Check the ignition for a good spark, check the timimg and the firing order.
If they"re OK than suspect the carb probably, especially the transition port.( I think it may be called the "economizer") It"s the one just above the idle port and provides fuel off idle. The float level could also be low.
It could also be an air leak around the carb metering section. Try pulling the choke out a little when putting it under load. If it improves than check for an air leak around the throttle shaft, at the manifold to carb interface or at the block to manifold interface. Also look for rust holes or cracks in the manifold. CAREFULLY spray some starting fluid around these locations with the engine idling. If you hit a leak spot the engine will speed up.