Merc 9.8 hp spark test?

In my quest to smooth out the idle on my '74 Merc 110 (9.8) I discovered last night that the bottom cylinder is not firing properly. <br /><br />Last fall, it idled rough, but functional, and ran at high speed better. At trolling speed, it was a little rough, but worked fine for several long days of trolling.<br /><br />When I first got it I did the spray type decarb. treatment as described in the FAQ and instructions on the can of Seafoam Deep Creep.<br /><br />I checked the compression last fall as well as yesterday. Both times, both cylinders were approximately 125 lbs, within two pounds of each other.<br /><br />To smooth out the idle, I disassembled, soaked and blew out the carb this spring and actually thought (still think) that it is a little smoother idling afterwards. I can idle down a little slower without it killing. Running in barrel, it actually starts fine (just a couple of pulls choked, a couple unchoked and it fires right up) It still idles rough, but functionally OK both in neutral and in gear and still seems to smooth out quite a bit when I give it some gas. I can't run it WOT in the barrel, but certainly well above a fast trolling speed. I'm guessing about 50%<br /><br />Really the thing doesn't run horribly at idle and fast idle and I was really planning on just using it as-is.<br /><br />But, after running at a fairly fast speed in the tank for about 35 minutes, just as a matter of curiosity, having never done it before, I pulled the spark plug wire on the bottom cylinder - and it made no difference! Replaced it and then pulled the top cylinder and it stopped dead. Also, not knowing any better, I pulled these wires from the plugs with my bare hands. The bottom did nothing. The top wire shocked me when I pulled it off. <br /><br />Pulled plugs - both were brand new and still looked it. They are surface gap plugs. Can't remember the number, but they are the spec. from the owner's manual and the shop manual.<br /><br />The bottom one perfectly clean and really didn't look like it had a lot of fuel on it. The top one pretty much the same thing with a little sign of combustion (tiny bit of dark, oily residue)<br />Pulled plug wires. They look original, but no visible cracks. Terminals have no obvious corrosion.<br /><br />Next step is I have bought an adjustable spark tester and test tonight.<br /><br />Questions:<br /><br />What gap should I set on the spark tester?<br /><br />Is there anything else that would cause one cylinder to not fire? I know that if the spark strength is bad, I'll need to do some more troubleshooting and will seek guidance at that point. This is a single carb engine. I have no idea what the ignition components are having never dealt with any of this stuff before -but I do have a Merc. OEM shop manual and know how to use a multimeter.<br /><br />Should I replace the plug wires on general principle? If so, do I need to get Merc OEM wires - or are they generic?<br /><br />Thanks for any advice you can give. I'll report back on the spark test, but am trying to look a couple of steps ahead if possible.

Re: Merc 9.8 hp spark test?

Spark gap 3/8". Any metal core wire. Try changeing the input wires to the coils, if it stays on the same cyl. it's the coil, if it goes to the other cyl. it's the switch box--Bob

Re: Merc 9.8 hp spark test?

Was very straight forward with the advice I received. Learned something and hopefully will be able to get needed part to repair. Even got to buy a new tool, but it was only $10.50. Thanks much!<br /><br />Both coils would spark the plugs visibly. In fact to my untrained eye, actually looked about the same - a little blue spark across the surface gap plug. I'm thinking a trained eye would have noticed one was visibly stronger or more energetic than the other.<br /><br />Used spark tester at 3/8. One coil had strong blue spark at 3/8", other coil nothing until less than 3/16, and then only sporadically. <br /><br />Switched spark plug wires, no effect.<br /><br />Switched input wires - good spark stayed with good coil.<br /><br />Conclusion: Bad coil!<br /><br />I'm guessing that even though the bad coil might have appeared to be functioning with the spark plug, it wasn't supplying power enough to fire the cylinder at all (see below).<br /><br />Is this unusual - <br /><br />(a) for a coil to visibly spark the plug, but not be strong enough? <br /><br />(b) for a two cylinder engine to actually run pretty well on one cylinder?<br /><br />Went ahead and ran motor with coils opposite to what they were yesterday (last time good = #1, this time good = #2). Motor ran exactly the same as yesterday - still actually starts pretty easy (4 pulls from dead cold) and idles rough, but definitely usable - doesn't die. Accelerates strongly in the barrel in gear. All on one cylinder - tested this by pulling #1 (bad coil) and engine didn't change (same as when I pulled #2 yesterday). When #2 was pulled, engine stopped dead (just like pulling #1 yesterday). Plugs again look like unused/new on "bad" cylinder and slightly used on "good" cylinder.<br /><br />While I'm waiting for the coil to come, I might actually go down to the river and give it a test on the boat. My theory is that I trolled for several days last year on one cylinder without even knowing it! Also did WOT last year and got 6-7 mph with my 21 foot 2500+ lb rig.