Jerry,
Put a vac guage on the intake manifold and adjust timing to highest vac reading ~ on a new engine it should be easy 19-22. If not then drop back and adjust the valves, then go back and adjust for highest vac again. I would also open up the carbs and check float drop and level settings and trim the gasget if they are hanging up. Is there junk in the bottom of the fuel bowls maybe the fuel pump is worn out from a dirty gas tank. Put a clear inline filter somewhere before the fuel pump and one between the pump and carbs. Try running on one carb at a time, disconnect the linkage one at a time. Contrary to popular belief I found that a 235 with properly set up 2x1 bbl carbs don't like a lot of ignition timing. Good luck.
