I have a Troy-Bilt self-propelled 21" walk behind lawn mower (model #12AV566N711183621). It has auto-choke and is self priming. It was purshased new, used two or three times, and then stored in a shed for 2-3 years. I took it out of the shed, pulled the cord three times, and it fired right up. I emptied the gas, changed the oil, and tried to cut my lawn. The mower will start and idle but thats it. It won't rev up enough to cut the grass. I changed the spark plug (looked fouled) and tried it again but same thing. I try to cut the grass and the mower boggs down and dies. Any ideas? I think it may have to do with the auto choke but I'm not sure. Thank you. (It has a briggs and straton 6.75 HP motor)