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Compression test results on my 1994 7.4L in 2855-gctid403618

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    Compression test results on my 1994 7.4L in 2855-gctid403618

    Hi all,

    Took the boat out last week, seemed low on power.. had to get it right up to 5000rpm just to get it on plane.

    Boat would not idle after..

    Here are the compression results: Note cylinder 5 is down, is this a big deal?

    cyl 1 131 psi

    cyl 2 134 psi

    cyl 3 131 psi

    cyl 4 132 psi

    cyl 5 100 psi

    cyl 6 131 psi

    cyl 7 131 psi

    cyl 8 134 psi

    Doing wet test on cylinder 5 shortly.

    Please advise what the problem could be..

    Many thanks,

    Jon

    #2
    How old is your gas? Filter clean, all the plugs wires ok, pulled dist cap to see if it has a forrest of penicillin growing on the contacts in there? Seems to me that that one slightly low value wouldn't matter that much to me. I had similar issues 5 years back and had to open up the carb top and clean out the flora and fauna that was growing around the jets.

    Comment


      #3
      driz wrote:
      How old is your gas? Filter clean, all the plugs wires ok, pulled dist cap to see if it has a forrest of penicillin growing on the contacts in there? Seems to me that that one slightly low value wouldn't matter that much to me. I had similar issues 5 years back and had to open up the carb top and clean out the flora and fauna that was growing around the jets.
      Hi thanks for the info,

      How did you clean the carb?

      Jon

      Comment


        #4
        me&myboat wrote:
        Hi all,

        Took the boat out last week, seemed low on power.. had to get it right up to 5000rpm just to get it on plane.

        Boat would not idle after..

        Here are the compression results: Note cylinder 5 is down, is this a big deal?

        cyl 1 131 psi

        cyl 2 134 psi

        cyl 3 131 psi

        cyl 4 132 psi

        cyl 5 100 psi

        cyl 6 131 psi

        cyl 7 131 psi

        cyl 8 134 psi

        Doing wet test on cylinder 5 shortly.

        Please advise what the problem could be..

        Many thanks,

        Jon
        You should not be able to get a big block to 5000RPM, you have prop problems for one. Engine troubles do NOT raise the RPMs required to "get the boat on plane"... props are directly connected to engines, if everything is okay with the prop/drive/coupler/etc., if it makes the RPMs required to get on plane, it should be able to get on plane. Engine troubles will make it harder to GET to the RPM required to get the boat on plane.

        Yes, it would still idle fine with 100psi on one cylinder, but it's rather amazing that the rest are all within 3 psi of each other... I'd want to know what's up with that cylinder... hopefully something on the top end. You may have some air/fuel or ignition problems causing idling problems.

        Comment


          #5
          ishiboo wrote:
          You should not be able to get a big block to 5000RPM, you have prop problems for one. Engine troubles do NOT raise the RPMs required to "get the boat on plane"... props are directly connected to engines, if everything is okay with the prop/drive/coupler/etc., if it makes the RPMs required to get on plane, it should be able to get on plane. Engine troubles will make it harder to GET to the RPM required to get the boat on plane.

          Yes, it would still idle fine with 100psi on one cylinder, but it's rather amazing that the rest are all within 3 psi of each other... I'd want to know what's up with that cylinder... hopefully something on the top end. You may have some air/fuel or ignition problems causing idling problems.
          Thanks for the comment.

          Actually, i have always had trouble getting on the plane, even since i bought the boat.. It has no trouble revving high, it will even get up to 5500 rpm.. With a light load (only 2 people) it will plane once its at 5500, then i can keep it skimming on top of the water at 4500rpm. I never realized it could be prop problem.

          So, i guess the misfire/idle issue is a completely different problem and maybe not connected to the low compression on cylinder 5.. ?

          Working on it tomorrow, so will update..

          Pls have a good listen to the engine here at youtube...... http://youtu.be/q3EM0-2tHIs

          Comment


            #6
            5500RPM's?

            I suspect your gauge is incorrect. How long have you been able to achieve and run at that RPM?
            Phil, Vicky, Ashleigh & Sydney
            1998 3055 Ciera
            (yes, a 1998)
            Previous boat: 1993 3055
            Dream boat: 70' Azimut or Astondoa 72
            Sea Doo XP
            Sea Doo GTI SE
            Life is short. Boats are cool.
            The family that plays together stays together.
            Vice Commodore: Bellevue Yacht Club

            Comment


              #7
              me&myboat wrote:
              Hi all,

              Took the boat out last week, seemed low on power.. had to get it right up to 5000rpm just to get it on plane.

              Boat would not idle after..

              Here are the compression results: Note cylinder 5 is down, is this a big deal?

              cyl 1 131 psi

              cyl 2 134 psi

              cyl 3 131 psi

              cyl 4 132 psi

              cyl 5 100 psi

              cyl 6 131 psi

              cyl 7 131 psi

              cyl 8 134 psi

              Doing wet test on cylinder 5 shortly.

              Please advise what the problem could be..

              Many thanks,

              Jon
              I'm confused! Isn't this the same boat that wouldn't even turn over last week(slow cranking)????

              Comment


                #8
                3 wide wrote:
                I'm confused! Isn't this the same boat that wouldn't even turn over last week(slow cranking)????
                No need to be confused Sir...

                Different thread, different boat!

                Comment


                  #9
                  itsabowtime2 wrote:
                  5500RPM's?

                  I suspect your gauge is incorrect. How long have you been able to achieve and run at that RPM?
                  A stock Chevy 454 will rev to 6000 no?

                  The gauge could be faulty, yes it is possible..

                  I never keep it at WOT, just for a few seconds to get me skimming the water, then to keep it planing, any lower than 4000, it sinks down.

                  Like i say, we are working on this engine today.. will update what we find,

                  Jon

                  Comment


                    #10
                    me&myboat wrote:
                    A stock Chevy 454 will rev to 6000 no?

                    The gauge could be faulty, yes it is possible..

                    I never keep it at WOT, just for a few seconds to get me skimming the water, then to keep it planing, any lower than 4000, it sinks down.

                    Like i say, we are working on this engine today.. will update what we find,

                    Jon
                    Depending upon which 7.4 you have the recommended renge is:

                    7.4 MPI 4200-4600.

                    7.4 MAG is 4600-5000.

                    In either case you're above the limit which can damage the engine.
                    Phil, Vicky, Ashleigh & Sydney
                    1998 3055 Ciera
                    (yes, a 1998)
                    Previous boat: 1993 3055
                    Dream boat: 70' Azimut or Astondoa 72
                    Sea Doo XP
                    Sea Doo GTI SE
                    Life is short. Boats are cool.
                    The family that plays together stays together.
                    Vice Commodore: Bellevue Yacht Club

                    Comment


                      #11
                      me&myboat wrote:
                      No need to be confused Sir...

                      Different thread, different boat!
                      O.K missed that, my Bad !

                      Comment


                        #12
                        3 wide wrote:
                        O.K missed that, my Bad !
                        Problem resolved..

                        The engine is now fixed, just emptied a whole can of carb cleaner into the pots while revving the engine.. It's just cleared it's throat and roared into life!! Very Very happy!!!

                        Thanks guys!

                        Jon

                        Comment


                          #13
                          me&myboat wrote:
                          Problem resolved..

                          The engine is now fixed, just emptied a whole can of carb cleaner into the pots while revving the engine.. It's just cleared it's throat and roared into life!! Very Very happy!!!

                          Thanks guys!

                          Jon
                          Great, easy fix!

                          Comment


                            #14
                            And that low cyl was probably caused by all your rings happening to be staggered at the particular time you shut it down before the test. It means nothing at all. It's amazing what a can of carb cleaner will do. BTW when my carb was full of flowers it was BEFORE they were using booze in the gas. I'm guessing you will find less of a tendency for the stuff to grow in there now days.

                            One thing you want to do sometime in the future is change out those fuel lines of yours with new lines rated for alcohol if yours aren't rated for it already. You won't see the deterioration of the inside liner and it will begin to start doing exactly what yours was doing as the tiny bits of black goo block up the jets. They tell me the goo goes right by all the filters and it's only a $20 upgrade after all. If it was mine that's what I would do as well as run a couple cans of Sea Foam through with your next couple tanks of gas to continue cleaning out the jungle.

                            Comment

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