Classic symptom of a bad accelerator pump.

A small pump that squirts a dose of gasoline into the airstream when the throttles are opened to skirt over the lean condition that develops before the main venturis can deliver fuel from the increased airflow.

You'll have to find a carb rebuild kit.......good luck with that.......and replace the pump diaphragm. A good cleaning of all parts will do the carb wonders. There is a special "carb cleaner" for that purpose........rather caustic stuff........don't know if it's available anymore.