1986 Mercury 75 HP- 4cyl- No spark from cylinder 3 and 4- Unique issue

On lake yesterday and was only running on 2 cylinders. Bogged down with throttle so I initially assumed it was a carb or fuel pump issue. Decided I would look at everything including electrical and noticed no spark on #3 and #4 with spark tester. I am no professional mechanic by any means but I am pretty logical in the way I approach things. For some strange reason, I felt the need to cut into the wiring harness of my trigger to see if anything might be shorting out. This is what I found. I have a theory. There was a huge dirt dauber nest under the flywheel. When larvae hatch, can those things eat wire insulation. I don't think it was mice because the harness was too small. Another theory is moisture or heat but the pic looks like something was munching on one of the wires going to the switch box. Wouldn't want to wish this one on anyone. Thanks.

Wires can be switched on the switch box for testing purposes.----And ---cdielectronics ----has a trouble shooting guide for that motor

Do a continuity check on the trigger coil wires.

Jeff

Not a trigger problem....switchbox or coil grounding issue

faztbullet said:

Not a trigger problem....switchbox or coil grounding issue

Click to expand...

Well, when I cut into the trigger wire harness, the white/black was corroded or eaten all the way from the switchbox to the trigger. I replaced the trigger and now have spark on all 4 cylinders. Left strapped to the trailer and submerged lower unit so I could put a load on the motor. Continues to bog down at full throttle but idles great. Spent much time trying to dial in fuel/air mixture with adjusting screws but to no avail. No change with tilt up and down but gas running out of the front of both carbs. Boat has been sitting up in the top of a boathouse for a few years. Things I've done so far in this order:

1. Replaced diaphragm in fuel pump- the triangular one. Old diaphragm was stiff. Corrected problem with starting and idling. Starts like a top. Boggs down at higher RPMS.
2. Replaced Stator- wires corroded or eaten
3. Replaced Trigger- black/white wire corroded or eaten. (wanted to include pics but would not let me upload)
4. Blew out carbs with compressed air and sprayed with carb cleaner. Did not replace jets, needles or floats (original cork floats). Probably next on the list. As stated above when adjusting air/fuel mixture unsuccessfully, gas coming out of the front of carbs thru brass outlets at times.

Now that I have spark on all 4 cyls, could bad continuity in one of the trigger wires be contributing to this as mentioned above.

Thanks

Thanks. I have it and will take a look. Switchbox replaced as well as stator (eaten through by rats?) and trigger (corroded black/white). After replacing I have spark on all 4 cyl but continues to bog down. Now thinking fuel issue. Spent much time trying to adjust fuel/air mixture without success

On the to do list. If bad, I may fabricate my own since they run about 70 buck for each harness X 2.

Well, after complete rebuild of electrical, thanks to the dreaded dirt dobbers, she runs like a top. Replaced coil wiring harness, 2 coils, stator, trigger. Was only getting intermittent spark on bottom 2 cylinders so I replaced switch box and this fixed the problem. Thanks for comments and guidance.