Im working on an 06 450 rhino, it is a job and not my own. It is all stock. The valves were just adjusted. The air filter is clean and oiled. The gas smells good but i dont know the age. Before i adjusted the valves i started it up when he dropped it off. It fired right up with the choke. I then turned it off so the engine wasnt hot for the valve adjust. After the adjust, i tried to start it and had an exremely hard time starting it. I finally got it started and it idled for several minutes then the idle fluctuated and stallled. Once it stalls, i sometimes start it easily and sometimes its hard. Also after stalling its idle fluctuates and stalls typically within 30 seconds. I played with the fuel screw and it seemed to get better almost all the way out. The throttle response is good. I installed a 22.5 pilot from the stock 20 pilot and checked all the passages while at it. The carb is perfect. It still does the same thing. I can play with the air box flow and it seems to get better with less air. If i pull the choke slightly it sounds great. I installed a clear hose on the fuel line and the gas flow looks good. I am doing all this with the gas cap removed to bypass vaccum lock. I dont hear too much difference when playing with the fuel screw. I dont see any reason why it should be running lean while stock and with a larger pilot. No air leaks in the intake also. Please give any advise as to why the rhino could be stalling. The compression is good and i am going to try a new spark plug tomorrow. Thanks in advance!