Lately my 1996 JD425 AWS engine started misfiring under load. A year ago I totally rebuilt the Kawasaki FD620D-AS02 engine with oversize pistons and rings, new intake and exhaust valves and springs. I only use 91 non-oxy gasoline and tried changing the spark plugs from NGK BM6A to NGK BM4RA as recommended by Kawasaki. So I now wonder is there something that changed in the timing!! Is it possible to adjust the engine timing without adjusting the timing gear? Any suggestions appreciated. Thank you.