At a loss - Briggs 21hp surging - can't solve. FIXED!

I'm at a loss for what could be the problem. I refurbish or fix riders and yard equip. & tractors for a hobby. I've got a Husqvarna YTH2148 with Briggs 21hp twin that won't stop surging at high throttle. I've tried almost everything.

It runs pretty much ok until 2800 rpms when it'll start surging with the govenor bouncing back and forth from low to high rpm with a cycle of about second or two aka surging. There might be a cough out the carb at idle or high speed once in a while as well. Adding choke reduces or eliminates the surging for a moment but usually winds up flooding the engine to low rpms then choke to off and black smoke till rpms up and starts surging again. A classic dirty or leaky carb or bad fuel scenario. I'm not unskilled at problem solving and have tried eliminating everything but the problem persists.

Here's what I have checked:
Gas seems clean. It's fresh. No contaminates seen. New fuel filter with visually 85% full of fuel. New fuel shutoff. No air filter in.place. Changed fuel pump. Appears adequate flow at carb intake. Tank cap removed no change. Vent seems ok. Fuel level seems adequate. Maybe pinhole in old fuel line but no significant evidence so far.

New oil and filter. Proper level and clean. Both ignition Coils swapped for other used ones. Spark plugs are new. Left cyl. might be fouling rich a bit more. Flywheel key ok. Charging ok. Battery ok.

Carb cleaned 3 times. No change. Did not replace seals etc as looks acceptable. Swapped with carb and intake manifold that had new rebuild no change. Throttle and choke linkage ok. Govenor- tried many adjustments to idle and wot springs and tabs. No positive effect. Internal govenor parts seem to operate to spec. Shaft turns about 10 or 20 degrees as normal. I don't suspect issues here. Idle ok about 1800 rpm or.lower. WOT set at between 3000 and 2600 rpm. Surging exacerbates with higher rpm.

Rider travels about the yard ok with occasional loss of power or cough whereas a bit of choke recovers after a rich moment. Usually doesn't have significant surge but can still tell not running smooth. Rpms vary up and down a couple hundred under slight.load. chassis doesn't seem to drag in any way.

Valve lash set and reset again with intake about 5 thou and exhaust 8 ish. They're on the tight side. I'll try making them sloppy and recheck for the ships and giggles. Springs and guides seem ok. I'm thinking of pulling heads grinding valves etc on idea maybe a valve is sticky or fouled and cannot be seen on inspection yet.

Intake manifold not leaking. When running wot or idle no change when ether or brake cleaner or carb cleaner sprayed about in copious amounts.

Engine runs otherwise ok. Maybe slightly hot but there's no shroud so explainable due to poor airflow. I suspected over sensitive govenor but don't really think so. I'm pretty good at adjusting but haven't replaced Springs. Looks identical to surplus govenor assembly on hand.

What am I missing?
Any ideas would be appreciated.

I'm really at a loss. I'll pull the heads and work them over otherwise swap the engine as last resort. After that I'll split the case and inspect govenor fly etc but don't think there's anything inside as a culprit.. yet.

Last- surging is guaranteed if throttle opened quickly to 2800+ rpm. If opened slow surge may not occur as drastically at least until approaching higher than 2300 rpm. Even when not surging dramatically there's a perceived up and down rpm change but very subtle with occasional pop out the carb.

Sorry for the long dissertation but I'm looking for a clue before the dynamite option...

Thanks.
Dave