Exstened tip spark plug advances timing

champion RV15YC4 plugs in it, i have never changed the plugs in it, they were in it when i bought the boat. not sure why they went to this plug???

??? hows that possible?

my understanding was extended tips are to try to get more into the center of the chamber, and they also tend to foul less?

What carb and intake is on it and was there a carb adapter to go from a weber holley square bore bolt pattern to the stock gm spread bore if so that is your lean problem it is a
sh!tty set that mercruiser will do to make the cheapest set up to make it run. they did that with all the 330's

stock carb and manifold on it.

well heads are off, cylinders 1 and 8 are the leanest cylinders judging by the combustion chamber. i'm surprised how clean the chambers are for a 500 hour motor, actually everything on this motor is clean clean clean inside. bores look mint, still see nice cross hatching in them/ luckily no surprises have been found!

gibson exhausts

anyone know about magnaflow, or kooks exhaust systems I was thinking of those?

Extended tip spark plugs bring spark event closer to piston, similar to timing advance.

Extended tip spark plugs bring spark event closer to piston, similar to timing advance.

+1 I have seen similar effect on circle track engines where the plug seat was machined deeper to more the plug closer to center. It does effectively "advance" the timing.
Sorry, didn't mean to hijak.
The light color holes do look a bit leaner, but the heads and ports look overall pretty good! By any chance you didn't happen to run a tank of race gas did you? Leaded fuel will give a white/grey color to exhuast when tuned properly.