Duster running rich

You have to begin with the timing in the correct ballpark.
Do what Crackedback posted.
I'm going to change the order a little bit since this is all new and the engine starts for you.
A. Get a Timing Light, Tachometer, and buy or make timing tape/marks.
The timing tape or marks go on the damper so you can read timing more advanced than 10* Before Top Center (BTC). If youy happen to have a 'dial back' timing light, then you can skip this step and it will generally be accurate enough to get you by. Many also will show rpm. RPM is critical since the timing changes with rpm. You must know the rpm of each and every timing measurement.

B. With the engine running, get the timing at the slowest idle you can, and also at some other rpms such 1000, 1250, 1500, 2000. That's enough you can call it a baseline reference. You know the engine started and ran with this timing curve.
Side note: Normally timing is measured with the vacuum advance disconnected and the vacuum line plugged.

C. The 'idle speed' screw on the carb adjusts the throttle position. There's a limited range of adjustment where the throttle opening is in the working range. Too far open or too closed and the idle and off/idle will fuel mixtures will get out of wack.
Even without removing the carb you can get some idea of whether its in the working range. For example, if there is no or little response when turning the idle mix screws, then the throttles are probably too far open.