Small Engines (Lawn Mowers, etc.): 1715 Cub Cadet, forward gear, driven equipment

Question
Bought used, model 154081, in any forward gear under a load it's not smooth, jerks or skips especially under a load.  Have replaced worn idler pulley and drive belt.  Is there any adjustments on the drive belt, other suggestions on what to check?  Thanks.

Answer
Bill
The adjustments are the idler pulley it keeps tension on the belt.Have you checked the main pulley and belts? you can try these hints out as well.

Problem: Engine vibrates excessively.
Possible causes:

Engine is not mounted securely.
Blade or other driven equipment is unbalanced.
Crankshaft is bent.
Counterbalance shaft is not timed correctly.