HELP! Nomad Overheating and Backfiring.

[sfisher59]

Hi All,
I'm really pulling my hair out on this problem. I've made some changes that have incrementally helped, but the problem still occurs.
My Nomad is a '99, completely stock. It had a problem of running lean at idle (i.e. I needed to keep the choke on for a couple miles), then about 8-10 miles out, the overheat light comes on, I felt a momentary drag, and then the backfire. If I roll off and ride slow (55-60), I can go probably 10-12 miles, before it acts up.
I have changed the pilot jet from a 45 to a 48, made sure the rubber oring/washer/spring was in the right place. The main jet is a 42 (original). I reset the float, as it looked like someone set it too low. It now rests on the needle with the float arm parallel to the carb body. The pilot air screw is at 1 5/8ths turn.
I want to go to 2 turns with the pilot air screw (with the larger pilot jet).
The thermostat seems to be working normally, and I've flushed, and refilled with fresh coolant. The coolant is flowing, and the fan cycles normally, and I checked the thermostat last night. It starts to open about 155 to 160, and is full open by 180.
What am I missing?