How can you adjust a Briggs and Stratton 16 hp twin mower engine speed It revs and slows revs and slows. Is OK at full throttle?

It sounds like the carb needs to be cleaned , it is surging for fuel .

Yes.

I have cleaned the carb, put in a new petrol filter and renewed the diaphrgam of the petrol pump but it still surges.

Any other ideas please? Is there anykind of adjuster on the machine . if there is try adjusting . If there is a fuel inlet on top of carb , there should be a plate that you can take off and that is where the jets are , try cleaning them .