1960 40 HP – Bottom cylinder misfire

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  • October 27, 2026 at 2:18 pm #330598
    TJ Brandon
    Participant

      US Member

      I have a 1960 40HP that I cannot figure out the bottom cylinder misfire. In low throttle the spark is consistent and strong. From about 50% up to full throttle there is a misfire that comes and goes.

      Things I have tried;
      – Moved all top cylinder components to the bottom cylinder and the problems stayed at the bottom cylinder
      – 3 sets of ignition points
      – 3 condensers
      – 2 ignition coils that tested good
      – disconnected the runaway safety switch thing
      – ensured the wire going to the point to the safety switch was not grounding
      – new spark plug
      – new plug wire

      This thing is driving me crazy!

      October 27, 2026 at 4:54 pm #330603
      JACQUES
      Participant

        any chance of arcing… try to run it in full darkness to spot arcing.   or poke around the wiring while skips

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        October 27, 2026 at 5:10 pm #330604
        Buccaneer
        Participant

          US Member

          Just out of curiosity, are the cylinders close in compression?

          Prepare to be boarded!

          October 27, 2026 at 5:56 pm #330605
          David Bartlett
          Participant

            US Member

            How is the clearance between the chankcase and the magneto. Is there excess slop?

            David Bartlett
            Pine Tree Boating Club Chapter

            "I don't fully understand everything I know!"

            October 28, 2026 at 8:15 am #330617
            TJ Brandon
            Participant

              US Member

              Thanks for the responses.
              I have checked for arcing and jiggled the wires while running and can’t see anything.

              For cylinder compression, I am not sure. I have not tested that. Can that make a difference in the ignition?

              The magneto plate does wobble very slightly, I want to say less than 1/32 of an inch

              October 28, 2026 at 8:30 am #330618
              Buccaneer
              Participant

                US Member

                Testing compression when troubleshooting an engine is
                just a basic first step. Kind of like when going to the doctor,
                the pretty nurse checks your blood pressure right away.

                At the least, just guessing that a low compression may
                perhaps cause a miss-diagnosed interpretation of an ignition problem??

                Prepare to be boarded!

                October 28, 2026 at 9:40 am #330621
                Tubs
                Participant

                  I’m mostly into pre-war motors, don’t know anything about the motor you have, but had a Mercury Super 10 that developed a bottom cylinder miss similar to what you have described. After completely changing the ignition from a Bendix to a Phelon, at a considerable expense, I found the problem was actually the lower crank shaft seal.

                  A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

                  October 28, 2026 at 1:16 pm #330631
                  Don
                  Participant

                    US Member

                    OK, not familiar with just which cylinder is cancelled out by the vacuum switch.  How did you disconnect it?  Did you disconnect the knife connector on the lead coming out from under the mag plate?  Or, did you just disconnect the lead from the vacuum switch?

                    How is it you have determined it is #2 that is misfiring?

                    I would run in at dusk using a timing light or those inline neon testers to ensure you actually have an ignition issue, and on which cylinder.  Keep in mind that a shorted/fouled plug will lower the secondary voltage enough so the timing light/neon testers will show a “miss”.

                    PS:  Forgive me if you have already described your testing procedures, I will try to review this post.

                    October 28, 2026 at 1:19 pm #330632
                    TJ Brandon
                    Participant

                      US Member

                      I will test the compression when I get it put back together.

                      I am sure it is the bottom cylinder because I have spark testers. Top is nice and consistent and strong while the bottom cuts out. I swap testers and the problem stays with the bottom cylinder. When the bottom does decide to fire it is nice and bright as well.

                      For the cut out switch, i disconnected from the switch itself. There is a nut on a stud the ignition wire connects to. I removed that.

                      A lower crank case seal can lead to poor ignition? How does that happen and how can I check? Upper crank seal I have replaced before and is easy to diagnose because of the mess it leaves on the magneto place

                      October 28, 2026 at 2:46 pm #330636
                      JACQUES
                      Participant

                        I know you switched stuff around…maybe  try a ohms test from boot to engine frame body.   stabalize the reading (low ohms)  and  start wiggling stuff around

                        check for sudden drops in continuity.  needle meters  are best for this as the needle swings react faster to ohm changes on digital meters (JMHO)

                        do same OHMS test  from  mag plate  to engine frame  as you rotate  the mag plate  or wiggle it… ground continuity is essential

                         

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