Zero Turn Mower. I'm stumped.

I have a Toro Timecutter Zero turn mower. I've always hated....no LOATHED zero turn mowers, but it was given to me a long time ago and it's always run very well until lately. The one thing I can brag on it about is, it cuts FAST. It cuts good, too. However, now it will not run unless the choke is engaged. When it does run, it will not get up to speed enough to engage the pto and blades. The extra load just makes the mower die. Of course, that's classic carburetor symptoms, right? I thought so. I pulled it and soaked it and put a kit in it. New float and all. Same thing. So I said to hell with it and got a new carburetor. Same thing. It's a Briggs 16.5 HP engine and it's always run very good. I've checked and adjusted the valves, put a new plug in it, air and fuel filters. Blown the fuel hoses out. Just done everything I can think of to cover all the bases. Has anyone else had a similar issue?