My sled has one cylinder that is laying down under mid-range operation. It doesn’t completely fall off to one cylinder, but it kind of pops and backfires. The situation will occasionally resolve itself after about an hour of running, or putting in fresh plugs normally cures it. I am confused because the plugs look good, and I am fairly sure that I don’t have fuel problems. I should also mention, this seems to happen after every cold start. Any suggestions would be greatly appreciated.
First off add some isopropyl to the fuel and see if it works better after the engine has run long enough to work it through the fuel and combustion system. Getting deeper, you could check the compression on each cylinder to make sure they are within 10% of each other. If they are, remove the carbs and check to see that all of the piston intake skirts are intact to allow proper intake vacuum for fuel. This could also be a matter of sucking snow dust into the combustion chamber, giving the impression that a plug is fouling. Plugs can’t burn water, and will act like they’re fouled.