Small Engines (Lawn Mowers, etc.): B&S 18.5hp rebuild, camshaft timing, combustion chamber

Question
John:

I tried your suggestions no luck. It will backfire sometimes from the muffler and from the carb. I've checked the cylinder position when  the amature is lined up with the coil and I think it is firing at the wrong time. The cam and crank timing marks line up and the flywheel key is new and not sheared. What else might it be. Thanks
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Followup To

Question -
John:
Recently my Craftsman riding mower abruptly quit. B&S 18.5hp Intek (single cylinder)31p777 0348e1. This unit is 3 months out of warranty so no help from Sears. As I have done in the past with smaller engines I tore this unit down and replaced the broken connecting rod. I also replaced the crankshaft because of journal damage and used a new piston and rings. A complete inspection of the block did not find any damage. When I reassembled the unit I couldn't get it to crank due to the compression buildup. so further investigation revealed that the compression release device on the camshaft was missing. I replaced the camshaft (timing marks matched as required) and resolved that problem. I adjusted the intake and exhaust valve tappet clearences as per my B&S manual. The unit will not start. I have good spark and the carb appears to be supplying the necessary gas. Even putting some starter fluid directly into the combustion chamber it is not firing. Sometimes I get some blowback from the intake. I've added fresh gas so that should not be an issue. The flywheel key is new however I still find myself questioning the timing even though the marks on the crank and tha cam are matched. What am I missing. I have extensive rebuild knowledge with the smaller engines but not as much with the OHV units. Thanks in advance for your assistance.

Answer -
 Hello Bill:

 Pour a Small Amount of Engine Oil Into the Spark Plug Hole and try to Start the Engine. I Set the Valves on Rebuilt Engines at .013in Intake and .017in Exhaust. Sometimes the Setting Briggs Gives are too Close to allow the Valves to Seat Properly. What was the End Gap on the Rings? Did you Offset the Ring Gaps? Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John

Answer
 Hello Bill:

 I have Read and Re-Read Our Conversations and am Siding with You on this. If the Engine has Good Compression, Is Getting Gas and Is Firing at the Proper Time; then there is No Reason the Engine Should Not Start. What is the Compression? Is the Cam Gear the Plastic Gear? Are you Able to Send Pictures if I Supply an Email to Send them to? Hope this Helps. I am here if you Require more Assistance. Dont Give Up. I have Engines Like this All the Time. Let em know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John