FR691v loss of power/backfire

[VRR.DYNDNS>BIZ]

I have a toro ss5000 with a Kawasaki FR691v that started popping and surging under load last season. Seems to idle fine and will drive okay, but with the blades engaged or on a slope it will start popping (I assume backfiring) and the power will noticeably drop. Power will come back for a few seconds and then will drop again.
I changed both plugs, air filter, fuel filter, oil, and fuel with no change. I pulled one plug at a time and found cylinder one would run solo, but not cylinder two. Removed and replaced both coil packs without any change. Removed and replaced the carb and now both cylinders run independently, but the mower still is popping and losing power. Verified both valves were within spec (approximately .005in). Running out of ideas, any words of wisdom?

Still probable fuel delivery. Do not forget throttle level means nothing of carb throttle opening as the governor changes throttle opening to attempt to maintain the same rpm. Most carbs have a low speed and a high speed circuit. Most twins will come to almost full speed without a load using very little high speed circuit. Another, hopefully not the case, bad mechanical cam decompression system, or sticky valves or valve train full travel issues due to guide migration. Run it for an hour with a heavy dose of Marvel's mystery oil in the gas and oil.

 

[Auto Doc's]

If it blows the gasket again, you will need to resurface the cylinder head. A nice flat surface and 400 grit wet/dry sandpaper work perfect. I use and old stove tempered glass panel for my flat surface. Proper head bolt torque is also important.