Did you try larger pilot jets? I had this problem on a 580 and had someone tell me to go up one size. Seemed backwards at the time but it worked. Also,, check the E clip position on your needles, rule of thumb is to start with them in the middle position. Not too familiar with the carbs on this model but look for a plastic washer under the E clip for the needles, if they have them if one is missing this will cause an imbalance in your fuel delivery probably would not cause a bog but with carbs every detail counts. Not sure how long you have owned this sled but if it is a recent purchase check and see if the previous owner ran race gas with this setup. Also your pilot circuit might be clogged or partially blocked.
