Surging at idle? High speed? While accelerating? Surging = lean running, often caused by a vacuum leak. Have the customer put his hand over the top of the carb (air filter removed) and see if it does anything other than kill the engine within 2 seconds. If not, then vacuum leak somewhere (could be carb, could be somewhere downstream). If not a vacuum leak, check for blockages in the high speed, idle, and power circuits.