I don't see any problem with the O2 sensor being in a primary, and if the slip joint is downstream of the sensor it's not likely the source of your problem. If the O2 were two or three inches from the open end of the pipe you might be getting backwash, but not likely with a slip joint leak. The only issue with having your sensor in a primary is that you're only sensing one cylinder but that's generally not an issue unless you happen to get a firing problem in that one, like a bad plug or wire. Looking back at post #6, is your FAST a self learn/self tune system and is your O2 sensor wide band or narrow? Also from post #3, what ignition are you running. Keep the dialog going and we'll try to get some focus to your solution process. Hanging a carb on should be a last resort, IMO.