Re: 1975 Mercury 650 65H.P. fouling 1 plug
It could be metal and not have a magnet stick to it, if it's aluminum. Like burnt off the top of the piston.
These 3-cyl models have a lot of trouble with heat issues so you need to make sure the cooling system is up-to-snuff. Generally the recommendation is to replace the impeller every season.
If it's got a thermostat you should remove it.
You should check the Wide Open Throttle timing and set it to approx 18-21 deg BTDC. Not the original 23+ setting.
Use premium fuel only and a quality oil.
If I recall, there was a modification made to the block which moved the water telltale discharge point from the side of the exhaust manifold to the top of the block. This ensured that the block was completely vented & flooded with water so no "steam pockets" could develop atop #1 cylinder. When these motors overheat, the block usually cracks, not a good thing! To do the block mods, you'd have to know exactly where to drill/tap the hole, perhaps one of the other guru's here has access to the Merc TSB.
Last and very important, prop to the hi-end of the mfr's recommended rpm range at Wide Open Throttle, with the boat lightly loaded. Lugging these motors with a large-pitched prop will kill them in short order.
The fact that it'll idle smoothly for a long time but fouls after a hard run leads me to believe it could be melted-piston-aluminum on the plug but if not, I'd certainly give it a good tuneup, impeller, do the other things and see how she does.
BTW I'd also take a compression check to see if any one cyl has significantly lower readings than the others, indicating a problem. If you have any dlrs near you who could do a leakdown test, more's the better (leakdown test is a more accurate indication of internal conditions than compression check).
HTH & G'luck......ed
