JohnDeere X320 Losing Power

I have a X320 mower with 285 hours on it. I has just started to lose power when mowing, blades on. If I turn them off it goes back up to speed but after a minute it bogs down again and still does it with the blades off and not moving. Fuel pump works, I replaced the coils and sparkplugs. Still bogs down, removed and cleaned carb, looked clean inside, no debris in bowl. Also replaced fuel filter. It does backfire sometimes when trying to get it going.checked valve clearance, in spec. not sure of the next step other than replace the carb.

Pull the plugs off with the engine running, one at a time.
The mower should slow down when running on one side only.
If there is less difference one side to the other then one side is not working.
Usual suspect is the valve going out of adjustment.