The enging in my riding mower is a 17.5 hp B&S. It is backfiring from the exhaust and the throttle is hunting. Originally I thought I had carb problem. Have had the carb off several times to check, recheck. Have checked, set, reset the valve clearances. Have checked and replaced the flywheel key. Air gap been set. Before all this, it was blowing back through the carb. Isn't doing that anymore. According to owners manual, valves should be set with piston 6mm past TDC. Since I don't have a dial indicator, am guessing at this. But I know that this is a very small measurement (@1/16"?). How critical is it that the vales be set at exactly 6mm past TDC? Since I have ,IMHO, check out the minor stuff, I am thinking that there may be something internal wrong. Possible burnt valve, blown head gasket, crank timing gear key broken. This thing will not win the battle. Any suggestions?