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1983 Mercury 50HP Ignition problems

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  • 1983 Mercury 50HP Ignition problems

    First, thanks to all of the contributors on this forum; the information given has proven very useful in repairs and maintenance.

    My current problem is with my motor's ignition system.

    1983 Mercury 50HP blue band 4cyl, S/N 641XXXX

    Motor will start and idle OK, but won't just start without using the 'fast idle' lever. Doesn't sound the smoothest either, and often when coming back to an idle from higher rpms, it'll miss and quit. Fires right back up when I idle it high again. When I give it gas it will accelerate, albeit slowly, but won't run very fast at all - about 1/4 throttle. If I put the hammer down, it just runs about 1/4 throttle until it floods itself. Now, if I gently ease the throttle, it will start to go higher than 1/4 throttle, but only for a little bit - then it will bog down and return to the 1/4 throttle speed. Super annoying. Sometimes it will run better and hold around 1/2 throttle or so and I'll think that I have everything fixed, then it will bog down again.

    Initial instinct was a fuel delivery issue. Bulb remains hard throughout this process. Fuel is new. I re-plumbed all of the fuel lines with quality 3/8 fuel line, and new lines from the tank to the motor. I tore apart the carbs and thoroughly cleaned them - all the passages, everything. Let them sit in a bucket of carb cleaner, scrubbed them out, and dried them using compressed air. Rebuilt the carbs with all brand new parts - seals, gaskets, diaphragms, floats, needles, etc. Almost everything except the body was new. Was cheap enough maintenance and they needed it anyway. Adjusted them to a neutral setting based on manual. When I use the adjustment screws, I can sometimes get the motor to run a little better, but mostly, I think the changes are random and not based on fuel.

    Compression test: Cylinder 1: 120psi Cylinder 2: 115psi Cylinder 3: 115psi Cylinder 4: 120psi

    New plugs too.

    Went on to check the spark. Cylinder 1, 3, and 4, all have spark that will jump 7/16". Cylinder 2 showed no spark on the adjustable tester - wouldn't jump even 1/4", but lit the bulb on an inline tester. When I pulled the plugs, it didn't look like cylinder 2 was firing at all - plug looked brand new. Now, watching the spark, I noticed that sometimes it's much stronger than other times (sometimes a very thin line, sometimes nice, thick, and bright blue), and it doesn't spark in the way that I would expect - it tends to miss a spark or two. It'll spark three times in quick succession, then a pause, then 2 more, then a pause, then 1, then a pause, then 4, etc. Cylinders 1, 3, and 4, were all the same in regard the above statement. Swapping the stator wires on the power pack had no change on cylinder 2 - still didn't fire.

    Resistance measurements on the trigger were low - 745 ohms - manual specs 800-1400.

    When I took apart the flywheel to inspect the trigger and stator, I noticed that all the wires seem pretty old. The jacket is faded and supple as they've been exposed to a lot of heat. Stator is one that has what looks like black plastic covering everything.

    Now, I did order a DVA adapter for testing purposes, but unfortunately it didn't arrive in time to make measurements. The motor is currently not near me so troubleshooting involves a lot of driving and time. I have a fishing trip planned in early August, and would love to have this motor up and running by then, so I need to get parts on order sooner rather than later.

    All this being said, anyone have any thoughts on what the problem might be? I'm leaning towards an ignition coil on cylinder 2, and a trigger and stator based on what looked like weak and intermittent spark. However, I'm not entirely sure what the power pack does and whether or not these symptoms are due to a failed pack. Seems like the motor has good ground, although I plan on replacing all the ground wires with new.

    Thoughts?

    Thanks in advance for any and all help. This motor has been a relatively annoying challenge.


  • #2
    Re: 1983 Mercury 50HP Ignition problems

    Anyone have any suggestions?

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    • #3
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      Re: 1983 Mercury 50HP Ignition problems

      This was definitely an electrical issue.

      I was able to find a switch box, stator, and trigger all for around $150.

      I swapped out the switch box, stator, and trigger, replacing wiring (to the coils, rectifier, etc.) as I went with new.

      Now motor has a bright blue, fat spark that will easily jump 1/2" on all cylinders.

      Runs like a new machine.

      Since I replaced all the parts in one shot, I don't know exactly what was causing the problem, but since all the parts looked original, I would suspect that they were all due for a swap anyway.

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