I have a 2n that generally gives me no trouble at all. Yesterday I started it and it ran smooth for a couple minutes. Then it started to run rough and black smoke was coming from the tail pipe. I had to increase throttle to keep it running. I decided to shut it down and do some further checking later in the day. My shut-down procedure begins with closing the fuel shutoff. Almost immediately after shutting off fuel the engine smoothed out. I opened the shu off and it ran rough again. Any ideas on my problem? Seems like fuel delivery but looking for guidance from you experts.