You time schedule for jetting for starters. Have you done any internal mods to the engine? what type exhaust are you running? what filter? a stock 400 carb is jetted at 148 main and 38 slowjet I will almost gaurantee you will have to go to a 155 and 42 with no mods maybe more if you can't get it to run great. All I can say is order a hand full of jets (slow and main) from rockymountain ATV (Cheapest I've found) and start riding and changing. It's going to be a long process so be paitent.