Another couple of things to consider. Typically if an engine starves for fuel under heavy throttle it will just bog or lose power and slow down, as soon as you back-off the throttle the engine will pick right back up. If it is popping out of the carb under heavy thottle, I would suspect weak ignition - can't produce enough secondary voltage to fire the plugs under load. When was the last time the plug wires and coil wire were replaced? How about the coil itself? If the engine has points ignition a loose or failing condenser will give the same symptoms. Another less common possibility is a flat camshaft. An engine with a cam going flat will run great until loaded up then it will pop or bark back through the carb. This can be checked by pulling the valve covers and watching the rocker arm movement at idle. They should all open the valves the same amount. If the cam is flat you will find one that is hardly moving at all. John