Lawnmower drive works on pavement not grass
 

My self propelled Murray (about 9 years old) stopped propelling. I
changed the belt and cleaned the pinion gears (one was really gunked
up with grass & dirt). The belt is set to be the tightest setting
according to the holes on the idler arm. The mower now will move
forward on it's own, on pavement but still must be pushed on the
grass. When I push down on the back wheels to lift the front wheels
off the ground, the right wheel spins fast as does the shaft. The left
wheel (which is by the belt) turns, but slowly. What could be the
problem? JB in CT