Well, I have a craftsman briggs & stratton 500cc 17.5 hp. Silver edition riding mower. I have cleaned the carbs on it three times already. I can only get it to start if I take off the airfilter and choke it by covering the intake hole. Only leaving a small gap. Then only will it stay on. Don't know what to do next?

There is a leak somewhere, did you replace the gaskets at carb.