When a sled sits for a while, the oil that is in the crankcase and on the bottom of the pistons and on the crankshaft settles to the bottom of the crankcase. When you first start the thread, the airflow picks up that oil and adds it to the oil you are giving, and you get a lot more oil than necessary. The plugs foul. That's more common on a sled that has been fogged.
Also, the sled is jetted for cold conditions. Running those jets in warm conditions makes the air/fuel mix really rich, which also contributes to plugs fouling. I've had sleds that could not move onto a trailer in the hot summer sun that ran perfect when it got cold, simply because of the jetting.
Both those conditions usually clear up when it gets cold and you run the engine up to operating temperature while riding. You could try smaller jets as a test, but when it gets cold, you will need to jet appropriately for the temperature to prevent overheating of the pistons because of the lean condition.
2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah