My stx 15f sat up during the winter and the gas gunked up. I ended up replacing the fuel pump, fuel filter and plugs and gas. I cleaned out the gas tank. It initially ran fine for several hours. It then bogged down and would not accelerate. and then would die. I replaced the plugs which were black. It then ran again as long as you kept it at full throttle. When you idled down it die again. I could not keep it running. Anyone have any ideas?