Later 1200s have an idle-cut solenoid, engine won't run under 1500 RPM if this is not connected to 12V. It will run when the choke is on.

There are two idle screws
* Idle-mixture screw
* Idle-speed screw
Both need to be adjust on the car, AFTER the rebuild.

Yes, timing can cause problems. If it is OUT, then adjust it to what your cam grinder recommends. There is a range of timing that works, it doesn't have to be stock. If it is too far advanced, it will idle well and have plenty of power at low RPMs but won't run over 3000 RPM or so. If it is too far retarded, it'll be low in power and tend to run hot.