Briggs & Stratton Engine, Can not keep it running?

I have a great little 12 horsepower Briggs & stratton garden tractor. I have used it for the past several years to cut the grass and it has never caused any problem. This year I picked up a snowblower attachment for it. I have used the blower once already
and it had no problem moving the snow. The problem is I had to play with the choke and throttle to keep it running. This is the first year I have ran the tractor in the cold???
Also, under the carbeurator there is a round, what I believe is the fuel filter. On the bottom of the fuel filter there is a screw with a spring on it. This screw vibrated loose I guess as it came out while running the blower. Is there a setting for this or just tighten it up. I thought it was for adjusting amount of fuel the engine gets so it should have a certain setting, I do not know what that setting is.

THANKS