I have the same problem with 2 of my 120's every year. I first fill the tank with fresh gas, with a little Sea Foam added. When preparing to change the plug for the season I use a small syringe and put a VERY small amount (half a thimble full) of gas in the chamber and put in the new plug. I raise the track off the ground with a jack, then with the choke pulled, give the rope a pull. It will start and just give it a much throttle as you can and push the choke in slowly as you can.
I have only had to repeat the process just one time on one sled.
For some reason whether its the float sticking or what I don't know. My sleds use to be the same way...fill it with fresh gas, first pull and the kids were gone.
Good luck... I know its enough to make you wanna scream. A little gas engine getting the best of you.
Dog