[oxocart]

I have good spark on the top cylinder but intermittent spark on the lower one.

I've now switched the coils side to side to see if the lower plug gets good spark now. I've tried two pairs of new plugs.

My question is: do the spark plug wires simply pull out of the coils or are they screwed in?

The wires both turn half a turn or more freely but I've given them a light to medium tug and they have not popped out.

One plug wire is longer than the other so I would like to switch them for the test and also try new plug wires.

The coil numbers are both 339 5288. All wiring etc. is original as I have owned it since new.
The compression is 105 on one cylinder and 112 on the other. The engine has very low hours on it certainly less than 200.

This is a Canadian motor serial no. 7063179

[7920hpMercury]

You are on the right track swapping plugs to see if the problem moves or not.

The plug wires pull straight out of the coils. Sometimes it is hard to pull out. Some WD40 might help. On the end of the plug wire that goes into the coil, there is a expanding spring that is soldered onto the plug wire.

Hope this helps.

I would also suggest that you make sure the little ground wire that is between the coil and the aluminum mount is intact and acts as good ground.

[oxocart]

Thanks for the reply and advice. The plug wires came out okay with a couple of squirts of WD-40 and heavier tugs. Those spring clips are stronger than I've seen before.
I think I will have time to hook up water to it and do a test run tomorrow.

The ground wire on the coil on the problem side looked slightly like a possible poor connection to the engine so I polished the wires and aluminum engine surfaces with fine emery cloth.

Would it be a good idea to put a little die-electric grease in there?

[oxocart]

When I ran the motor this morning with the coils reversed in position the lower cylinder was now firing okay but almost no spark to the top one.

This is the opposite of before so it tells me that my plug wires are okay but I have a bad coil.

Better than needing to replace the switch box or parts under the flywheel so I will order one.

Any suggestions as where to buy one other than from Mercury which would cost $85.00 Canadian or about $77.00 US?

[7920hpMercury]

Di electric grease is good.

And that is a pretty common coil. Coils last a long time unless they get wet or the little wire on the back corrodes out. Coil no. is 339 5288 and the + to - terminal should be less than 1 ohm. the plug wire to ground should be 900 to 1200 ohms. Check your plug wire too for continuity. I would go to your local Mercury independent repair shop and get a used one, just bring your ohm meter to check it. Or check ebay. That sounds very expensive for a new one to me.

Good luck.

[Yepblaze]

I've "pitched" (taken out of service) one of those switchboxes before only because it "wasn't right", and in turn brought a motor back from the possesed.

It was like night and day.

[oxocart]

Thanks gentlemen for the advice.
I'm going to purchase a coil, they aren't in stock locally so I'll either order one or get from the USA from an on line retailer.

Any other thoughts on this motor are welcome.

[oxocart]

Just an update on the 20 hp ignition and another question.

This afternoon I replaced the defective coil with a new one from Mercury and had excellent results. Strong ignition to both plugs and the motor purrs. I will test the old coil resitance later.

Do these engines have a thermostat or can one be put in?

I read that some of the old motors didn't come with one but it could be installed as an option for northern climates.

[7920hpMercury]

This motor did not come with a thermostat. You could probably rig one up off the 1/4"npt tale tell connection on the powerhead, but it would seem to be of marginal benefit to me and probably work marginally as well. Not all of the flow goes out the tale tell as there is also water that goes out the bottom of the powerhead and out the mid section.

Glad to hear you got it running great!

[oxocart]

Thanks for the thermostat information. I'll leave it the way it is.

I'd used this motor for trolling on my old fiberglass boat (sold a couple of years ago) but want to use it on my new WC-14 Lund.
I'll need to buy or fabricate a trolling plate for it as I need to be able to troll at 1 mph. This motor does run well at low rpm without fouling.

We usually troll with a Minnkota but often have windy conditions requiring the gas motor.

I've been using a 5hp Honda 2007 4 cycle on the Lund this fall. It's been my experience that the 4 cycle motors won't run as slowly as the older two cycles. I need to drag a sea anchor or sock to troll with it.