I know this might sound stupid, but are you 100% sure that the sled is running on BOTH cylinders? I've seen people turn the idle up on everything under the sun (from chain saws and weed wackers, to cars, trucks, and farm equipment), all for the sake of compensating for other more serious problems that they didn't have a clue how to fix.
If you don't know, there are two real easy ways to find out.
#1.) while the sled is running, pull of one plug wire at a time from the spark plug. You should hear a noticeable drop in engine RPM and engine sound when you unplug each wire, and then an increase when you re-attach the wire. Again, do one at a time. If you are worried about getting "zapped", use some insulated handled pliers and wear a DRY leather glove also. Othewise, just grow some balls and just pull 'em off. It only hurts for a second anyhow. LOL
#2.) You can put your hand on the side of each cylinder jug. One side will be stupid hot and the other will be warm to cool if there is a dead cylinder. Otherwise, both should feel roughly the same temperature. An easier way to do this is with an infrared heat gun, but still not everyone has those these days.
Ok, so that's just my first thought. Not saying that's what it is, but it's a starting point.
Judging by your comments about the carbs, it's sounding like they've possibly been all monkeyed up by someone? If so, time to get a manual, remove them, clean them out, inspect them, re-adjust them, re-install them, re-synchronize them, and get her up and running again.
I would also take a compression test on each cylinder and see what I have. A blown head gasket can cause lots of strange problems, and sucking air can lead to high RPM's as well.
There's also the outside chance that you have bad crankshaft seals. They will suck air when bad, and create a lean run, higher RPM condition.
What else? The boys already covered checking your carb intake boots. They should not have any tears in them.
If you remove the carbs and the intake boots, might as well check the reed petal cages. The petals should not be burned or broken on the reeds. (However, that shouldn't have anything to do with a high idle problem.)
Sounds silly, but check your throttle cable routing. I've seen people route the cables incorrecty sometimes, and that causes the throttle pistons to be pulled up by the tension of the cables themselves accidentally, even when you aren't touching the thumb-throttle lever.
There's probably more, but these are just some ideas off the top of my head.
Good luck hoss.
