Mercury Spark Plug Question

[FloraBamaFishing]

I have a 2005 Mercury 115 2 stroke 2+2 outboard. When looking up parts I noticed it list two different spark plugs NGK BPZ8H-N-10 ($7.50 ea) and NGK BP8H-N-10 ($2.50 ea) My engine seems to leave an excessive amount of carbon on 3 of 4 plugs every trip. So much so that I keep extra sets to cleanup and reuse. I’ve noticed if I use the same set 2 days in a row, my engine gets very sluggish running at any level and will start dying at idle wake. I have used both types of plugs with the cheaper performing better. I’ve never known any engine manufacturer recommending cheaper items that perform better. Usually they want to sell you what costs more. Does anyone know if maybe I’m supposed to be using both types because of the 2+2 design?

 

[racerone]

There should be no reason to fiddle with sparkplugs after every fishing trip.------Do some trouble shooting on your motor.-----Post compression test numbers.----Is thermostat installed and working ?----Is oil pump output verified correct ?

 

[Texasmark]

I'm not a mechanic, did not attend school nor get certified so you can take what I say with a grain of salt. I only talk about my experiences.

Last year I bought a 2002 115 2+2. I don't run full throttle for extended periods of time. Usually I run 3000-4000. I went through numerous things to get this engine in working order to suit me but had a lingering problem. Mainly 3 and 4 stayed wet and insulators were grey, not tan like is the color of plugs running at the proper temp. Other thing was when I punched out, it took a minute or two for 3 and 4 to clean up, come online 100% and get my rated WOT RPMs. I decided to go with Iridium plugs for my own reasons and also decided to go to a higher heat range.

NGK gets hotter with lower numbers so I found (in the online NGK catalog) Iridium plugs that met the physical dimensional requirements for the engine with a 6 heat range rather than the 8 for the OEM recommended plug. The other thing I did was that any time I was below 2k RPM I tilted the rear of the engine up slightly. Results were that the engine idled like a sewing machine and when punching out it didn't take a minute or two for the plugs to clean up to get my WOT RPMs. Next thing was that when I pulled my plugs for inspection after several outings, all the plugs were dry and insulators were tan. Mission accomplished.

OEM per service manual: NGK BP8H-N-10. The -10 is for 0.040" gapped

Iridiums I use: NGKBPR6HIX (NGK part number 4085) off ebay priced right.....non counterfeit....NGK tells you how to determine the real thing! The OEM plug is gapped to 0.040 so regapping from 0.030 is required....maybe not but I did and not sorry. Use a wire gauge and be extremely careful as the tiny iridium tip is easily broken.