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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:

Do the sparkplug test.
Remove the plugwire from one side and try to start, then try it on the other side. See if it will start on either side.

the engine starts on either side

What color is the spark, blue or orange?

orange in color

Take off the valve covers and slowly rotate the engine. Make sure that all of the valves open @ the same distance.

valves move the same distance and proper gap setting

Start the engine and then engage the blades
Now manually work the governor arm
If the engine will run happily & smoothly up to 3600 then the governor need adjusting
If the governor adjustment does not fix the problem then it looks like the governor is not working so that is a split the cases job.
They do VERY OCCASIONALLY fall apart.
IF the engine does not run smooth then please tell us exactly what it did.