Most carbeurator problems are ignition related. Most ignition problems are fuel related...
Champions used to be great, they're are absolute junk now. If you have one in a running engine, take an oilcan and drip a little oil at the base of the insulator while it's running, you'll see it leaking compression right through the crimp. I had one crimp fail on the 350 in one of my boats, it spat MOST of the core out into the bilge, but a fragment did NOT go out, instead, it buried into the piston, and opened up the #1 cylinder wall. Ruined the engine, bent the rod and crank. No more!
The K-series Kohler has other habits that you should be aware....
The ignition points and condenser are frequent suspects. The points are unique in that the SPRING for the points, are IN the ignition circuit. THat means, when the points are CLOSED, current flows from the coil, through the screw terminal, through the SPRING, to the moving contact, which then contacts ground.
The problem here, is that if you leave the key on for any length of time, AND the engine is NOT running, AND the points are closed, that spring represents a load resistance, hence, it gets hot.
When you heat up a spring, it becomes un-springy. This results in an engine that will start, and run, but not run much past idle, all because 1) the points don't close tight and 2) will not cycle FAST ENOUGH to speed up... instead, they 'FLOAT', just like valve float.
Solution here is... at LEAST... replace points and condenser. Usual warnings of modern ignition 'kits' being low grade import, and frequently the condensers are junk right out-of-the-box, so don't be surprised if you get a bad one.
The ignition COIL of a K-series looks like any OTHER cylindrical coil, but it is NOT. a 4, 6, or 8 cylinder coil operates in an entirely different electrical circumstance than a single-cyl... a 4, for example, fires TWICE in each revolution... a SIX fires THREE times, while an 8 fires 4 times. The K-series single fires ONCE... EVERY OTHER ROTATION. This means, that at any given speed, the K-series coil is spending a considerably larger amount of time with the points closed (longer dwell) than any other application.
Longer dwell time at slow speed means considerably more heat developed... and the K-series is a vibratory torture platform for ignition coils and carbs, so don't use a generic coil... it will die very young.
The K-series, like any OTHER flathead, is subject to carbon gak buildup on the exhaust valve stem. When the engine is cold, it will seem okay, but once you get it warmed up, that gak will start to bind up the valve stem to guide, and the valve will fail to SEAT... and the result is a loss of compression pressure, poor scavenging, and no power. This usually happens when you're doing real work, and once it starts fussing, if you throttle it down, it'll idle (because the valve has more time to close), but you just can't get it back up to working.
Solution: Get yourself a bottle of Marvel Mystery Oil... and dump about three shot-glasses worth into a gallon fuel tank. When you run it, the whole yard will smell 'minty', and the mosquito population will go away soon... it WILL clean up the valve stem about halfway, and if you do that every 6-10 tank fulls, it'll be perpetually fine. If you choose to pull the engine down partially for a cleanup, yank the exhaust valve, chuck it in a cordless drill and introduce it to the wire wheel with a spritz of carb cleaner... and if you can find a proper guide cleaning tool, that'll do well for getting the ID of the guides clean.
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