. #2
    Join Date
    Sep 2010
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    ontario
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    12,314

    Default Re: Correct procedure for compression test

    All spark plugs out.-----Good battery and starter motor.-----Throttle does not need to be wide open as some will say !-----------Won't hurt to make sure kill switch is on or spark is disabled.--------You could leave ignition off and just use a jumper wire at the solenoid to crank it over.--Crank till gauge stops moving up Usually 2 seconds is enuff.

  3. #3

    Default Re: Correct procedure for compression test

    Quote Originally Posted by racerone View Post
    All spark plugs out.-----Good battery and starter motor.-----Throttle does not need to be wide open as some will say !-----------Won't hurt to make sure kill switch is on or spark is disabled.--------You could leave ignition off and just use a jumper wire at the solenoid to crank it over.--Crank till gauge stops moving up Usually 2 seconds is enuff.
    So if I leave the key turned off, is that good enough to avoid maybe doing damage to the electrical, and with key off will jumping the solenoid still turn engine over to check compression?

  4. #4
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    ontario
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    Default Re: Correct procedure for compression test

    With the key turned off you can safely use a jumper wire at the solenoid to crank it over.

  5. #5
    Join Date
    Apr 2010
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    2,775

    Default Re: Correct procedure for compression test

    When you use the jumper wire on the solenoid, make sure you put it on the small terminal marked "S' on the solenoid and the battery terminal.

  6. #6

    Default Re: Correct procedure for compression test

    Have you done any trouble shooting at all to figure out why it won't run? (Check for spark, fuel system woes). I'm going to advise you NOT start jumping things at the solonoid to prevent further heartache, because for one, one of the small wires at the solonoid is a ground wire. You don't want to make the mistake or accident of jumping or bumping 12 volts into that wire or you might end up with further electrical heartache that you weren't planning on. I've had to repair a couple that the customer did just that and ended up loosing a hundred dollar pack in the process.

  7. #7

    Default Re: Correct procedure for compression test

    Bought a tester and going to do the test today, got the motor running, turns out, if you kink the fuel line by pushing the battery up against it, the motor won't run, who'd have thought? But still worried about doing electrical damage,, so rather not try jumping solenoid, and no kill switch, and trying to keep 3 plugs grounded on a shaking motor while 8ft away turning the key doesn't sound do-able, so which is the easiest wire to unhook, read somewhere to just un-hook the coil, would this do the trick? if not, which wire, in layman terms.? Thanks

  8. #8

    Default Re: Correct procedure for compression test

    Quote Originally Posted by ali'i View Post
    Bought a tester and going to do the test today, got the motor running, turns out, if you kink the fuel line by pushing the battery up against it, the motor won't run, who'd have thought? But still worried about doing electrical damage,, so rather not try jumping solenoid, and no kill switch, and trying to keep 3 plugs grounded on a shaking motor while 8ft away turning the key doesn't sound do-able, so which is the easiest wire to unhook, read somewhere to just un-hook the coil, would this do the trick? if not, which wire, in layman terms.? Thanks
    Had an idea, rather than un-hooking any wires, what about a piece of wire like maybe a coat hanger, bending it around all 3 plugs, then just ground the end of the wire? If this will do trick, how far apart should I space the plugs? Thanks

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