I have a 23 year old MTD riding mower(B&S engine) that needed a new electrical fuel shutoff solenoid in the base of the carburetor. I was told that they didn't make the part anymore but gave me a comparable solenoid. The plunger looks to be the same height as old one plus the solenoid functions when hooked up. But mower barely starts and only stays running if I disconnect battery (which I assume disengages solenoid) - and runs rough usually with some choke... and a little smoke/smell of burning oil. It will run for 15-20 minutes but seems to go through a lot of gas. Time for another solenoid... or another mower?

Make sure your engine crankcase isn't full gas.
Sounds like valve problem, or possible coil/magneto.