What Tinkerboater and Gordo said.

Make sure you run the correct heat range plug for your motor.

Check the compression. Hopefully you don't have a detonation problem.

Are you sure you didn't run out of fingernails and start
chomping on the plug

Do you guys think that all of those 4-5 mile WOT trips finally caught up with it?

No wonder you are sexually frustrated if my my hands looked like that my cock would never let me touch it either.

This stuff is the best after a $hitty day at work!!!

Good one Lapse of Reason, I'm LMFAO !!!!

always use new plugs after fogging oil. not only does it glaze the plugs it can effect the heat range. an earlier post said it correctly. run the engine on the old plugs to burn off the fogging oil. due a summerization tune up,ect. take it easy on the old fuel. there are different fuels for winter and summer and around here we start getting into winter fuel in October,so keep that in mind as well. make sure to wash your flame arrestor. fuel filters should be changed after you run it for the first time in the spring, the reason to do it now rather than in the fall is because all the sediment,water and debris will migrate to the bottom of the tank. the boat sit's still all winter so it is like vacuuming the bottom of you tank and replacing the bag.

i have also seen that condition i a 4.3 before. this one was a 1988 model,middle cylinder port side. checked out ok and still running today.

get rid of the champion plugs, ngk v-grooves are good.

enjoy

5 mile WOT runs should be no big deal on a stock motor running in the recommended rpm range, assuming clean oil and proper state of tune (and fresh gasoline). I had a Wellcraft with a stock 260 Mercruiser and we put 700 hours on it, probably a full third of it WOT. It was not unusual at all to run 4600 rpm for over an hour at a time. The boat was parked under a gutter and was accidentally "sunk" and after that, the rings didn't seal well and there was excessive blowby so I pulled her down for a look-see. Cylinder bore had a noticeable ring ridge at the top of the stroke, but was otherwise in good shape (except for the rusty rings). Rings, bearings, a pass with the ridge reamer, and a few strokes of the hone and it was still going strong at 1100 hours when I sold her (she got a Comp 268H cam, a set of cheap springs, and some hand-lapping on the valves, too, while down).

As far as plug brand and style, I'm not a huge proponent of any particular plug on a stocker - whatever the parts house hands you will work okay as long as it is a direct crossover for the Merc recommendation..