Merc 9.8 ignition problem

[dajohnson53]

'74 merc 9.8

I have only rudimentary knowledge of ignition stuff. I know this ignition has parts under the fly wheel with wires running to a switch box. The switch box has + and - wires running to each of two ignition coils which are also grounded to the mounting bracket.

I was losing a cylinder intermittently only at lowest rpms. I noticed this late last summer trolling - sometimes it would get really rough, running on one cyllinder when I wast trolling - but only intermittently. It would reun consistently at higher rpms. This spring when running in the barrel, it starts fine, but when idled down as low as it will go, it would be fine for a while then lose the cylinder and die unless I revved it up. The cylinder would kick back on if I gave it a little gas to higher idle.

I exchanged the plugs - no change in cylinder affected.

Exhanged coils and the same cylinder still went dead (lower). Got out the spark tester. One coil would give strong, blue 3/8 inch spark, the other nothing most of the time, occasionally a good looking spark.

Switched coils - spark did NOT follow coil.

Switched plug wires - spark din NOT follow plug wire. The spark stayed with the switch box leads that fire the top cylinder regardless of coil or plug wire. So I'm thinking switch box or the parts under the fly wheel, neither of which I can really analyse..

Not having any more skill, I simply disconnected, cleaned up and reconnected every ignition wire I could find, testing the spark as above each time I cleaned up something. None of them were obviously badly corroded.

Cleaned the coil grounds and leads, spark plug leads, no effect.

Cleaned the switchbox ground - no effect.

I found two brown wires coming out of the flywheel and leading to the switch box via connecions (each wire) screwed into the block. After cleaning up both of them (they looked OK, but I worked them over anyway), I now have STRONG BLUE spark from both coils at 3/8 inch on the spark tester, every pull of the starter rope.

I haven't had a chance to put it back in the barrel to see how it idles but what do you think? Have I solved the problem? Or is it a cruel coincidence and I'll run into the intermittent problem again.

If not- - I guess I'll hav eto take it into a pro because I don't knw how to test the switch box and don't have the skills or tools to pull the flywheel to look at or test that stuff.

Thanks for any feedback you can give.

 

[tashasdaddy]

Re: Merc 9.8 ignition problem

you very well may have found your problem. throw it in the barrel and see. a good reason to be late for work today. better than a kid being sick.

 

[dajohnson53]

Re: Merc 9.8 ignition problem

For what it's worth, I guess the clean-up I did on the conacts did the trick. I fired it up in the barrel and although it's "no swiss watch", it idles down without losing a cylinder. The lowest idle (at the shift position) is still kind of rough, I tend to think it's probably just a 70s era 2 stroke kind of thing rather than a defect. If I idle up just a little bit - say extra slow trolling speed - it smooths out quite a bit. Bottom line is that it seems to idle indefinitely on two cylinders without losing a cylinder and dying (which is what it used to do).

I might take off the carb next chance I get and clean it out again. The float needle seems to be sticking a bit - if I really pump hard on the bulb, it occassionally squirts out the carb. Maybe that will smooth out the idle as well. But in the mean time, it starts fine and runs well, so I think I'll actually do some boating!

 

[Motor Boater Bill]

Re: Merc 9.8 ignition problem

My 76 Merc 200 doesn't idle all that great either--good, not great, and cleaning the carb will be my next step also. I tend to think that these were designed more for top end than idle. My buddies with old Evinrude FastTwin motors have the smoothest idle I've heard, but I blow them away at WOT!

My guess is that the signal from the trigger may be pretty low voltage, so it wouldn't take much resistance to degrade that I would think. Cleaning those contacts seems like a good bet. I think I'll have a look at mine also.

By the way, the book says adjust your idle mix with the motor well warmed up, in forward gear, in the water (not with muffs). Adjusting it in neutral will likely give you a lean mix in gear.

 

[dajohnson53]

Re: Merc 9.8 ignition problem

...

By the way, the book says adjust your idle mix with the motor well warmed up, in forward gear, in the water (not with muffs). Adjusting it in neutral will likely give you a lean mix in gear.

I run it in a barrel and am able to put it in either forward or reverse without any problem. So my general technique is to let it warm up, then adjust the lean/rich screw while it's in gear at idle. I think that adjustment is pretty much as good as I can get it - just a 1/4 turn either way makes it change noticably for the worse, so I think I've found the sweet spot, which for this engine is probably a relative term. thanks for the comments.