I have a '98 Big Boss 500 6X6 which has plug fouling issues. The machine will run well for several hours and then suddenly start fouling plugs very badly. I mean they are coated with soot and the bike won't get off of idle. I put a new plug in it once and it fouled that plug in about a minute...
I've gone through the carb and all of the jetting is correct for our elevation and temp, the floats are set correctly, needle clip is spaced properly...plus I've changed all these things an none of them make any difference. Air filter and breather filters have been replaced along with a new foam pre-filter.
I pulled the cam out and made sure all three lobes were well within tolerances and then set the valves.
By the specs the machine should be perfect...yet something happens after running for a while that causes this. After several hours it seems to go away and the quad runs great again. ('course the plug needs to be changed as it's totally fouled).
Been wondering about the oil system and wondering if it is possible that the oil is backing up into the motor instead of making it back into the tank...I'm new to these dry-sump polaris systems...
Any thoughts on this one?

 

Air/fuel mixture screw has been adjusted correctly I assume. Try a new pilot jet(I've seen them wear to the point of running rich, unlikely though) ETC adjusted right? Choke adjusted corretly? Check for a pinched plug wire(They had issues with that, though it was later years) Runs good throughout the full RPM range? Check the diaphragm on the slide, is it loose to the slide?