I have a commercial grade ZTR that has 21 hp Kawasaki FH641V engine that has started stalling. I put stabil in all of my gas, and it doesn't begin stalling until I'm about 1/2 way through mowing my 2 acres. So, I don't think it's the gas. After it stalls I get the backfire smell. After it stalls, it starts much easier if I give it full choke. I was able to keep the engine going if I dis-engaged the blade. That worked for a while, but it just stalled sitting there without the blade engaged. Does it sound like a problem with the carb? Does anybody know if this engine vapor locks? It's extremely frustrating and take a LONG time to mow.