Had a serious bog about 150 km out on first day of 6 day trip. Had filled up with gas about 15 km earlier (used different pump from other 3 riders; they had no problems). Was leading our pack, and stopped for next sled to catch up (engine still idling). When I gave it gas, I immediately noticed no power above 1/3 throttle. After a few kms, engine bogged right down. Stopped & checked/changed plugs (both were fine). On restarting, had power to 1/3 throttle (about 75 kph) but not above. Soon figured out I could ride about 3 km at 1/3 throttle, until engine started bogging, then stop/kill engine/restart immdiately, and I was good for another 3 km. So I rode over 100 km this way (on a twisty trail, so I didn't slow our group down much). The restarting sequence was like giving it a cup of gas; when the cup was empty, the sled bogged down (but I was still restricted to about 1/3 throttle).
Took the sled into the Val D'Or dealer next morning, and their computer showed nothing. They said it could be bad gas, fuel pump, or computer. They had dozens of sleds needing work, and advised me to get new gas, and add fuel stabilizer (bought some from them) and ride to next town/dealer. So I filled up with new gas, added stabilizer, and went back towards our hotel. Once on the trail, the sled worked flawlessly -- the problem had gone away. So we headed out, and I had my old performance back. No further problems on our 1660 km trip.
I've never experienced anything like this before, and have driven more than 50K km of sledding.
Would appreciate suggestions re what was happening.