Outdoor Power Equipment and Small Engines - b&s backfire at full throttle

briggs and stratton 25 hp intek v twin ohv,model 445777.
when running at full throttle it backfires. runs good at idle.when i ground out one plug,it runs same. when i ground out other plug it sputters and dies. got spark,adjusted valves,new plugs,new fuel filter,new air cleaner,adjusted armature air gap. what next? any help would be greatly appreciated.

About the next thing to do would be to try and rebuild the carb, get a carb kit, soak it in carb cleaner and blow it out with compressed air and rebuild it and try it.

By grounding out the one spark plug and the engine still running indicates that this ignition coil is no good. When you ground out the other and it stalls means that the first one is not firing and trying to run on one cylinder. Before performing a carburetor rebuild, test each cylinder for sufficient spark using a Briggs in-line ignition tester, part number 19368. If you find insufficient spark at the afflicted cylinder, you will want to remove the kill wire at that coil and then perform the test again, to eliminate any bare wire possibility. If needed, the ignition coil part number is 691060.

thnx a lot guys