Have Montgomery Ward lawn tractor. Engine lost RPMs and will barely pull the tractor. Changed the plugs and tampered with the carburetor. I can get it to rev ? spring missing. Can't find a schematic for the carburetor or throttle mechanism. Power Kraft 18/42, model TMO-3500006, with a Briggs and Stratton model 422707, type 1214-04 engine. Appreciate any recommendations. Thank you