[gorco]

I need help identifing what's the correct type of spark plug for my 1990 Mariner 115 4cyl.

On one page of the 90-93 Clymer manual I have states the motor takes a standard gap NGK BP8H-10 plug. Another page states it takes a surface gap NGK BU8H plug.

Also, the manual states all other hp models above 5hp take surface gap plugs.

I find it odd that the 100 and 115 take standard gap plugs when most of the rest take surface gap plugs.

Any help or guidance is greatly appreciated.

ps. I am new to this forum and I think it's great.

Thanks

Steve

[wbblc87]

Originally Posted by gorco

I need help identifing what's the correct type of spark plug for my 1990 Mariner 115 4cyl.

On one page of the 90-93 Clymer manual I have states the motor takes a standard gap NGK BP8H-10 plug. Another page states it takes a surface gap NGK BU8H plug.

Also, the manual states all other hp models above 5hp take surface gap plugs.

I find it odd that the 100 and 115 take standard gap plugs when most of the rest take surface gap plugs.

Any help or guidance is greatly appreciated.

ps. I am new to this forum and I think it's great.

Thanks

Steve

It depends on the serial number of your engine whether it takes BUHW or BP8HN-10. Get me your ser# and I can tell ya. Bill

[gorco]

Bill,

The serial number is OC227571.

Thanks for helping.

Steve

[gorco]

Any thoughts Bill?

Thanks

[Texasmark]

My last 3 mercs have had surface gap and I am still trying to figure out the advantage over standard plugs; Mid 80's 4 cyl 85, '89 115 6 cyl, and '02 90 hp 3 cyl.

Only thing I can tell is that they run forever. My 115 ran the same set for 7 years with stellar performance.

Mark

[gorco]

My 115 runs great too, but I can only get 4100 rpms at WOT. It has a Quicksilver Laser II 20" prop on a 18 foot 900 lbs flats boat.

I suspect the prop is too much for the boat/motor combo, but before I buy a new prop, I want to make sure I have the correct spark plugs in it.

Steve

[phatmanmike]

all the mercs run on surface gap NGKs. dont question it! it works great, JUST DO IT !....... or else.......:devil:

[gorco]

Is there any chance I'll damage anything if I switch from standard to surface gap plugs?

[Chris1956]

Mike and Gorco, Mercury changed the Spark Plug numbers for the newest inline sixes, and certainly for motors that superceeded the classic inline 4s and 6s. It is a simple matter to look on the spark plug web sites, or at a local auto parts place to get the correct plug number for your motor. If I had to guess, Gorco's motor will use the gap-type plug. I recommend that Gorco and Mike run what is recommended for their motors. I also think that surface-gap plugs stink, but I am stuck with them in my inline, unless I am willing to take a risk with someother plug.

[Texasmark]

As far as hurting somehing your piston could come up and strike the plugs protruding element, or if the wrong heat range there is a possibility of a burned piston crown.....picked that up from listening to others talk on here.

Mark

[Texasmark]

Hey Gorco, You are probably correct. Flats boats have a lot of hull in the water, hence a lot of drag. I ran a 19P HP SS back in '89, was a merc prop but don't remember what the name of it was.

It was the 115 tower and was on a heavy 17' Ranger Fisherman (680) which was padded and I would run 6 grand at WOT consistently. The pad (and trim) did the trick.

Mark

[gorco]

I'm going stick with the standard plugs.

Thanks for all the comments.

Steve

[emckelvy]

If you go to the www.mercurypartsexpress.com site and plug in your ser #, you can access parts diagrams for your motor.

The "electrical components" parts diagram shows the spark plugs as "Ref # 10".

Click on the yellow highlighted # to get specific footnotes as applicable to each part.

In the case of the spark plugs, it lists #'s for standard, RFI supression, and Canadian/Belguim motors.

It lists the standard plug for your motor as a surface-gap NGK BUHW.

It lists an RFI Supression plug for your motor as standard-gap NGK BP8H-N-10.

If you do a lot of idling, the standard-gap would probably do better. If you do a lot of hi-speed running, the surface gap plugs may run a bit cooler and last longer but overall it's a tossup. If you've gotten good service out of the standard-gapped plugs then there's no reason not to continue to use them.

BTW I've worked on a number of the Big-Bore 3's and 4's, and they've all had standard-gapped plugs.

Here's links to specs on each type of plug:

http://boating.aquamoon.org/product_...g-BP8HN10.html

http://www.boating.aquamoon.org/prod...park-Plug.html

HTH........ed

[gorco]

Ed,

Thanks for the info and the link to www.mercurypartsexpress.com. I didn't know about it.

I'll stick with standard plugs for now. My next step is to try different propellers.

Thanks
Steve

[emckelvy]

One thing about these motors, they are not rev-happy as were the older Inline Six Towers of Power.

Best performance will be a prop whose WOT rpm falls within the mfr's recommended rpm range.

Whether you prop to the top or bottom of that range depends on what type of boating you do.

Cheers..........ed