I have a Craftsman yard tractor with the B&S 18.5HP Intek OHV motor in it. Last fall, as I was driving it to the storage shed for the winter, it started spitting and sputtering and almost sounded like the was a leak in the exhaust system. It sounded similar to a car when the gasket blows out in the exhaust system. I shut it down immediately and pushed it into the shed and never had a chance to get back to it until now. I looked everything over and I didn't sse anything obvious on the outside of the motor like oil running down the side or something like that. There was very little gas in it so I thought maybe I mistook running out of gas for a problem. I add some gas and started it up. At idle it seemed to run fine but as soon as I moved the throttle to a higher speed it started running rough and blowing gas out of the carburetor. I pulled the spark plug and it was covered in carbon. It almost acts like the motor cannot burn all of the fuel that is going in and is running rich. I would like to try to rememdy the situation myself before taking it to the repair shop if I can. Any suggestions as to what I should look at? Could the coil have gone bad and it's not getting enough spark? Maybe the plug (original, at least 5 years old) just needs to be replaced? I've seen a lot of talk about valve adjustment on the site for other Intek motors. Could it be that? Any ideas as to what I can try would be appreciated. Thank you.

A little additional info. Mower is a DYT4000 (917.273632) and the motor is a 31P777 0348E1.

With 5 years of runtime, it most certainly is in need of a valve adjustment. The symptoms also SEEM to indicate that as well