1985 Merc 20hp - possible electrical issue?

[311Hemi]

I have a 1985 Merc 20hp tiller and ran into issues last year with it. Waiting for the snow to melt to start troubleshooting, but looking for some help before I get started.

Motor starts fine and runs prefect on WOT.

The issue:

At idle the engine will sometimes idle fine and other times it starts missing, which obviously causes it to run really rough and eventually die if we don't throttle up (which suck when trolling). When it starts to miss at idle there is a very apparent "ticking" sound that is constant with each rotation of the engine. You can hear the engine run fine for one minute and then the next minute you hear that ticking sound and the engine starts running rough. Again, no problems at WOT.

I believe this to be an electrical issue, but do not see any sparks jumping anywhere. I also believe a repair guy last year tried new (used) coils and new (used) stator on it and did not see any difference, although I am not sure he was seeing the issue to begin with because it does not happen 100% of the time, and sometime the engine has to run a while to reproduce it.

Any thoughts on where to begin trouble shooting this issue?

[boatfixer]

It does sound to me like you are hearing the spark jump to ground. Where, is the question. 1 simple way to determine if you do have an ignition problem, is to use a timing light. Watch for the light to quit flashing in unison with the miss. If you determine the ignition is the problem and like you mentioned before about hearing the tick, one thing I will do is to wait until it is dark out to find the spark. Much easier to find with out the sun shining down. Areas to look at would obviously be the coils, wires, and boots(especially near the metal shroud). If it is loud enough to be heard, it is on the secondary side of the ignition. Look for burns on the plug wires and anywhere the wires or boots come close to metal.

[311Hemi]

Thanks for the info!

What components are on the secondary side of the ignition? I would assume all these component would be suspect because they are high voltage, vs the standard small gauge wiring which would be low voltage and should not be audible if arcing?

I really hope I can reproduce this in my driveway so I can check it at night.....otherwise I guess a late night out on the lake would work also.

[boatfixer]

Coils, plug wires, boots(both ends), and dont forget the plugs-cracks in the porcelean. One other thing to take note of, would be, lets say you've ran this for an hour at home and no problem. you take it to the lake, and guess what, problem. Was the shroud on or off at home and on the lake. Sometimes you can short to ground to the removable part of the shroud. Just something to keep in mind. And since you say this happens at an idle and not at WOT, without the engine running, move the throttle position and watch all the wires. Do they come into contact with anything?. This all assuming that it is shorting to ground and causing your problem.

[311Hemi]

This does happen with the cover off, so I can rule that out. This happened on our Canada trip last year and I was trying everything!! Trolling while watching this engine with a ticking sound trying to see if something was arcing.

I will have to check for any secondary ignition sources near the gear lever, but I don't believe there are any. The coils and plug wires are on the very rear of the engine and there is not much for them to arc to, but I will have to look close and I have an idea after looking at a diagram of a few things to check. We have checked the plugs and did not see any issues there either, but plugs are cheap so I can try new ones.

Could you hear sparking inside of the switch box (I would not think so) or on the stator?

[marine_man]

The other thing you can do to try and isolate which cylinder is causing problems is to remove a plug wire when it's missing - if it dies right away you've at least narrowed down which cylinder is causing the problem.

Good luck!

marine_man

[311Hemi]

Finally got around to troubleshooting things and I believe there was a spark jumping from the plug wire/coil area. I pulled the plug wire off one plug while it was running and it started jumping directly from the plug wire to coil. I know this is not the most accurate or best thing to do for the coil, but it recreated the exact sound I was hearing before and I saw the spark when doing this. Decided to just replace both coils, got new plugs, and cleaned all grounding connections and everything ran good this past Sunday. I now have a spare coil in case one goes bad....just not sure which one is bad!!! Hopefully I never have to figure it out.

Thanks for the suggestions!

[marine_man]

Glad you got it figured out!

marine_man

[boatfixer]

yea, good to hear!