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76 Johnson 15hp "Lurching"

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  • 76 Johnson 15hp "Lurching"

    Finally got my new to me 12ft tinner with 76 15 out this week.
    It was a little hard to start at first the bulb takes alot of pumping. It was stalling out while trolling every so often. Then we (my brother in law and I) experienced a drop in piwer yesterday morning. I pulled the plugs to have a look. They were an NGK and not the equivalent to the recommended Champions... there was some gunk on the one bridging the gap and the other was a little oily looking. I picked up a couple of NGK B7HS plugs at a local (to cottage I am at) Johnny dealer and replaced them.
    Power restored.
    Went out again this morning and after anlbout 30-40seconds running at WOT the motor kind of lurched like a sudden loss of power then shortly there after restored (like someone putting on the brakes at highway speed then speeding back up). From what I have been able to find on it. Seems like a carb clean/restore is needed which is fine as Id planned to do that this winter.
    The other thing is when slowing down quickly from a WOT run and the wake catches up with the transom it tends to stall out the motor.
    If anyone has any ideas on vause of my two issues would be much appreciated.


  • #2
    lot's going on there. Re. the primer bulb -- should be firm after a few pumps if the carb float valve is closing, and if the fuel pump is not leaking (may not see a leak). Firm doesn't necessarily mean rock hard, but noticeably different from the first one or two pumps. I think I would rebuild or replace the fuel pump first, then rebuild the carb, like you plan. Losing power or faltering sounds like it might be a pump issue.

    Will want to get into ignition and compression tests as well, so just a case of what you want to do first.


    • #3
      You have a points motor. Pull plugs and check spark. Need 1/4" blue snappy spark from each plug lead consistently. Also, check for lateral play in armature plate..any lateral movement will change point gap and will not run consistently.
      Google "leeroy's ramblings" and look around..Tons of great info on your motor!
      author unknown: It's almost beyond the point of bearable how torturous it is to try to have any dignity in being a Vikings fan..


      • #4
        So the bulb will get firm... pull the cord a couple tries or so and firmness in the bulb has dissipated.

        Ill be checking compression at some point in the near future. As i currently dont have a tester.

        Will look into fuel pump rebuild ss well. I had a weird feeling it could be a fuel pump issue.


        • #5
          Just FYI, I recently replaced the square pump with a new Sierra one, and have had no problems. Cost is about half of the oem pump (I don't rebuild the little square ones -- assume that's what you have).


          • #6
            Its a square pump. BRP part number 0386979.
            I cannot find a sierra equivalent.
            Though I did find a web retailer that has the OEM on sale for $70US... Im in Canada and not going to US any time soon though.


            • #7
              good choices. I don't like to work on the little pumps, although kits are available.


              • #8
                As a follow up to this. I did a compression test today and got 110psi in the top cylinder and 105psi in the bottom cylinder.
                I have one of the lightup spark tester and it indicates spark on both, but I do not have a gap tester, so I suppose that will be the next thing to check.

                These are the plugs that the PO had in there after I took them out (they were NGK BR6HS); based on recommendations from reading leeroysrambling, I purchased a set of NGK B6HS as they run a little hotter for my climate leading to less fouling. I am keeping the B7HS that I used to replace the PO's plugs as backups.

                Click image for larger version

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                Also after running the motor in the garbage pail today, I noticed all this oily grime on the surface. Any idea what it might be?
                Based on the colour of the grease the PO used on the "Steer Tube" I was thinking it might be that.
                I was also afraid it might be LU oil, so I checked the LU oil and it ran yellowy/gold like new oil (changed a couple weeks ago), didn't appear to be milky.

                Click image for larger version

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                • #9
                  It is normal to see sludge in a test tank.---Just use some dishwashing liquid to control it / make it go away.


                  • #10
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                    OK. First of all you fouled a plug. If improperly burning fuel created enough carbon to foul your spark plugs, you can be sure it sprayed it all over your pistons and their rings. I would do a decarb for sure to clean that crap out.

                    Use the NGK B6HS. The B7HS will work but are better for the smaller 9,9Hp carbs. On the 15HP, the hotter B6HS plugs will work better. That motor has a poorly designed cylinder head and is prone to spark plug fouling. It is worse on the 15Hp then the 9.9Hp because of the larger carb throat. The only cure is to use the NGK B6HS plugs and "stop trolling". They don't make a plug that will turn that motor into a trolling motor. You might get 6 to 10 hours of trolling, with the best spark plug you can find, before one fouls with that model of outboard. In any case, make sure you open it up to WOT for a 1/2 minute or so before you come back to the dock, to hopefully burn off any carbon that might have started to form on the plugs and in the cylinders.

                    As for the 30 second run, then a bog and resurgence of power. That sounds like a fuel pumping issue but that does not mean it is your fuel pump. Those pumps are not cheap and the rebuild kits only work for people with the hands of a 5 year old. They are very annoying to rebuild, so I would recommend you narrow down the culprit first. A fuel pumping problem can be an improperly vented gas tank, a fuel hose where the check valves are malfunctioning, a fuel pump malfunction, a clogged up fuel filter screen, an air leak in the fuel line or a sticking carb inlet needle. Before I spent much time on eliminating any of those I would wait until it happens again and if you can quickly squeeze the primer and make the motor revive, it will verify a fuel pumping issue. Since it fixed itself we don't even know if you still have an issue.