The Classic Briggs and Stratton valve guide sliding out of place. You will find that the guide migrated out of its place and the pushrods are all bent up not allowing the valves to open at all.
Flyin2low is on to something. That B&S twin had a rash of problems with Bad pushrods also. They used aluminum tubing with billet ends to save a buck. I'v done 3 in the past couple years, all the same,Bent exhaust pushrods that popped outta the rocker arm cups. Deceptive, cuz you's still get full compression readings. With the exhaust valve not opening, You'll definately get backfiring thru the carb. I've even seen the air filter tops blowing off under that situation. They even ran [poorly,on just one Cylinder firing. Easy to check, just pop off the valve cover tops.The local dealer didn't have any T.S.B's, but 3 of the same motor is more than a coincidence.The sad part is that the replacements are still aluminum w/ the billet ends. I think the only way to avod a repeat [if thats the problem] is annual valve lash adjustment, as I believe that too much clearance is what allows it to hop ot & get mashed up while running. If that's it, I'd love a reply,outta curiosity. Double check, but if memory serves, the correct lash is .004 to .005 thousands clearance, Also, has any of you all ever encountered that ? Good luck, keith / Sgtrudz. Semper Fi.