I have a 15 h.p. envinrude outboard motor.it started skipping so i checked and one of the coil packs was bad.changed it and it and it ran fine.then the next fishing trip it started skipping and tottaly quit running.i took the carb. off and cleaned it and it was perfect.when you try to crank it it want and it back fires.the plugs seem to be firing but it will not crank,just back fires through the carb.can you give me any suggestions...thanks and god bless jimmy