It fell over...

Hope someone can help. So, I have this '78 KZ750B3, owned it for almost 10 years. Found it as a pile of junk, stripped it down to the frame, rebuilt it, including a top end rebuild, aftermarket electronic ignition, VM34 carbs, stock pipes. Valve clearances in spec. By all accounts been a great bike to me, until...My buddy helped me trailer it to my new place, so we had it strapped down nice and tight but when we were unloading, he jumped the gun and undid one of the ratchet straps before I could get a hand on it and it fell over and slammed into the trailer rail. No cosmetic damage, amazingly, but I cannot get it to run right since. Symptoms: Starts up fine, very easily, as it always did, with chokes on. But the left cylinder seems to be completely out of whack, seems to be running way too lean to idle, backfiring badly, of course. Right cylinder will warm up after a while and go on its own, pulling the left cylinder along with it, but, obviously, it won't run long this way.I've checked and re-checked, cleaned and re-cleaned the carbs, thinking a piece of debris got knocked loose and clogged a fuel or air passage. No dice. Did a cursory check of exhaust pipes, nothing seems cracked but will likely need to pull them off and do a more thorough inspection. Changed the sparkplugs, no change.Has anyone got any suggestions for other things I should check? It's a bad deal and I don't want to tell my buddy. He felt bad enough when it fell over. Any help is appreciated and I can provide more info, if wanted.

Welcome, how about the fuel Petcock. Possible debris in the tank was dislodged and is caught in the petcock. Or maybe in a fuel line.

EDIT: I’d check the carb bodies for hairline cracks. Also check the carb manifold boots, could have gotten knocked loose and leaking air.

What side did it fall on?
Show photos of points of contact on the bike.