Most likely a stuck inlet needle valve in the carburetor. The needle (and seat if it has one) could also be worn out. I suggest pulling the carb bowl, removing the float, pull the needle valve then spray with carb cleaner.
Clean the bowl, replace and try again. You might need a new bowl gasket depending on the age of the carb/gasket.
As you are doing the work, look for signs that the fuel lines may be degrading. They can shed and the debris can prevent the needle valve from seating. If they are in poor condition, I would replace them.
Clean the bowl, replace and try again. You might need a new bowl gasket depending on the age of the carb/gasket.
As you are doing the work, look for signs that the fuel lines may be degrading. They can shed and the debris can prevent the needle valve from seating. If they are in poor condition, I would replace them.
