Engine speed cuts in half after 15 minutes of use, out of ideas

[hovercouch]

Got a push mower front wheel drive with a Briggs and Stratton engine model 122L02-0115-F1...last week halfway through the job the engine speed just suddenly cut in half and was on the verge of stalling. I took apart the carb and blasted out the jets with an air compressor, cleaned out the gas tank and fuel line, reassembled and it ran fine for a couple minutes then got the same problem again. So I bought a new spark plug and air filter, ran fine for about 15 minutes, same problem. Bought a used magneto/wire, ran fine for 15 minutes, same problem. There's no throttle or carb adjustments and putting extra tension on the throttle cable changes nothing. :confused2:

 

[Rivets]

Will it run longer if you do not engage the drive. I'm think you have one of two problems. Either the gas cap vent is plugged, loose the cap and see if it runs faster after it starts to slow down. If you run it longer without the wheel drive, you may have a tranny going bad or a bad belt seizing up.

 

[hovercouch]

Sorry forgot to mention that I tried cycling the drive on and off while it was sputtering, with no effect...I've got the carb off again and gonna soak it, if it still doesn't work I'll try the gas cap thing but I'll probably buy a new carb like Briggs/Stratton and a couple shops around town recommended.