If you've got a manual choke, try pulling the knob out a little to create a slightly rich mixture. It's possible the power valve in the carburetor isn't opening up and the drop in manifold vacuum as you go to open throttle is causing a lean-out. Have you checked the fuel volume to see if the pump is delivering enough gas? Disconnect the fuel line at the carb, run the line into a 1-quart catch can, and crank the engine up. It should flow at least a pint in 30 seconds at idle speed, running on the fuel in the float bowl.
Also, try a cylinder balance test, where you run the engine at a constant 1,000-1,200 RPM with a tachometer and a vacuum gauge connected. Disconnect one spark plug wire at a time and record the RPM and vacuum drop for each one. If you've got one with a leaky valve, etc. the RPM and vacuum will not drop as much as a healthy cylinder.
Jerry
