Small Engine Troubleshooting Problems? Ask a Mechanic Now.
What deck do you have? 290, 54 inch deck? or 291, 60 inch deck? The gearbox is mechanical, but the blades are belt driven. The belt has to be tensioned by the spring on the deck. Is the belt nice and tight before you close it up with cover? All spindles are free and blades are in good shape? I don't see this as drive problem, but deck problem.
Is the actuator moving and tightening the pto drive belt? Does it have vacuum from engine, or is vacuum line leaking?
Actuator may be leaking, not keeping belt tight