I just bought a 48" Scag hydro W/B. It has Velocity deck which makes it at least an '06 if my info is correct. It ran great first few times I ran it, but I started it the other day and the idle has turned rough. Almost a repeating "surge", not a steady idle. When I throttle up the pattern happens faster. I changed plugs, fuel filter and hose.... No change. I'm seeing some black smoke puffing out on start up also. Plugs were dry, but obviously old. Could you guys help me out?