An engine includes a throttle, a drive shaft, a driven shaft, a continuously variable transmission coupling the drive and driven shafts and providing a mechanical advantage therebetween, and a governor that operates the throttle as a function of rotational speed of the driven shaft. The transmission includes a first wheel coupled to the drive shaft, a second wheel coupled to the driven shaft, and a belt coupling the first and second wheels, wherein a working diameter of the second wheel changes as a function of torque acting on the driven shaft, thereby automatically changing the mechanical advantage in response to torque experienced by the driven shaft, and wherein a working diameter of the first wheel changes a function of tension in the belt.