Uh-oh...flooding

When the engine starts up just fine but then runs poorly, it suggests that the carb isn't boiling but the line between the fuel pump and the carb is getting hot. As soon as the engine starts running, that boiling-hot fuel is pumped into the carb and things get rough and choppy until cool fuel from the tank replaces the hot fuel. Work to reduce in-line heating of the fuel: slit a length of 3/8" fuel line hose lengthwise, push it over the pump-to-carb line, and secure it at bends and ends with hose clamps. Remove any metal clips holding this line to the engine.

Tell us more about the upgrade you did to the fuel tank venting system. What fuel tank did you use, and where did you run its vent? Does your 2280 have an external bowl vent (some do, some don't)? If so, where is it routed?

On your '71, the original fuel cap would be a pressure-vacuum vented item. If you are using a non-vented cap of some kind, that's wrong.