Small Engines (Lawn Mowers, etc.): murray riding mower, hydro drive, shift keys

Question
I have a 2003 murray 21hp-46"deck.  it was fine but now wwhen i drive forward it is herky jerky??

Answer
The first question is do you have a your drive or a hydro drive the first things to check would be your belt, wheels, bearings, brake pad, brake adjustment and fluid level if a Hydro.  If it is a year drive unit it sounds like the shift keys are damaged.  This normally comes from shift on the go.  New shift keys are not difficult to put in the transmission will have to be taken apart.  Please get back to us if you need more assistance.