The header with no heat riser absolutely makes a difference. Choke to start then push it in half. a few minutes at least with the choke at 1/2 to 1/4 until warm. I would usually start rolling after a couple of minutes warm-up with the choke 1/4 then open it up all the way once I was down the road. We've had these Butt Cold temps lately too. That 40 spent winters hibernating in the garage. I would get it out for snow runs occasionally and it always started and ran even after sitting weeks in the garage in the cold. It only needed choke in the summer for initial start up.