How do I adjust the throttle on a Qualcast 43S petrol mower?

My 43s is an old model, well old, black engine with green body. Throttle assembly is connected to an arm which is connected to a small shaft protruding from the crankcase. I cannot find out how to adjust the throttle, but the engine will start and run on the throttle stop adjustment. The cable is okay and levers move when it is opened. The arm is not loose on the shaft from the crankcase. It is just that nothing seems to happen on the carburettor when the throttle control is moved. It seems that there is some sort of governor inside the crankcase, but is does not appear to move when the engine is speeded up by manually opening the carburettor butterfly valve.