Now that you know the kill wire (black cord) is the problem, you still need to fix it. Most likely the problem is where the grounding contact on the governor base grounds the kill wire when the speed control is set to minimum - in other words the engine stop control. It is a pretty tin-pot piece of gear (in my opinion) and also is affected by the adjustment of the speed control cable. When you look at the contact and slide the speed control between max and min, you'll probably see what is wrong. (Having said all that, I've never even seen a Quantum 50 - I'm describing the smaller vertical crankshaft engines).
If your engine was a bit hard to start and then blew black smoke, it was almost certainly too rich, probably due to the amount of priming you did (pumping the primer bulb) and the use of starter fluid - it seems to have been too rich before you used the fluid and really messed it up. If you close it down properly, let it cool completely, and then use the B&S starting procedure, it should start either first or second pull. Don't prime it unless you are cold-starting it, and don't ever use starter fluid unless you are pretty sure it's broken and isn't going to start otherwise.
