If you look at rudderless notes. He is running a NGK b6 on top and b5 on bottom. I am guessing he is running B6HS top B5HS bottom You might consider trying what he has learned about the plugs.

I think I was running a heat range of B7HS in both plugs and I used them in all 3 motors both cylinders which is a bit cold for that motor in my humble opinion but they are being ran in Key West its a bit hot down there and I pulled the thermostats on them one of them had sand up to past the bottom cylinder in the water jackets when I pulled the head. If you hit a sandbar down there it will fill one up with sand quick. I decided to pull the thermostats to help more water flow and less likely hood of that happening. Again just my humble opinion my wife hit a sand bar in VA Beach once and filled the motor water jackets with sand so fast that we had to walk like 8 miles down the beach to base to get the truck. I bet I pulled the heads on that 140hp 3 times to finally get all the sand out of water jackets. It would start and circulate water great then 2 minutes later stop. The sand got hot and formed like concrete in the water jackets and would stop water flow.

You might try his heat range on your plugs might help dial it in.

I did find out the BR7HS didn’t work as well the R is for resistor. and wasted power less spark.

You don’t want a resistor in the 9.9s just my humble opionion. As NGKs numbers get lower the plug gets hotter. A 7 might be a tad cold for that motor? again just my humble opinion. A ngk B6HS may give you better results it would be closer to an 8 in Champion.

Like a champion L77JC4 the heat index on it is almost an 8 which is way hotter than a NGK 7 again my humble opinion. I would try what rudderless did he has 3 years of messing with these. Don’t re-invent the wheel.

Grinding the side electrode and orientation when plug is tight in cylinder are great old timer tricks to keep plugs from fouling especially when a lot of trolling is involved.

Hope that helps,

Joe