Well I do appreciate the effort Texasmark! I ended up with the noninductors as I wanted to run the boat. I will look around for an old version of the manual and see if I can find it in there. At least I now have some confirmation that it is possible that it had conventional plugs in it at some point as I remember. I had watched a video where a boat mech said that the conventionals would start and idle better. I don't know personally but figured it would be a cheap test. LOL. Now that I think about it I think a buddy has an old Mercury manual. Maybe it will have it in there. Sorry I took awhile I lost my login somehow and had to do a password reset again. Tried to upload pics of my sparkplugs but keep getting errors.
Starting and improved idling was my reason for trying the gapped plugs. My recollection of why the gapped plugs is that they are used as they don't have heat problems like gapped plugs for long term high rpm running.
If you go to the NGK spark plug manual, you can find your existing gapless plug and obtain some information for use in finding a suitable gapped plug. Things you need to know are threads per inch and hole diameter, heat range and length of threads protruding into the combustion chamber. The NGK catalog shows you where in the part number those parameters are located.
Your recommended surface gap plug:
BUZHW-2
B: 14 mm diameter thread, 13/16 hex
U: Surface gap
Z: If used, inductor suppressor
w: Tungsten electrode
-2: No information....probably special processing of some sort
1987 through 1999 series engines are not listed in their applications charts for Merc 75 and 90 hp engines. For earler engines your current plug is the only listing. A suitable slightly projected insulator plug similar to the one used in the 4 cylinder engines would be:
BPZ8H-10-N:
B: 14mm diameter thread, 13/16 hex nut
P: Projected electrode (not surface gap)
Z: Inductor for interference suppression if needed otherwise omit
8: Heat Range 8 a medium number...2 is their hottest, 12 their coldest...you be the judge of what you want to run and why
H: 12.7mm (½" thread depth)...same as surface gap but protruding ground pin and insulator project farther into combustion chamber.....I ran them in my 90 but you will want to check your clearance with the piston at TDC to ensure. Should have plenty of room.....if they were to impact the piston, they wouldn't have been listed as an alternate in the service manual in the first place...........
-N: Strengthened G.E. whatever that means...guess E means electrode, no idea as to what the G stands for.
10: 0.040 gap which Mercury engines use. No number puts it at 0.030-35 inch range.....larger gap allows the energy to rise to a higher level aiding in firing dirty plugs. Since Merc boasts of 40 kv open circuit, no problem jumping that gap.
Final note. What you do to/with your rig is your doing, not mine. I tossed out some information in an attempt to help you in your quest for faster starting and better idling possible improvements. Have no idea as to whether or not they will help your situation.
