my 27 hp Briggs and Stratton engine backfires when under load and smells of sunburn gasoline when under load. idles fine and at full throttle engine runs at 3150rpm. when engaging mower deck rpm drops to 2600 rpm and starts backfiring thru carburetor and smells of sunburn gasoline, changed plugs, replaced flywheel key, recapped the flywheel to .010, replaced carburetor, fuel filter, starts fine, checked and replaced deck spindles and grease new spindles, checked spark to individual cylinders and they are OK, what could be the problem with the engine?:confused2: