mercury plugs

[island guy]

I thought I would put new plugs in my merc 18xd. (18hp) It's been a while since it was serviced. It has run Ok on the plugs that are in it. To my surprise the plugs the dealer sold me were the type you set the gap on. The ones I have in there now are not your regular plug. I wish I were more mechanically inclined so I could describe it better but there is no piece on the bottom that you would set the gap. It is just a flat bottom plug. Is it possible I've been running the wrong plugs these past few years? It has run well at high rpm's but a bit rough at idle. Any thoughts??

[Whoaru99]

My engine has the surface gap plugs in it (the flat ones), but it's a way different engine - a 90HP Merc.

Don't know what to say except that I'd hope the Merc dealer would sell you the correct plugs...

Have you done any searches on Google or on any of the sparkplug manufacturer's websites to see if you find a recommendation for your engine? Try NGK HSOforum...

[eurolarva]

The plugs you currently use are gapless plugs. According to the merc site your plugs are 33-97180 for US made motor and 33 - 14103570 (these are mercury part numbers) for belgium and canada models. It could be that they crossed the parts over to the wrong motor. What is the number on the spark plug that was on the motor and what is the number that is on the plugs you just got. The number should be something like L72C or it could be a NGK number. I would not change the plugs until you are sure you got the right ones. The part that is at the end is called a ground electrode. You bend this to set the gap for the plug. If this is the wrong plug you could run the risk that the electrode goes too far inside the cylinder and could be struck by the piston when it hits top dead center. You might want to call a NAPA store tomorrow and see what they say is suppose to be on your motor.

[island guy]

Thanks for the info euro!

The plugs I had in there were NGK BUHW. They were the gapless. The ones the dealer sold me were NGK 4838 BP8H-N-10. They were gapped. I've not used the motor a lot the past few years and am surprised it would run on such different plugs. I put the new 4838 in and hooked the hose to it and it started right up. I only let it run a few seconds. Now I'm not sure if I should use it till I know for sure!

[Surface Tension]

My search found the NGK BP8H-N-10 for your outboard.

[Rivernut]

Islandguy, my in-laws are looking at a boat with an 18xd. Do you happen to know what year motor yours is. This has the brown/gold striping. Also, has it ben a good motor for you. Thanks a lot for any 411.

[island guy]

Yep, the BP8H-N-10 is the right plug! Put two in and it runs great! I'm surprised it ran on the others at all.

To answer the other question, it has been a good motor. It's heavy to store if you need to take it off the boat. It is not very economical on gas. I bought it 12 years ago and really have not had any problems. It doesn't idle down real well but not bad. I use the electric trolling motor for that anyway.

[eurolarva]

gapless plugs run good at wide open throttle, however are prone to fouling at slow speeds. You might have better luck with it at slower speeds with the new plugs. NGK recommends the gap be set at .040 however it is probably factory set at this.