My John Deere 425AWS lawn tractor with a 60" mower deck and the Kawasaki FD-620D-AS02 engine has started misfiring under heavier load with mower deck engaged. It runs perfect without mower engaged. It was recently overhauled with oversize pistons and rings, and new valves and springs. I replaced the existing NGK BM6A spark plugs with new NGK BMR4A plugs and a Briggs & Stratton fuel filter. I am wondering if the fuel filter is too small and restricting the gas flow, or possibly the intake and exhaust clearances are to wide. Presently set at .020" cold. Or possibly could one of the ignition coils are weak. Also maybe the problem is with the high speed carburetor jet. Any help before I have a John Deere dealer look at it. Thank you.
