Motor pulsing usually indicates lack of fuel supply, remove the carburateur and clean the jets, clean the jet set pilot, clean the inside of the carbi with carbi cleaner and blow with compressed air, flush the petrol tank and fill with fresh fuel. It is important not to change the original setings of the air fuel mixture on thiese mashines, as well as the maximum and idle revs as altering those settings can cause the engine to pulse.

It sounds like you either have a clogged nozzle or one that is too small for your pump. At lower speeds the pump is pumping less gpm. So your unloader valve does not cycle. Then when you go to full power. The engine is running faster and pumping more gallons per minute. The nozzle is too small or clogged and so the bypass opens and closes ( surges) Try a new nozzle that is the right size for your machine.