Having to "choke" the motor to keep it running indicates fuel is not getting through the carb properly.
Now, if the carb has recently been worked on it may be something as simple as an idle adjustment.
If it hasn't been touched they do NOT go out of tune on their own and that would indicate either a clog/crud in the carb or a bad seal/gasket letting extra air in.
In either case the carb(s) should be removed, disassembled, cleaned, rebuilt (or parts replaced as appropriate).
You could try some Seafoam in the gas or get a can of Powertune from a Merc dealer and follow the directions, but these products are more for "maintenance" and will generally not correct an issue that has got to the point where the motor won't stay running.
