That carburetor is too big for that engine. That's most of the problem.
Here's a simple formula:
d = Displacement in Cubic Inches. (d x 2) - 15% = CFM
EXAMPLE: d = 273 (273 X 2) = 546 (546 - 15%) = 464.1 CFM
I bet a 450 would make it run like gangbusters.