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How to test compression?

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  • How to test compression?

    I have a 98 Evinrude 250 (carb) and would like to test the compression on it. I bought a kit, that has the gauge, a rubber bung, and two sizes of screw-in adapters that would go where the sparkplugs were. Should I run the engine before testing? I've also heard about spraying some WD-40 in the cylinders.. And where would I put the spark plug wires to avoid any personal injury/damage to the motor? Do I turn it over WITH the fuel lines connected or not (i'm assuming leaving the fuel line connected may start it up, or remove all the plug wires?) Are all the plugs removed at once, or one at a time for testing?Any step by step guide will be of great help.I'm also investigating why the output of this 250 is rated (printed on label) at 167KW~223 HP , So if anyone who knows about these Evinrude/Johnson engines can shine some light.. I've called Evinrude and the rep didnt have the slightest idea what i was talking about... so I'll call again and hopefully get one who knows his stuff.


  • #2
    Re: How to test compression?

    Oh yeah, here are 2 pics of the rig taken last weekend. (copy and paste into a new browser, Angelfire sux) http://www.angelfire.com/ny/somie/PC090024.jpg http://www.angelfire.com/ny/somie/PC090009.jpg

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    • #3
      Re: How to test compression?

      Start and warm the engine up to operating temperature. Shut down and remove all the plugs. I've heard about reconnecting the plugs to the wires to avoid damage to the coils. I'm not sure about this but it can't hurt. Put the gauge with correct threaded adapter to fit your spark plug hole into the first cylinder you're going to check. I never heard about disconnecting the fuel lines and since all the plugs are out you don't have to worry about the engine starting. Turn the key and crank the engine over several times. Record the pressure and go on to the next cylinder. All cylinders should be within 15 lbs of each other. Good luck. And let us know what the results are. Dave

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      • #4
        Re: How to test compression?

        Somie, I believe that you must have the leads all connected and the spark plugs earthed whilst doing the compression check otherwise the power pack/s may fail. I guess disconnecting the power pack whilst spinning the motor over with the starter would overcome this problem. If the motor is warm you won't need to oil the bore as the fuel or oil would have done this for you. I would leave the fuel lines in situ when testing compression. Hope this helps someRoss

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        • #5
          Re: How to test compression?

          Somie, got another couple of suggestions, if you have time to do it.Just for kicks, check the compression with the throttle at idle, and also at wide open throttle. I've noticed in the past it is higher at WOT (wide open throttle), and have never heard an answer from a trained mechanic which value is the correct one to use. On a boat, most have mechanical timing advance, so that will probably affect the compression too. The timing will definately be different at WOT than at idle.You will have to put the boat in gear to go to WOT, so be careful of the prop turning while you hit the starter.If you know how to use a remote starter switch (they are $5.00 or so at parts places), it makes it easier, because you can connect it across your starter solenoid right at the engine and see you compression guage easier.MAKE SURE IT WILL NOT START, as it won't if there are no plugs in the engine. And again, be careful of moving parts at that end.Hope this helps........ And please post your results if you decide to try this.

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          • #6
            Re: How to test compression?

            Thanks for the tips! Right now I'm relocating the fuel/water separator to the stern and replacing the inline fuel filter, so when that's completed, the fun will begin. Will post results.

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            • #7
              Re: How to test compression?

              HIYou guys are all right!>crack the plugs and re-snug(prevent thread damage)>warm up engine>remove all plugs>ground plug leads to block>crank engine over with wide open throttle>crank equal number of hits per cylinder(5-7 hits minimum)>don't use WD-40, as it will increase your readings and give you a false sense of security>generic spec = 10% variation from highest to lowest cylinderjust my .02 cents worth..good luck!
              Alias "schematic"

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              • #8
                Re: How to test compression?

                Unfortunately, I was unable to perform the test today, One thing i want to completely understand though, is to make sure the spark plugs are earthed? That means running a wire fron the leads to the block of the motor..?I'll also look into the manual to see about disconnecting the powerpack. I don't think the starter will engage if in gear, so that'll mean no WOT test (for now at least)

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                • #9
                  Re: How to test compression?

                  somie, I believe that you have an ignition safety switch ,just pull the lanyard and the ign will not spark, hence no damage to ign components

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                  • #10
                    Re: How to test compression?

                    Somie, Johnkc makes a lot of sence. it was my advise that the power pack be disconnected so if the kill switch would prevent a spark then it should be safe to do compression test that way. Obviously no good for testing for a spark however.Ross

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                    • #11
                      Re: How to test compression?

                      Somie, you should still be able to use the ignition starter switch at your motor, even with the boat in gear. You just need to connect it so that you get 12 volts to the + side of the starter.Just make sure you DON'T connect the starter switch from the positive 12 volts to the negative (ground) side of the solenoid, or you will create a direct short and burn up the test switch. And probably make a big arc or two.You should be able to look at the wires and see which one is the "hot" wire there.If you are uncomfortable using the starter switch, get any of your electrician friends to help you, if you show them this e-mail, thay should be able to help you, even if they know little about boat motors.Just 2 more cents worth!!!

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                      • #12
                        Re: How to test compression?

                        Season's greetings to all. Well due to my ISp screwup and going to orland for a few days, I finally have the results to post. Here is a diagram of the cylinders (looking at engine from back) and their average PSI after 3 cranks each (at idle). Oh yeah, I disconnected the power pack.100 | 10099 | 10395 | 95

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                        • #13
                          Re: How to test compression?

                          Normally the shifter has a "button" to permit accelerating throttle without engaging into forward or reverse gear. You would use this to "juice" it when starting so you can rev the engine without it in gear. Use this to go to WOT without prop spinning. Engine won't crank with shifter in gear. You must check compression with WOT. Using oil in cylinder is part of troubleshooting AFTER you determine that a cylinder has low compression. You spray a couple squirts of oil in and recheck compression. If compression is higher with oil, low compression problem is caused by bad rings.

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