I've rebuilt several K-series, I have a beastly little K201 in my Cub Cadet plow/puller tractor... and it is REALLY happy to work hard.
The K-series Kohler went through lots of little improvements over time. The location of spark plug was ONE of those improvements. As you noted, the burn efficiency is better with the ignition source located over the exhaust valve... that's because, being an L-head, the valves exist in a 'pocket' off the side of the chamber.
L-head engines naturally don't develop much compression easily, and trying to increase compression by decreasing the head-to-piston clearance puts the chamber in a 'shrouded room', so expanding chamber pressure builds up in that pocket, but the piston is 'over there' and doesn't rapidly receive combustion pressure as it would in an overhead-valve design.
What they found, is that the central-location design would present good ignition in the cylinder, but there would be a significant amount of unburned fuel (carbon deposits) building up on the exhaust stem and port interior... and this was caused by the flame of the centrally-located spark spreading out to the edges of the CYLINDER, but not continuing into the VALVE POCKET. Effectively, the exhaust gases of the flame front would effectively 'isolate' the pocket, preventing any fuel in that area from getting enough oxygen to burn, so instead, it just sent it out the valve to become black goo.
By moving the plug to above the exhaust valve, IT was the first area to burn, and the flame propogated from there, into the chamber.
The downside, is that starting 'in the pocket', would be slowed because it was stuck travelling through a 'tunnel' into the chamber, but this doesn't become a serious impedance to power output until the engine's speed is significantly higher than it's intended powerband... which L-head engines naturally suffer anyway.
Basically, moving that plug brought up the horsepower a LITTLE, but reduced the fuel consumption very noticeably, and eliminated a significant amount of carbon fouling of the exhaust valve.
There's also an LP-gas head for the Kohler K-series... difference being interior volume. Gaseous setups require higher compression ratio, so they simply made it in reduced head volume. Competing guys will look for the LP head specifically for this reason... and then they'll go to steel crank and flywheel, forged aluminum rod and piston...
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