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How to - outboard compression check

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  • How to - outboard compression check

    Can someone explain how to perform an outboard compression check? Also, does the outboard need to be in the water during this check?

    I'm not sure, but are you suppose to remove all the spark plugs before the compression test or one from each cylinder at a time? I recalled something bad if you remove all the spark plug wires at the same time...

    Also, if the outboard is a 60hp, am I suppose to see a compression close to 60? I understand that it is the difference in compression cylinders that matters.

    What about when i put the spark plugs back, do you apply the same method as in care engine. Hand tighten then 1 full turn with the tool?


  • #2
    Re: How to - outboard compression check

    Originally posted by salmonee View Post
    Can someone explain how to perform an outboard compression check? Also, does the outboard need to be in the water during this check?

    I'm not sure, but are you suppose to remove all the spark plugs before the compression test or one from each cylinder at a time? I recalled something bad if you remove all the spark plug wires at the same time...

    Also, if the outboard is a 60hp, am I suppose to see a compression close to 60? I understand that it is the difference in compression cylinders that matters.

    What about when i put the spark plugs back, do you apply the same method as in care engine. Hand tighten then 1 full turn with the tool?
    Yikes! you have some reading to do...... but I'll get you started.

    1st, anytime the motor's turning, the impeller needs water. This can be done w/ "muffs" or stick it in a bucket of water. This assumes your engine's water cooled (vs. air cooled).

    Next, compression does not correspond to HP rating of the motor. 2 strokes should give you >100psi each cyl, preferably more, and should all read w/in 10-15psi of each other.

    You will want to remove all spark plugs, but ground the leads to the engine block. I use a wire shoved into the boot. Read compression with the intake at full throttle.

    Hand tighten the plugs, then tighten using a torque wrench to specs. If you were to tighten 1 full turn over "hand tight", that's WAY too tight and you'd be stripping threads out. Use a torque wrench until you have the feel for where they need to be.


    An alternative is have a mechanic do the comp. test and you watch him. Then you'll know for next time.
    The fleet:
    -1979 18'6" Starcraft Super Sport, 1985 Mercury 60hp http://forums.iboats.com/showthread.php?t=260400
    -1994 Kawasaki Super Sport X1 (750cc)
    -1998 Arctic Cat Tigershark 640L
    -Sevylor Voyager 9'2", Tanaka 1.75hp

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    • #3
      Re: How to - outboard compression check

      Probably best to post this in the outboard repair forum for your engine make.

      - Get a engine compression tester
      - Test can be performed out of the water on the hose
      - Warm up the engine
      - remove the spark plugs
      - screw compression tester into one of the cylinders
      - crank the engine a few cranks
      - write down the number on the gauge
      - move the tester to the next cylinder and repeat

      The numbers for all cylinders should be about the same or within about 10% of each other.

      The typical number for each make of engine will vary. For example, my mercury outboards have typically had results of about 118 psi in each cylinder. This was last done for a 1982 150 V6.

      The test itself takes about 10 minutes.

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      • #4
        Re: How to - outboard compression check

        Good point about grounding the spark. I forgot to include that.

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        • #5
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          Re: How to - outboard compression check

          When testing motors in the shop here, I usually bypass the controller and harness by hooking directly to the starter, that way I eliminate any chance of the ignition being 'live'. I normally just hook a jumper switch right to the starter terminal from a battery, that way even a magneto motor won't make spark and it eliminates worrying about stray sparks or possible damage to any electronics.

          I either run the motor, of if it's been sitting a long time, I'll pull the plugs, give it a shot of light oil to make sure everything is well lubed. If the motor can't be started, I'll then do the compression check cold but only once I know all the cylinders have good lubrication. Don't over oil the cylinders, it will give a false compression reading and I try to make sure they are all lubed about the same amount. I'll crank the motor over several times to purge any extra oil as well before installing the gauge.
          It's common for a motor that's been sitting a long time to read a bit low but what you are looking for is a consistent reading. You want to see all the readings within 10% of each other. For example, a 4 cylinder with compression of 120, 121, 123, 90, would indicate a problem being that the last cylinder is way lower than the others, but a reading of say 119, 123, 125, 120 would be well within range.

          Keep in mind that compression gauge accuracy often varies, so stick to one gauge and look for consistent readings, not just pressure. I have 4 or 5 different gauges, I find that the cheaper ones tend to read lower for some reason. I like the Snap On set the best but it's a bit pricey for the do it yourself mechanic. One from Sears or similar will work fine for most outboards.

          When cranking the engine to get a reading, crank the engine till the gauge stops advancing, and crank each cylinder the same amount of time.
          The faster the motor is cranking, the faster the reading will climb, a motor that won't crank properly will often give a lower reading. This is one of the reasons I go right to the starter when checking compression, it eliminates all else and gives me an honest reading.

          If you don't have a torque wrench, buy one. In general, spark plugs in an aluminum head are only tightened a fraction beyond hand tight to make is simple. I lube the threads with Never Seize and snug them up by feel. Most engine makers say to torque spark plugs in any aluminum head to 7 to 15 ftlbs with 15 ft lbs or about 180 in lbs being the absolute max most aluminum heads will take. This equates to about 1/16th of a turn beyond hand tight.

          The majority of all outboards will give readings in the 115 to 150 psi range, any less and either the motor is well worn or the compression gauge may be suspect. A lot has to do with knowing your compression gauge and knowing how it reads as well.

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