Your problem is either-
1. a clogged jet
2. the float is full of gasoline
3. a bent flywheel key

The fact that it will only run on full choke points toward a clogged jet.
The fact that it backfires points towards a bent flywheel key (i.e. the timing is off)

My bet is the jet is clogged. At the bottom of the bowl, there is a bolt. Pinch off othe fuel with vise grips at the rubber fuel line. Remove the bolt. You will find that the bolt is hollow with a pin-sized hole on the side of the bolt. Soak that bolt in carb cleaner and blast it with compressed air. Make sure the pin-sized hole is clear as well. put it back in.

If you want to check the plug gap, it should be .30
If you want to check the air gap on the magneto, a business card sandwiched between the magneto and the flywheel is the proper gap.

If the jet is clean and it still backfires, I would take the float out and shake it to see if there is any liquid inside it.

__________________
Ben
1987 190d 2.5Turbo