1994 15hp merc coughs at idle

Hey guys 1st set of ?s here.
What is the correct spark plug for my 94 15hp merc and is there a better plug than
the original? Also what is the gap?

I just put in NGK5510 B8HS and gapped then as close to the old ones as i could now
the motor is running rougher.

There isn't a better plug then what the manufacturer recommends which is a Champion L92LCC gapped at .040

Whoopbass i went to a site supplied by cajun and the manufacturer calls for an NGK-BP8hs-15 gapped @ 0.060
So im going to try it and see what happens.
Thanks much for the reply and ill let you know how it goes.

Just call your local Napa and they will look it up and get you the right plugs.
I always hear to use exactly what the manufacturer recommends.

Never new a plug that was gapped at .060. All NGK;s I've used are gapped at .040, Can't swear to a 4 stroke though.

Why did you change out the plugs? Start of the season maintenance?. Don't overlook your fuel system, clean out tank, fresh gas mix. Carb set all winter, now is showing signs of a lean pop at idle. Just saying!

I was changeing the plugs because the guy i bought everything from said he hasnt
done any maintenance to this rig in 3 yrs.
I am takeing it to a dealer today and haveing them go thru everything but
I want to learn to do all the required maintenance myself and any future repairs.
As far as useing a better plug, (and my way of thinking my be wrong) a little hotter
plug makes for better burn off in the chamber/ better combustion thus better performance.
I know this is not an old hot rod im takeing to the drags every once in a while but the basics
of a gas engine are the same.(BTW im a HP junkie also

FishMar74 said:

Whoopbass i went to a site supplied by cajun and the manufacturer calls for an NGK-BP8hs-15 gapped @ 0.060
So im going to try it and see what happens.
Thanks much for the reply and ill let you know how it goes.

Click to expand...

I assume your speaking about the sticky link to the public free manuals at the top of the Motors section. Because, I didn't remember responding to your post. I have some knowledge on Evinrude and Johnson, but somewhat ignorant when dealing with Mercury. I'm a JohnnyRude dude. :LOL2:

But I gandered a look at link you were looking under. It seems you found the information under the 25hp Mercury 2cyc (link has NGK-BP8hs-15 gapped @ 0.060 ), but dang......that seems a bit long of a gap.

I did look up your motor (I assume it is a 2 stroke). For the year and make of your Mercury these are the 2 suggestion at www.iboats.com

Mercury
15hp 2-Stroke 1989 - 2004 Champion plugs L92LCC gapped at 0.040

here is the link
https://www.iboats.com/Mercury-Cham...4471833--session_id.216991611--view_id.669917

Mercury
15hp 2-Stroke 1989 - 2000 NGK BP8HS-15(1) gapped 0.060

here is the link
https://www.iboats.com/Mercury-NGK-...4471833--session_id.216991611--view_id.669910

If your motor was running fine and you changed your plugs, then your gap or plugs or wrong. You need to purchase a spark plug feeler guage to accurately gap the plugs. $3 or 4 dollar at any auto parts store.


Good luck!

Get a Marine plug, don't let any auto parts store 'cross reference' a plug for you, spend a couple extra dollars for the right plug. Also, get what was designed to work best in that engine, which would be the one the manufacturer recommends.

Call a Mercury dealer, they will tell you the plug and gap needed....probably even sell ya one.

Bufford

Thanks for the replys glad to see yall here to help. I called 2 local mercury dealers/repair shops today both said the plug was an NGK-BP8HS-15 but 1 said the gap was .040 the other said .060 so i got that plug and gapped them @ .050 and will try this out saturday and see how it runs.
I also cleaned out the fuel tank, replace the fuel line, replaced the fuel filter, changed the gear oil (used quicksilver GL5) greased the fittings. I also bought a can of sea foam to use saturday.
If im missing anything please let me know.

AND AGAIN THANKS FOR ALL REPLYS

In a 2 stroke engine, hotter is not better, lean running is also not better for the same reason. Unburned fuel in the cyl head is also a cooling process. It maintains the combustion chamber temp at a certain degree. Leaned out cyl's will cause detonation/pre-ignition and the piston to come apart.

The misconception of hot plugs in two strokes has nothing to do with the spark. It's in the insulator and how quickly it can conduct cyl head temp to the water jacket through the plug wall for the circulated water to carry the heat away.

A perfect example of this is in those engines with a surface gap plugs, no electrode sticking up, just a small dot in the center of the insulator. They were engineered for hot running engines. The insulator is right at the surface of the plug.

This is not theory, but facts, as I learned them from Champion spark plugs. If you want to test out these facts for yourself, get two plugs for the same application, One hotter than the other, and compare the two. See where the insulator contacts the plug wall.

Thats very good points Bobg and thanks very much.
Theres alot of differance in a .060 over chevy 350 and a 15hp 2 stroke
I found a manuel online it was writen by mercury and covers 2.2hp to 3.0 litre EFI
it seems to cover everything ill need. Was a very good deal to only 15.99 and down loads to your comp.
outboardexpert.com is the site if anyone would like to check it out.
Im excited tomorrow is the big day, hoping all goes well and i dont find any probs with this motor.

.060 it is, I'm not arguing with the engineers that designed it. Most of my experience is in working with Yamaha's and never ran across a plug gapped at .060.

Got out yesterday and the boat ran great im very happy with it .
But i do have another problem now it is leaking around the transom not to bad but its something that needs
attention soon. Im going to try and post some pics today.