look up the engine  model by serial #  in marine.com to confirm the suggested  plug type.

Mercury Marine Outboard Motor Parts by Engine Model (marineengine.com)

on the 7.5  I see some  ser#s     with conventional gap plugs   yet my  1970’s   little 4.0 hp  uses surface gap  plugs  and runs all day long idling without a hickup.

https://maxrules.com/oldmercs/timing/55.pdf

outbnut2, The best way to check outboard performance is on a boat. Winter in the north, we have to use a test tank. The best is if we could afford a factory style outboard test tank, with test props, that would be ideal. Did you use two separate inline spark testers when running motor, to make sure that you are not losing spark, at any time. When you rebuilt carb, initial idle needle should be 1.5 times out from lightly seated. Motor should start, let warm up, check RPM at idle, should be around 500-550 rpm in forward gear. Adjust carb, turning clockwise until motor runs lean, back out 1 turn counterclockwise. If engine hesitates during acceleration, carb is adjusted to lean, readjust carb 1/8 turn counterclockwise at a time, test motor for clean acceleration, full throttle to clean engine, then back down to idle. A tachometer is a must for tuning any engine. Doesn’t take much movement of throttle to come off of idle circuit to be running in high speed circuit. That’s what makes it so hard sometimes to adjust idle circuit, that’s where a tachometer does it’s job. The important thing is, make sure engine is in forward gear when adjusting carb or timing. I’ve got 3 of these mercs, 4hp, 7.5hp, 9.8hp, in my book, that’s the best fishing engine on the market. As far as spark plugs, I ran both spark plugs, surface gap, and conventional, both ran good. Seems to me I read someplace, if you ran these motors trolling all day they suggested using Champion J6J, instead of the surface gap plug, for running better, if you liked to troll. Gene.