Somebody messed up the carb rebuild, would be my first guess. Or maybe it was never adjusted properly.

 

i'm with scot, the float adjustment is not properly set, or the jet is partly blocked . it's running to lean making for the need to choke it helping over come the leanness

 

Also, double check for any intake gasket leaks and be sure there are no open vacuum connections.

 

Do you have a manual to show how to time the Mag..?? If not, here's instructions copied from the I&T manual, that should at least eliminate that part. How far did you tear the carb. down..?? If you found sediment in the carbs. bowl, then I'd suspect the little cross drilled holes in the discharge nozzle need cleaned. Just spraying carb cleaner through it won't do it. It needs to be removed, and visually inspected. Hold it up to a bright light, and see if you can see through the holes. A wire plucked from a wire brush, or strand of wire from automotive type primary wire makes a good cleaner. Then flush with spray carb cleaner, and blow through it with compressed air.