Typically this problem is caused by an issue with the carburetor. If the machine sat for any amount of time (more than a month) with fuel in it then we can be pretty certain the problem is within the carburetor.
Do check the fuel lines to make sure none are cracked. If there is a primer bulb on this model make sure to check it for cracks. When you pump the primer does it fill with fuel? If not then the problem is going to be with the primer bulb or the fuel lines.
If the fuel lines and primer bulb are in good shape then we can go in the direction of the carburetor. Stop by and auto parts store and pick up a can of spray carb/choke cleaner. Remove the air filter on the carburetor and spray a 3-4 second burst of cleaner down the throat of the carburetor and immediately try to start the engine. If it starts for a few seconds and dies, repeat the spray again. Do that 3 or 4 times. If the engine still will not stay running on its own then the carburetor is shot will need to be replaced.
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