We use to race snowmobiles but it's hard to figure out what's going on unless you are in front of the machine itself.
The low end may be flooded with gas, this happens sometimes when you transport the sled with the gas turned on. Also can happen when the sled is just setting if the float got stuck for some reason and let gas drain into the low end.
The way to clear the low end is to drain the gas out and you do that buy taking the drain plugs out of the bottom of the engine and then holding the rear of the sled up high so the gas can drain out.
Another way to clear the top end of gas is to hold the spark plug over the hole of the cylinder with the key on and pull the starter over+++++WARNING***** DO NOT stand over the engine while doing this, you loose your eyebrows and possibility some hair.
Like someone else said, be sure to have some new plugs. That is very important.
Jerry