You are pumping the gas pedal once before you try to start it? Is it setting the high idle right? Right after it starts, it should be more like 1200 rpm or so until you push the choke back in.
With a manual choke when cold, push pedal to floor, set the choke, let off the gas pedal, and then try to start it. As soon as it starts, push the choke in an 1/8th an inch or so.
When warm (and in warm weather this can be several hours after it has last run), then make sure choke is all the way in and just start it. Don't pump or set the choke.