Lawn Mower?

My harrier Hayter is only working at low revs and I don't understand why. There are some springs that seem to work the throttle at the carburation end near to the primer cold start. If I pull on one of the moveable spring levers it revs the engine but it tends to flucturate in the sound of the revs. Can anyone help point me to what is wrong with the mower. I change the oil regularly but I have never touched the carb. I regularly clean out the air filter sponge.
Thanks Chris