Use "normal" spark plug instead of surface gap?

[Whoaru99]

I was wondering about substituting the "normal" kind of spark plug in place of the surface gap kind in my Mercury engine.

I was thinking maybe the conventional plugs would run hotter and not load up as much when I'm just puttin' around? Those surface gap plug just have to be about as cold-running as possible, don't they?

Any takes on whether or not this would be a bad thing to try - as long as there is clearance in the cylinder for the different plug, that is?

[PierBridge]

NO....Not a good Idea.

[Whoaru99]

Care to elaborate further?

[PierBridge]

The projected nose on the plugs does not have much to do w/ heat range. Heat range is the plugs ability to transfer heat from the combustion chamber to the head, colder plugs transfer heat faster, hotter plugs transfer heat slower. Too hot a plug will cause preignition due to fuel burning from the plugs heat. See if there is a surface plug 1 step hotter than your recomended range, if you do a lot of low speed running.

[Macgyver55]

Actually, the primary means of adjusting heat range are by varying the length of the core nose and the alloy material used in the electrodes. Hot plugs have a relatively long insulator nose with a long heat transfer path. Cold plugs have a much shorter insulator nose and thus, transfer heat more rapidly.

[tomfromblaine]

You may also want to think about clearance issues, a piston hitting a sparkplug electrode would be very bad.

[Valv]

Which year is the motor ?

If it's in the '70s you might want to be careful, these motors were made for leaded gasoline, they run fine on unleaded but changing type of plug might change het dissipation and cause failures in the long run.

Load up issues is from carburation, not spark plug type.

If you have a large motor there's nothing you can do, just find a small kicker for trolling, otherwise I would keep using standard Mercury plugs, and kee a set along as spare.

[musky hunter]

I had an older Mercury 50 hp a few years ago. I could not make it run well at all with the surface gap plugs. I put in Splitfire plugs and it ran like a top. If the motor does not run well with surface gaps, what have you got to loose? But be sure that the plug you substitute does not interfere with the piston at top of stroke. You are correct about running hotter. The more plug reveal into the combustion chamber, the hotter the tip of the spark plug.

As the engine ages, compression is lost, and changing plugs is one way to compensate. Also, you might want to try going to 100:1 Amsoil as further compensation.

[Surface Tension]

What year and hp is this outboard?

[Whoaru99]

The engine is a 1996 90hp (3 cyl).

[Surface Tension]

Foiling the recommended plugs shouldn't be a problem if everything is right. Theoreticly at slow speeds your oil is injected at a lower rate then if your running at high speeds.

Thing is the low speed normally would plug. A dirty carb can starve an engine because of gumming or debris or it can run rich from a dirty inlet valve not seating.

Its a tough call because if you lean it out to much you'll do damage to the piston and cylinder walls.

Changing to a hotter plug or to a synthetic oil might help but your not fixing the problem.

If it were me I'd open it up from time to time and blow the snot out, in the back of my mind I'd be thinking its time to do an overhaul and good cleaning of the carbs.

[Whoaru99]

Thanks everyone for the responses.

I'm hoping it's not time for an overhaul - even though the engine is near 10 years old, it really has very few hours on it. Just guessing, but I'd say around 100 hours total, probably even less counting the last couple years it was not used much at all. It's been fogged and stabilized each and every year, etc.

The plugs don't look oily or all blackened, they just look wet when ever I pull them out - particularly the topmost plug of the three.

While not a marine engine mech, I do have some mechanical background - and a Merc factory service manual - and have done all the carb and ignition adjustments "by the book" but it always seems to be about the same.

Seems to start OK and runs above 2000 RPM just fine, it's just the low speed loading up/rough running where I seem to be having problems. Almost like it runs on two cylinders until it's good and warmed up, then the third cyl. kicks in.

I can set the carbs leaner with the mixture screws and it seems to help some, but then I start to get a flat spot in the acceleration until (presumably) the high speed fuel circuit starts to come in. Don't want to go so lean that I end up with a melted piston, etc.

Maybe time to rebuild the carbs (or maybe??) seek professional help...

Any more suggestion or thoughts? Any recommendations for a good marine shop in the West Central MN area?

[Surface Tension]

Sorry I meant carb overhaul not an engine overhaul.