Having trouble with me 22 HP garden tractor with B&S twin engine. It was running kinda rough, changed plugs, filters, gas filter, oil, but still ideld rough and lost power while mowing. Added Seafoam, as recommended by a brother, and now it idles much better, but still gets rough under a load, such as engaging the mower deck -- sputters, poor power, jerks and misses. Occasionally backfires when shut off. Any ideas, guys?