Simplicity Sun Runner Idle Problem?

I have a Simplicity Sun Runner with a 12Hp Briggs Model 289707 Type 011301 Code 9509074D. Last year I bought a new short block for it and built it back up. It has a newer carburetor set as well. It has never started very easy and now I cannot run it at a high idle without it being fully choked. If I just the needle valve and idle screw I can dial it in well for a slow idle, but then when I turn up the throttle it barely runs. This model has the gas tanks below the engine so there is a Briggs pump 808492. I checked all the hoses and it seems to be pumping gas OK. Can it be this pump is weak? I'm at a loss as to what to try next. Any help is greatly appreciated.

Regards,

Jim