I have a craftsman 18 HP Briggs and Stratton that I can't get started. Model #917.271760. I have been searching the forum and think it maybe the valves need adjusting. The mower ran fine last week and when I tried to start it this week, the engine cranked a few seconds and backfired. I took apart the carburetor and cleaned it, no change. I unbolted the muffler, no change. I replaced the key switch, no change. I replaced the coil, no change. I replaced the plug, no change. I pulled out the plug and turned over the motor, and got what looked to me like enough spark. I checked the electric plunger at the fuel bowl and it seems to work properly. I dumped a little gas down the air intake and it still won't fire. Just keeps backfiring. Does this sound like a valve problem? If so can I get instructions on how to adjust them? Any help would be appreciated.