Most likely, you have a stuck power valve piston that's holding the valve in the full rich position, and/or a warped carburetor body that is allowing a big enough vacuum leak that the piston can't overcome the spring pressure that pushes it toward the rich position.
If you've got about an hour to invest and some basic fabrication skills, that insulator is easy to make from a piece of phenolic plastic, rather than paying some outrageous amount for an original equipment part.
Jerry
