what jets?
After you check your plugs etc to see if you are running too lean, I would go to your friendly dealer and get 2 of each size of the next 3 larger sizes and do some experimenting with them. It doesnt take much to richen jets. You failed to mention whether you have modified the air intake on it or not, but I am thinking that if all you did was to put on after market exhaust, that just going up one size in jets will be sufficient.
Ive been through this with my 79 XS1100 Yamaha motorcycle. It has aftermarket headers and a UNI air filter. Surprise of all surprises, when I went just one step larger in jets, it got too rich. Wound up going back and putting the stock jets in and setting the jet needle one step higher. The needle sticks down through your jet like on a Carter or Edlebrock carburetor. Mine have 5 rings around the top where a snap ring holds it to its diaphram. The factory setting was the middle ring. Putting the snap ring on the next lower groove richens the carb up and the more you twist the throttle the higher the needle goes, richening it even more. That was all it took with mine. You may even want to try that before buying other jets. BTW, my bike has Mikunis on it from the factory, and I am sure they are the same design as on your Snow Mobile.