One of my customers just gave me this mower. It has been sitting for a few years and will start and die. I took the carb bowl off it was very gummed up . I removed the carb and soaked it for a few minutes and got some white chalky stuff out of it. Put it back together and it did the same thing. the fuel lines are clean and if I turn the gas valve on with the bowl off gas does run thru it. All suggestions would be appreciated. The mower was used just a few times and then parked.