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1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

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  • 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

    Engine starts and runs at all speeds OK (not really smooth acceleration) - would describe noise as tappet noise if the engine had valves. Engine is oil injected - may have a weak fuel pump. The noise seems to occur once per revolution assuming the idle RPM is around 900 from looking at the audio waveform because the noise happens every 65ms or so. I guess this also means that there is a problem with only one cylinder or the crankshaft? Earlier in the season, it did run on no oil for a few minutes - clear oil tubing cracked and drained the tank. Also, (not connected directly with the oil problem) the engine would stall at high RPM unless someone pumped the primer bulb. I bought a fuel pump rebuild kit and some new filters. Replacing the filter seemed to solve the problem so I haven't rebuilt the pump yet. When I took the video just before leaving the lake the primer bulb was not firming up even after a lot of pumps.

    I live about 6 hours from the boat, had to leave before I could look at the plugs or do a compression check. I'm hoping that from the description above and the youtube video someone with some experience might be able to give me an idea what is likely the problem so I might be able to order the parts that are likely the issue and have them when I get back up there.

    Thanks in advance,

    Greg




    1991 Mercury 75hp 2-stroke "knocking" or "banging" noise at idle - YouTube

  • #2
    Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

    Originally posted by propgun View Post
    Engine starts and runs at all speeds OK (not really smooth acceleration) - would describe noise as tappet noise if the engine had valves. Engine is oil injected - may have a weak fuel pump. The noise seems to occur once per revolution assuming the idle RPM is around 900 from looking at the audio waveform because the noise happens every 65ms or so. I guess this also means that there is a problem with only one cylinder or the crankshaft? Earlier in the season, it did run on no oil for a few minutes - clear oil tubing cracked and drained the tank. Also, (not connected directly with the oil problem) the engine would stall at high RPM unless someone pumped the primer bulb. I bought a fuel pump rebuild kit and some new filters. Replacing the filter seemed to solve the problem so I haven't rebuilt the pump yet. When I took the video just before leaving the lake the primer bulb was not firming up even after a lot of pumps.

    I live about 6 hours from the boat, had to leave before I could look at the plugs or do a compression check. I'm hoping that from the description above and the youtube video someone with some experience might be able to give me an idea what is likely the problem so I might be able to order the parts that are likely the issue and have them when I get back up there.

    Thanks in advance,

    Greg




    1991 Mercury 75hp 2-stroke "knocking" or "banging" noise at idle - YouTube
    just went through this, first do a compression check to know how the top end is. My knock was coming from the lower unit. I drained the lower finding metal shavings, so I know the gears or bearing in going out. My knock was very faint and across all rpms.

    Comment


    • #3
      Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

      Sounds like a rod knock.
      Ignition troubleshooting

      Comment


      • #4
        Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

        Dave and wilts,

        Thanks for the replies. I agree it sounds like rod knock or maybe a reed valve. If it is rod knock then the compression for all three cylinders would check OK? I was thinking that I might be able to quickly find out which cylinder it is by pulling the spark plug wire on one cylinder at a time and see if the knocking goes away? Does anyone know if the warn part is likely to be the rod, wrist pin, piston, or crankshaft? Would I be able to tell there was play by moving the flywheel back and forth a little while checking for piston movement?

        The sound doesn't seem to be coming from lower unit - I had read some posts about lower unit issues and that is why I rotated to prop - not that that makes any real sense...

        There is one other piece of information I left out. When the kids got back from skiing they said the engine made a strange noise when they slowed down quickly. After I heard it at idle, I haven't run it fast for fear of the rod breaking free so I haven't heard this first hand.

        I am leaning toward ordering all the gaskets I need to tear it down along with a rod and wrist pin. Are there any other things I will need to get it put back together?

        Comment


        • #5
          Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

          Originally posted by propgun View Post
          Dave and wilts,

          Thanks for the replies. I agree it sounds like rod knock or maybe a reed valve. If it is rod knock then the compression for all three cylinders would check OK? I was thinking that I might be able to quickly find out which cylinder it is by pulling the spark plug wire on one cylinder at a time and see if the knocking goes away? Does anyone know if the warn part is likely to be the rod, wrist pin, piston, or crankshaft? Would I be able to tell there was play by moving the flywheel back and forth a little while checking for piston movement?

          The sound doesn't seem to be coming from lower unit - I had read some posts about lower unit issues and that is why I rotated to prop - not that that makes any real sense...

          There is one other piece of information I left out. When the kids got back from skiing they said the engine made a strange noise when they slowed down quickly. After I heard it at idle, I haven't run it fast for fear of the rod breaking free so I haven't heard this first hand.

          I am leaning toward ordering all the gaskets I need to tear it down along with a rod and wrist pin. Are there any other things I will need to get it put back together?

          If it sounds like a rod knock its the lower unit.
          the sound is traveling up the motor making it sound like a motor problem. Take of the lower and hand rotate it, bet it sounds like gravel in there ! If left on the motor it won't sound or feel like anything is wrong so take the 10 mins and take the lower off. Let me know what you find out

          Comment


          • #6
            Ran Dunks Decarb procedure this weekend.
            Attached Files

            Comment


            • #7
              Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

              Originally posted by wilts42 View Post
              If it sounds like a rod knock its the lower unit.
              the sound is traveling up the motor making it sound like a motor problem. Take of the lower and hand rotate it, bet it sounds like gravel in there ! If left on the motor it won't sound or feel like anything is wrong so take the 10 mins and take the lower off. Let me know what you find out
              Wilts,

              I really don't have enough experience to identify rod knock but the people on this forum and other local techs have come to that conclusion by listening to the audio I posted with the youtube video in my first post. Did you listen to that? Did your engine sound like mine?

              Thanks,

              Greg

              Comment


              • #8
                Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                Originally posted by propgun View Post
                Wilts,

                I really don't have enough experience to identify rod knock but the people on this forum and other local techs have come to that conclusion by listening to the audio I posted with the youtube video in my first post. Did you listen to that? Did your engine sound like mine?

                Thanks,

                Greg
                your saying that your noise goes away when its in gear and that doesn't match up with what mine was doing. you could rule out the lower by changing the oil and checking the magnetic plug and oil for shavings or shiny stuff.. If you de carbed and it acted better maybe your on the right track.

                Comment


                • #9
                  Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                  1. Fix your fuel pump. You have both check valve disks installed, You only want the ethanol resistant ones in there. The black disks are distended, which will allow fuel to back into your vacuum port and cause ping (pre-Ignition) This sounds like a rod knock. Sound Familiar?

                  2. The two plugs on the left appear to be shorted to ground with carbon. Ditch those center fire plugs. They suck in older engines with carbon buildup because carbon has only a very small gap to cross before the plugs short out. A regular plug has sort of a tower of porcelain that takes longer to bridge. Open up the gap to about .050" Your CD ignition can handle that gap easily. Keep a spare set on hand and buy that little plug sand blaster and extra media fro HF they really work pretty well.

                  Report back after doing these two things. Dont go tearing your engine down just yet.
                  Last edited by jagans; September 12th, 2013, 09:56 AM.

                  Comment


                  • #10
                    Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                    Did you check compression? I beleive the fuel pump runs on the #3 cylinder, low compression could result in fuel pump not pumping enough fuel at high rpm. Also, something is wrong down there, may explain the sound as well...

                    Comment


                    • #11
                      Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                      Originally posted by jagans View Post
                      1. Fix your fuel pump. You have both check valve disks installed, You only want the ethanol resistant ones in there. The black disks are distended, which will allow fuel to back into your vacuum port and cause ping (pre-Ignition) This sounds like a rod knock. Sound Familiar?

                      2. The two plugs on the left appear to be shorted to ground with carbon. Ditch those center fire plugs. They suck in older engines with carbon buildup because carbon has only a very small gap to cross before the plugs short out. A regular plug has sort of a tower of porcelain that takes longer to bridge. Open up the gap to about .050" Your CD ignition can handle that gap easily. Keep a spare set on hand and buy that little plug sand blaster and extra media fro HF they really work pretty well.

                      Report back after doing these two things. Dont go tearing your engine down just yet.
                      Thanks for the response and suggestions. The first thing I did when I heard the knock was to rebuild the fuel pump and run a gallon of gas with a whole can of Sea Foam to try to do a decarb. I didn't put in new plugs given the decarb would probably foul them but the "knocking" noise was still there after that. Could be the plugs, I suppose. I finally pulled the power head today and have it in my car to take home. I'll post what I find after I get the crankcase cover off. I know there is still some carbon on the piston heads at least it looked that way with the inspection camera images posted in a different thread.

                      Comment


                      • #12
                        Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                        Originally posted by snowbrd84 View Post
                        Did you check compression? I beleive the fuel pump runs on the #3 cylinder, low compression could result in fuel pump not pumping enough fuel at high rpm. Also, something is wrong down there, may explain the sound as well...
                        Thanks for the response. The compression results with the throttle plates open and the engine warm were 110 - 105 -110 (T-M-B) which is on the low side but the HF compression tester I was using might be reading low. I went ahead and pulled the power head. Tomorrow I'll crack it open and see what I find. I hope it doesn't turn out to be the oil pump (fuel pump has a hose to cylinder 2, oil pump is lined up with the bottom cylinder). Of course, the middle cylinder might be cylinder 3 in firing order - I haven't looked that up.

                        Comment


                        • #13
                          Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                          Plugs can't make a knocking noise. Did you do as Wilt said and try running the motor with the lower unit off?
                          Ignition troubleshooting

                          Comment


                          • #14
                            Re: 1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise

                            Originally posted by Dave1027 View Post
                            Plugs can't make a knocking noise. Did you do as Wilt said and try running the motor with the lower unit off?
                            Dave,

                            It makes the same noise with the lower unit off. I was able to push a 5/8" piece of garden hose over the tubing in the exhaust housing after I pulled off the seal so I could run it for a while. Didn't need a hose clamp - nice tight fit. Had to lower the boat until the bottom of the exhaust housing was under water to hear anything. The power head is now in my garage - having trouble getting the crankcase cover to break free. Plan to pound "alternately sidewise" on the bottom of the crankshaft a little more with a bigger plastic mallet and see if that does the trick.

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                            • #15
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                              Resurrecting this post Have same motor with what sounds like the same problem & am wondering how this turned out

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