'03 Toro 36" WB (Pistol Grip Gear) w/ 15HP Kohler. It just started doing this on thursday. After being started, when you engage the blades, it drops in RPM, shoots back up with a little backfire / "puff" of black smoke from the exhaust, than runs and cuts fine. No problems while running, only when you first engage the blades. Any ideas? I loosened the deck belt as far as I could to see if it would stop "bogging down", but no luck.