Compression Testing

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    bill-m

    US Member - 1 Year
    Replies: 41
    Topics: 23
    #194824

    Can someone please tell me when testing compression, how many pulls of the rope would be needed to get a proper reading? Thanks.

    lindy46
    lindy46


    Replies: 312
    Topics: 23
    #194825

    6 or 7 pulls should do it or until reading doesn’t go any higher.

    • This reply was modified 5 days, 3 hours ago by lindy46lindy46.
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    fleetwin

    US Member - 2 Years
    Replies: 2868
    Topics: 37
    #194862

    What engine are you trying to check compression on?

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    gbm

    US Member - 2 Years
    Replies: 79
    Topics: 10
    #194865

    A far better indication of effective compression can be observed with a leak-down test. You can make (or buy) a gauged fixture that will let you fill the cylinder with compressed air so you can see if any leaves. It won’t take much pressure to make the gauge drop.

    Sufficiently advanced technology is indistinguishable from magic

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    billw

    US Member - 2 Years
    Replies: 1106
    Topics: 38
    #194906

    6 or 7 pulls should do it or until reading doesn’t go any higher.

    • This reply was modified 5 days, 3 hours ago by lindy46lindy46.

    Ditto from me. The key is at what pressure it stop climbing, and whether you have eaten your Wheaties. If your compression gauge does not hold a pressure indefinitely after you stop pulling, then you would need to clean the little Schreader valves. If it falls, back, you will not be able to get an accurate reading.

    Long live American manufacturing!

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    bill-m

    US Member - 1 Year
    Replies: 41
    Topics: 23
    #194926

    At present time I will be testing a 1956 Johnson 5 horse motor. What would be a good reading for this motor? I also just bought a 1960’ish Sears 3.5 horse motor that will not be delivered until May. My friend in California said the rope pull needs to be fixed but when turning the prop it seems to have good compression.

    lindy46
    lindy46


    Replies: 312
    Topics: 23
    #194927

    Minimum 75-80psi and even across both cylinders would be an acceptable starting point. Reading may go up after it is run for awhile. Oh, a 56 Johnson would be 5.5hp.

    • This reply was modified 4 days, 4 hours ago by lindy46lindy46.
    outboardnut
    outboardnut

    US Member - 2 Years
    Replies: 587
    Topics: 414
    #194934

    I always ground the plugs when doing a compression test. Thats correct? Right?

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    kirkp

    US Member - 1 Year
    Replies: 83
    Topics: 5
    #194939

    I seem to remember reading that if you don’t ground the plugs you can trash the coils???
    Kirk

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    Tinman

    US Member - 1 Year
    Replies: 233
    Topics: 41
    #194975

    I have never grounded the plugs on my omc motors when checking compression and I’ve never had a bad coil. But if it’s a possibility and I’m doing it wrong I need to start grounding them. Curious to know the answer.

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    dave-bernard

    US Member - 2 Years
    Replies: 984
    Topics: 12
    #194987

    Same here.

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    jeff-register

    US Member - 1 Year
    Replies: 1154
    Topics: 48
    #194988

    The magneto creates energy & does not go away. Needs to be discharged before it finds the easiest path thru ground….thru the windings & mag plate. It WILL blow tiny holes thru the coil to ground opening secondary windings causing an open or carbon connection.
    Also when checking compression I would say around 20% difference between cylinders is out of spec & needs attention.

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    Samuel Phelps

    US Member - 1 Year
    Replies: 129
    Topics: 22
    #194991

    I heard about grounding the plugs and I started doing that . What is the chance of fuel fumes coming out of the open cylinders causing a boom ?
    That’s always sort of freaks me out .. wrapping a wire around the plug bases then grounding the wire is how I do it . I
    Am I doing it correctly ?

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    JOHN HOLBIK

    Canada Member - 2 Years
    Replies: 21
    Topics: 2
    #195000

    I personally dont totaly agree that a magneto ignition must have the wires grounded I know electricity will take the path of least resistance but when the gap to ground is beyond the capacity of the coil the electricity is contained in the mag as the potential to ground is beyond the coils capacity. I have tested motors for 50 years pulling off spark plug wires etc.We ran a fishing resort for 49 years and probably hundreds of motors came home on one cylinder because the other was fouled. I never ever saw any electrical damage as a result of this.
    With electronic ignitions there is a greater chance for grounding issues as the voltage is much higher and the sensitivity of electronic components

    frankr
    frankr

    US Member - 1 Year
    Replies: 4715
    Topics: 47
    #195001

    My opinion on this:

    Not grounding the wires CAN bust through the insulation if the available spark is strong enough, AND the insulation is weak or compromised. But the chances of this happening are sort of iffy and the coils probably are crap anyhow.

    Chances of a fire: YES, YES, YES. The only reason I’m sitting here writing this is because I was standing to one side. And the Grace of God. See, I had this electric start V4 motor that wouldn’t start, mounted on the boat, and was about to check for spark by cranking it using a jumper switch to run the starter. Plugs were out. Open spark checker was on plug wires. What I didn’t know was that the motor was badly flooded. As soon as I hit that start jumper switch and the motor started to spin over, that flooding gas came blasting out of the spark plug holes an the spark checker ignited it. A column of fire came shooting out about 10 feet. If I had been standing one foot closer in line, that burning gas would have hit me and I would have been on fire. Another stroke of luck was that I didn’t set the shop on fire.

    Be safe, guys. And think.

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