I'm going to amend the symptoms here - actually I did get it to backfire once or twice. It didn't do it all the time though. There was still some stumbling though- as if the engine was running rough at a constant RPM. But it wasn't like it had a flat-spot then suddenly picked up RPM though. It immediately increased it's speed but it was sounding rough when it was doing it. I'm going to change the accel pump level setting back to it's midpoint, but it'll still leave me with the rough acceleration.