My little 9 hp keeps backfiring. It happens when I stretch her out, say above 4000, and while she's backing down it consistently backfires. Foot off the pedal, butterfly closed, ignition disarmed, (cuz foots off the pedal), and valves adjusted to .002 intake, and .004 exhaust. Valve settings are per the manual. Any idea why I'm getting the backfire? It sounds like it's in the exhaust and not the carb.