It needs to be replaced.
Exhaust leak possibly...
No, seals are cheap, engines expensive.......
Dirty or stuck open needle valve in carburetor.
Inlet needle in carb sticking open; it runs in through the carb.
If you tilt the mower to get at the blade for sharpening, or go down a particularily steep slope, gas will run from the tank out through the filter.
By backing the choke out I assume you mean turning it off. If it stalls when the choke is off but runs while it is on then you have an air leak somewhere
There are products at your local auto parts store such as Epoxy Ribbon, JB Weld, and others that don't require heating you can use to patch the leak. Be sure and follow the directions on the package. You can't BEAT KENT'S ALL IN ONE.
fill tank, to top of neck, watch the leak area, [don,t start it]
You may have a crack and/or leak in either the bulb itself or the fuel lines between the bulb and the gas tank and carb.
I would start at the carb take the air filter and clean it .Then take the carb apart and clean it also might be stuck linkage. Good luck
Lawn tractors are generally more time efficient than a push mower however, cost efficiency can be another story. Much of cost efficiency depends on how large a lot is and how often it requires mowing.
Is it a Craftsman? I have two of them and they are doing the very same thing. The carb is somehow allowing the gas to leak into the oil. When you start it up... does is smoke a lot? I fixed the issue by putting a on off valve in the gas line. Open it up when you are using the mower and turn it off when not in use. Problem was solved. However do check the oil occasionally