Engine backfires badly on start up. New plug and gas filter,also checked flywheel key and removed cyl head and checked valves, which are a little worn but no real problems. I also had tried a valve adj. first with no luck. I had thought I fixed it first with a valve adj. as it ran real good,but two weeks later when I tried to start it ,it would only back fire and wouldn't start. The push rod for the exh. valve is aluminum and was wore where it went through the guide plate. I think I can just turn it around to reuse it. The only other thing I haven't checked is the carb and fuel shut off.Haven't put the head back on to retry it, I had the valves reground and will reseat them first,any other ideas, like maybe a bad Ign switch or loose wire somewhere, this is in a 5yr. old sears rider.Model 3107-type 0136-EI-code 000822ZD,thanks

Gary, it that one of the engines that has the plastic cam gear, if so that is possible the trouble, Light Mechanic

The problem tuned out to be a ign. module that would short in & out. Was enough to drive a guy crazy,In the past ii seemed either they worked or didn't,not so with this one, thanks.