Rebuilt 273 won't idle

I have found a few carbs with messed up throttle plates, things like flipped plates, or miss adjusted, so they bind in bores. One way to inspect is with a light, look for gaps when closed.

How are you setting timing? If with a light running at 1500 RPM, it needs to be at least 20 to 25 degrees. Changing to 15 with no change indicates, other issues, 5 + degrees more should be noticeable.