I have a Grasshopper 725 with the Kubota gas engine. When I put the throttle to idle the engine dies. At half throttle the engine rpm's go up and down. I took the carb off and cleaned the jet needle, put a new fuel filter on, new air filter and changed the spark plugs. One thing I noticed was the fuel filter (its clear) is never more than a 1/4 full of gas. Could it be the fuel pump? The mower use to have a back-fire when I would shut it off. It doesn't do that anymore, it just idles real low until it dies.