I have an oneida royal oil Furnaces that keeps reseting?

Ok I think I have some kinda venting problem. I run my furnaces and it runs for a min then resets. I pulled the cap of the front of the furnaces. looks like the one on the rear that goes to the pipe that goes to the chimney. it runs fine with that off, but if I try to put it back on when its running, black soot start to pour out from around it. Is something plugged somewhere? Im thinking some kinda venting problem? im not running it right now until I can find the problem.