I have a 1998 Craftsman lawn tractor with a Briggs & Stratton 20.5 HP twin cylinder engine that seems to be losing power as my 42" cutting deck is really lugging. The deck is fine. the tractor we got with the farm last year. It smokes when engaging the mower. I put oil in in every time I cut the grass all 2 acres. Up until now its been fine. Today more smoke than ever. I did a compression test & both cylinders are at 90lbs & holding. I'm thinking the motor needs a little freshening up. What should the minimum compression be? The tractor had probably sat a couple years before we bought the place.