Need help on new rebuild

Lets break this down. That way you can isolate the issue.

Before that, clear away a few things.
Has the engine been broken in? Say 20 minutes at 2000 rpm or whatever your favorite procedure.
Initial timing was at what rpm?
- Was this measured with or without vacuum advance? (should be without)
How low of rpm would the engine run at before dying?

Take a deep breath.
Check the fuel level.
Get it running at the lowest rpm it will run smoothly at.
Check the timing again if needed.
Adjust the idle mix screws in and out.
- What happens? Is there response? Can the motor be shut off whith the idle mix run in? How many turns out to get it running smooth?
All of this ^ is to see what ways the carb and fueling may be the problem.

Add some timing and see if you can then slow down the engine with the throttle position (idle speed) screw.
^ This is to see if its timing related.

Obviously there's spark, so it comes down to fuel, timing or compression. So the third thing to check will be compression. But I would start with the two above. If you have vacuum gage, it will show poor valve seating, so thats an indirect way to check the cylinder is sealing.