1978 Johnson 25 hp 25R78C Electrical Fault???

[boatertom]

Hi. I recently picked up an old 1978 Johnson 25 hp 25R78C

After repairing the pull start I got it to run and it seems to run quite well, Except that when I decelerate a bright orange spark (about an inch long) leaps between the block and the plate (ignition advance?) under the fly wheel that moves (rotates) when you throttle up or down. It makes quite a loud crack and is only a couple of inches from the fuel pump! I have tested the magneto coil, sensor coil, stop circuit, and both ignition coils, with a multimeter for correct continuity and resistance. From what I can tell with my limited experience they seem ok which leads me to think it could be the power pack?
Does anybody have any ideas what would cause this? Any advice would be appreciated.

Cheers Tom

 

[F_R]

That is a really weird one, and I do not know the answer. Except I seriously doubt the power pack theory. There are only two electrical components on that plate, the driver coil and trigger sensor. Both are complete closed circuits, neither requiring a ground between the plate and block. In fact, neither of those parts are grounded to the plate either. Let us know what you find so we'll know next time.

EDIT: Still thinking about that one, and wondering if one of the spark plug wires is arcing through to the plate or some part physically connected to the plate (like throttle linkage)? Then the spark is actually between the plate and block as it returns back to the coil. Just something to explore.

 

[Crosbyman]

well such a big spark can only be traced to the secondary side of your spark plug coils. I doubt anything else would produce over 40,000 Volts snaps


if that high voltage isn't used for feeding your plugs and ending its life in the frame ground it seems to be back feeding

I would recheck all coil wiring .

 

[steelespike]

Run the motor in the dark with the cover off,look for stray electrical leaks.
Original plug wires are surely suspect.
Make a ground wire from the plate to the block.

 

[F_R]

I suppose that 40,000 volts in the coil secondary could be arcing internally to the coil's primary winding. But if it did that, it would feed back to the power pack, and I would think surely that would blow the scr's in the pack. Just thinking out loud here. Don't take this seriously.

 

[gm280]

Obviously the only way a spark could jump is with a seriously high voltage source. And that is only on the secondary side of the spark coils. So I would most certainly suspect the coils and spark plug wires myself. No other way to have such a spark. Sounds like break down of either the coils themselves or the spark plug wires break down from aging. I would do the night time run suggestion to actually see. JMHO!

 

[boatertom]

Thanks for the advise. I will put it all back together and try the dark running trick. I have to agree after thinking about it that it would have to be high voltage to jump that far and be visible in daylight. I will let you know how I get on. Cheers Tom

 

[kbait]

Should be a short ground wire connecting block to armature plate on port side of motor (opposite fuel pump). I've seem 'em not run if that wire is disconnected, but not big arc from plate to ground. Make sure that ground wire is connected, verify good connection from coil ground wires to ground (on upper coil mounting bolts), verify cd box ground wire is connected, inspect coils for cracks.. And then run in the dark and check for arcing..especially from coil to ground - or plug wire to ground.
Good luck!