20 years of acceptable performance on what was a "wrong" carburetor setup to begin with is much better that I would expect to happen. The carb(s) are worn out. Dual carbs on a stovebolt engine doesn't make good nonsense- - - -either run one carb, or three. Unless you have done a massive amount of internal modification to the engine, including virtually redesigning the cylinder head, the dual carbs serve no purpose. The shape and size of the ports are the limiting factor, no matter how much "improvement" is done to the intake and exhaust system. It's like joining two pieces of log chain with a paper clip.
A single Carter YF calibrated for a 235 will provide all the air/fuel flow the engine can handle, and it will be a LOT more reliable in the process.
Jerry
