B&S Engine Trouble

I am working on a Yardman riding mower with a Briggs 31C707 0603 B2 engine.

An older neighbor brought this to me for repairs. He said it started backfiring 10 minutes after he put in some suspect fuel (don't really think that had anything to do with it, but...). I have dismantled the engine. Cam lobes appeared normal as did the other internal parts. Reassembled with new coil, replaced intake pushrod (bent), adjusted valves (intake - .004/exhaust - .006), cleaned carb and fuel system. Exhaust valve guide had slid out toward rocker arm so I pressed it back in and had a spot of weld put on it through the exhaust port to keep that from happening again. Crank key was perfect so timing is not the issue. Started it and IT'S DOING THE SAME THING - backfiring at any speed above an idle.

I did notice that the exhaust valve does not open as much as the intake valve. Before I disassemble the thing all over again to recheck the cam lobes (again, they looked normal), how much should the exhaust and intake valves open?

Did the chat thing with B&S and they just sent me to a dealer. The nearest one had nothing useful to contribute. My simple "favor" for a neighbor has gotten way too complicated! Any help would be greatly appreciated!!