20 hp Briggs Backfires and stalls under load

[COLOR="indigo"]I have the cub cadet 2135 ... I was unable to locate the engine serial number, they claimed it was on the right side of the engine blower housing, but they don't show where that is.:NO NO NO: For a while the engine wouldn't start right off the bat ... I would get a series of mechanical clicking noise, after a few tries it would start ... well I tried to start it today ... got the clicking noise a couple of times then nothing ... :eek: I can't get it to start ... I turn the key and silence:help: I get a amp light on the dash board ... I tried cleaning the battery terminals with no result. So could it be the starter gone bad?:madhell::wall: Anyway I can check it to make sure?:confused:[/COLOR]

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.