how large of a carb can perform well on stock 273 commando ?

I'm at 4500 ft and think I got down in the neighborhood of 64 and 65s to get decent plug readings. It's been several years so I don't remember for sure.Try something a few steps smaller and watch your plugs closely until you get dialed in. Unfortunately with modern gasoline it's a lot harder to tell when you're "on". I know I had mine leaned down a lot more than you would think you should.