The black oil is from the backplate being loose. You'll want to fix that ASAP. The black color is from the metal that's been worn away already. If it keeps up, the backplate may not seat properly, or, if it's coming from a bolt hole, the bolt may strip out. If it just started, you probibly just have to tighten the backplate bolts a bit. If it's been going on for a while, you may need to use some RTV to make a good seal.

Either way, a loose backplate will certainly cause poor running.

So will a cracked fuel nipple. If it had been cracked for any length of time before it broke off, it would air in, causing running problems as well.