I have a 17hp Kawasaki FH500V-AS06 on a Exmark Turf Tracer that has an issue with backfiring when running unloaded. It backfires loudly with fire coming out of the exhaust every few seconds (but not every stroke). When running loaded with the blades engaged it does not backfire. I have replaced the spark plugs and adjusted the valve clearances and replaced the carburetor. No change so far. Any suggestions would be appreciated.