Ok all working on a kohler 15.5 cv15s engine. Motor was running good, all at once starting backfiring through carbuator. Pulled valve cover, exuast valve not closing all the way. Loosend the rocker arm, valve then closed all the way. Pulled the lifter out, it's ok. Pulled bottom of motor apart, acr working ok. Checked the valves, they are seating properly. Carbuator checked ok. Put motor back together, still backfiring through carbuator. What do i need to look for???????
Also pulled the flywheel, sheer key was ok.