If it has a Holley carb you may want tap on the float bowl with the back of a screw driver while it is running. The carb float may be stuck not allowing the needle and seat to open properly. This would not allow enough fuel in the carb bowl and the engine would idle fine but not allow higher rpm under a load.

Most Holleys have a float level screw about 2/3 of the way up the bowl remove that scew and the fuel should be at the bottom of the hole while running.