I purchased a used 52in Exmark Lazer last year with approximately 2000 hours on it. I believe its a 1996 model, I don't know the hp rating on the motor though it is a Kohler. The problem that I'm having is that its backfiring unless the choke is pulled half way out. With the choke half way out or when the motor is under full load (blades engaged moving forward , choke in) the motor does not backfire. I've changed the plugs and air filter but this seems more like a Lean condition to me. I was told by a mechanic at local landscape company that this could be caused by a leaking intake gasket. Does this seem like a cause or should I be looking at something else. Also what size motor is on this mower? Thanks.