Small Engines (Lawn Mowers, etc.): Briggs v- twin 21 HP I/C Intek, hairline crack, metal spacer

Question
I have a briggs v-twin # 407777-0121-e4. Starts fine and seems to run ok under load. That being said, the engine makes a popping/ backfire in the at idle all the way to full throttle. This gets worse once the engine is warmed up. The only time I don't really notice it is when the mower is engaged. You can feel the burst of exhaust from the muffler. When mower is disengaged, engine sounds ok for a few seconds and then the backfiring starts, as it gets worse, the governor kicks in and the engine starts hunting. Engage mower and start off and problem slowly diminishes. I have replaced air filter fuel filter, spark plugs, dissembled, soaked and rebuilt carb with briggs kit # 792455 , and gasket set. Carb kit is crossed to this number from 499811. Diagram shows it has the fuel transfer tube, nozzle and main jet but it did not include these items. Gasket set included a metal spacer which apparently restricts carb where it bolts to intake manifold, was not on carb originally, so did not use. I have removed heads cleaned out all carbon buildup and installed new head gaskets and set valves .004 on intake .006 on exhaust. Did not see any problem with carb cleaned all passages with compressed air and small wire, throttle shaft was not worn. Plugs look ok tan electrode but has some carbon around outer edge. When throttling down it backfires quite a bit, however since adjusting valve the backfire is not as frequent once it is at idle speed for about 30 seconds, throttle it up into the midrange and here we go again. Was thinking ignition problem, but spark appears to be good in both cylinders. Almost acts like it is getting to much fuel, bowl is not overfilling and fuel in the oil. Carb has no adjustment except Idle stop screw. Float is level when installed and carb is flipped upside down, no adjustment here either. Transfer tube and nozzle are plastic and may have hairline crack, but I can't see it, Only other main jet is for high altitude application. I am starting to think engine is possessed. Any help is appreciated. Thanks Mike in Oklahoma


Answer
First off, who cares how it runs at idle or mid range.  Wide open with deck on is good to go and really that's all that matters.

Briggs says to set the plugs at .015. I know that seems tight but it seems to help.

High altitude jets really help where I live, Kansas City.  We are at about 500 feet and I use the high altitude jets regularly.