Loose drive belt

So I asked my father-in-law to borrow his yard machine tiller. He said if I can fix it, it's mine. The last time it was used was over a year ago and the gas had been left in the tank. We were both confident that the carburetor needed to be cleaned or replaced, so that's where I began. After removing the carburetor I took it to a local shop where they cleaned it and replacement all the parts for it. They said with 100% confidence that it would work after replacing them...wish that were true. After putting it back together, it still wouldn't start. It definitely sounds like it wants to turn over, but won't. I took the carburetor back off and noticed some black gunk in the bolt that holds it on (not sure of the correct term). I sprayed everything out again with carb cleaner, cleaned out the gas tank and fuel line with an air hose and tried again. I also replaced the spark plug. Still nothing. When I removed the carb again I noticed the same blank gunk in the bolt. I'm a novice when it comes to working on engines and really don't know where to go from here. I'm guessing the gunk is the reason why it's not running, but I'm not sure where it's coming from. Any thoughts or other things to check? The engine model number is 0hh60.

Hi Gang. The Toro HXL 16-38 16 HP OHV briggs adn Stratton that dies after 15 -20 minutes of mowing continues to do so. Gas good, new plug, valves adjusted and proper, new carb, oil level good. What is the purpose of the selenoid at the bottom of the carb? If I unplg the wire connected to it, whats the likely result if I run the mower? When it begins to die, I stop the blades and it runs just fine, when I engage them again it wants to die again. The switch for the blades if engaged wont allow the engine to start, which is proper. Could this be culprit?