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1986 Johnson 20hp. Very weak spark

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  • 1986 Johnson 20hp. Very weak spark

    Hello everybody! Been working on my boat motor for a few weeks now trying to get it running with no luck! I've fixed one issue after another and now I'm trying to figure out why I have a weak spark on both of my cylinders and by weak I mean very faint. I have checked the ohm's on the plug coming from under the flywheel to the power pack and it was within spec. New plugs And cleaned every ground I have found so far, disconnected the kill switch for the time being so my question is what could be causing this? I figured the power pack would make it not fire completely and coil pack would be just one cylinder. Any info would be great.

  • #2
    The proper way to check the spark is to have the spark plugs removed... and use a spark tester hooked up to the s/plug HT leads whereas an adjustable spark gap can be set to 7/16" for that model.

    Checking spark with s/plugs is a waste of time!

    The spark should jump that gap with a strong blue lightning like flame... a real SNAP!

    You'll see a black/yellow wire leading from the powerpack which is the kill circuit. Have that wire disconnected temporarily. If you have poor or no spark with it connected but great spark with it disconnected, that indicates that the kill button or the ignition switch is shorted somehow. For now, let us know what you find with this test.

    Also let us know... electric start, ignition switch? Fast normal cranking, slow cranking, last time it ran okay, what work performed?
    Last edited by Joe Reeves; 05-07-2017, 10:29 AM.

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    • #3
      Done the test first thing this morning using a screwdriver and setting the gap with no luck. Had to get within about 1/16 for it to arc over and still very faint spark with kill wire both connected and dis connected. It's an electric start push button and brand new hot battery showing 14v on multimeter and the motor is turning over fast. Couldn't tell you the last time it ran good or work performed as I bought it from a individual that had it sitting in his shed for a year. He said it ran fine when he put it in storage and only used it fishing on the river just bought a bigger motor for his boat but you know how that goes. Thanks for your reply!

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      • #4
        I went ahead and took the flywheel off to see if anything had melted. Stator didn't have any of the ooze under it but the charge coil had some under it and the other sensor had the same green ooze on the sensor itself (between side of sensor and magnets on the flywheel) both the charge coil and sensor wires go to my power pack but read within spec on my multimeter could this be my issue?

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        • #5
          From what you've said, it is my understanding that with the black/yellow wire disconnected from the power-pack (disconnects the kill circuit), and with the engine cranking over fast, you still have extremely weak spark present at the spark plug leads.

          That would indicate that the charge coil that feeds approximately 300 AC volts to the power-pack capacitor is faulty... OR... that the wires leading from that coil to the pack may be frayed or semi-grounded (chafing) at some point which would have the voltage trying to go in two different directions.

          Your manual would show what color those wires are, however whatever the color, the coil would be the one with its wires leading directly to the power-pack.

          The rope start models normally have individual coil components (charge coil, stator coils, sensor coil), however the electric start models have everything sealed inside one round stator assembly where if a problem exists, the entire unit needs to be replaced.

          Check that charge coil for the proper readings as per your manual... and visually for any obvious flaws including the pin/socket connections, etc.

          One other thing, if your engine has a ignition switch incorporated (does it?), disconnect the black/yellow wire from the power-pack, hook up a volt meter with its setting so that it would register the lowest voltage reading, even a micro-volt, between the wiring harness portion and ground... now, turn the key to the ON setting (NOT START). If any amount of voltage registers on that black/yellow wire (The Wiring Harness End... Not The Power-pack), replace the ignition switch.

          NOTE: If using a ignition switch, it's usually a raised "M" terminal that the black/yellow wire attaches to... and a plain black wire that attaches to the other regular "M" terminal which leads to ground. Nothing, NO other wire should be attached to those "M" terminals.

          Let us know what you find.

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          • #6
            No key Start just push button( tiller handle). This one has the separate coils instead of all together, charge coil ohm's within spec but can't test volts now because flywheel key stock broke apart after removing flywheel. Charge coil does have signs of melting under it And so does sensor. Think I'm going to start here first by replacing both of them and go from there.

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            • #7
              That would be my line of thought also. I have encountered components that read the proper readings even though they had entered that meltdown scenario... and replacing them cured the problem.

              The flywheel key actually broke? I've never encountered that unless you're, in a round about way stating that it had "sheared". If this was the case (sheared), that would have thrown the engine out of time... BUT... the spark, if good, would have remained good.... just would fire at the wrong time.

              NOTE: Use a factory part numbered flywheel key... NOT one from a hardware store. And install the key with the flat straight side in a vertical position with the engine... NOT with the crankshaft taper.

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              • #8
                Yes sheared is what I was getting at. And thanks for letting me know about the key because I sure was about to find a piece of key stock at work and take a grinder to it. I appreciate all of your advice and help Mr. Joe and hopefully I'll be posting pics of me cruising down the river on my boat soon.

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                • #9
                  Also..... Note that the flywheel nut absolutely needs to be torqued to factory specifications (manual).... otherwise it will most certainly shear again.

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                  • #10
                    Well I got my Parts in today and installed them I put the flywheel on but didn't torque it down due to my socket that fits the nut is at work. I didn't try and start it for this reason but I noticed my throttle was stuck and traced the reason why back to the flywheel. The flywheel magnets seems to be sticking to the stator. What could cause this? I might be over looking something that's binding it up but definitely strong magnetic pull to the stator. Also I took the plate my stator, charge coil, and ignition sensor mounts to to run the new wires. Throttle moves fine with flywheel removed. Any info would be great thin advance.

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                    • #11
                      Originally posted by cvalentine72 View Post
                      I noticed my throttle was stuck and traced the reason why back to the flywheel. The flywheel magnets seems to be sticking to the stator. What could cause this?
                      Possibly the following may correct that problem.

                      (Magneto & Driver Coil Alignment)
                      (J. Reeves)

                      To align the coils properly, have the metal vertical portion of the coil yokes aligned with the inside edge of the bevel that exists on the top portion of the aluminum seat upon which the coils sit. This creates the proper distance between the coils and the flywheel magnets. Faulty alignment creates friction and the yokes of the coils heat up, turn blue and expand.

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                      • #12
                        Thanks Mr. Joe. It was sticking out a bit too much. Corrected that issue and runs pretty good thanks for your help. Now just need to find time to get it out on the water.

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