I have a 2009 foreman it is hard to get started and keep running when its below 20 degrees. I need to keep pumping the primer button the first minute or two to keep it from dying. Then it will idle but as soon as I touch the throttle it spits and sputters. If I leave it idle long enough it gets warmed up and runs great. Has anyone changed jets to help with winter starting? If I change jets, what size?

 

Colder = richer
Warmer = Leaner

Your main jet is probably Ok.....need to adjust your air screw a little richer. If it is an air screw, turn in to richen, if a fuel screw, turn it out to richen. Not sure which it is on your rig. Somebody will chime in and say. My guess is it is an air screw. Turn it in a bit and adjust your idle screw up abit.

 

Other things that might help....run a hotter plug in cold weather, and a proper weight winter oil.

Cold weather plugs have a shorter insulator, so they will get hot quicker and also you will be less likely to foul the plug. See your owner's manual. NGK Spark Plugs USA

A proper winter weight oil will get pumping faster and bog the crank less on a cold start. Synthetics also pump better in the cold than petroleum oils. I like Maxima 530MX and Maxima Ultra 0W30 for cold weather. Again, see your owner's manual for what is recommended.