My problem is the mower doesn't engage the wheel drive. When the engine is off and I pull the drive handle up it engages and I can't pull the mower backwards. With the engine running it either does nothing or makes a repeated clunking sound. On the occasion when it does engage it will pull the front wheels off the ground. I just replaced the belt, it was 21 years old, but that didn't seem to have any positive effect. So, my questions are; is the clutch in the transmission serviceable; this is a two speed manual. Finally could the drive cable be stretched? It's fully adjusted and there is elongation of the spring.