the SAI will cause the exhaust gas reading to show leaner at idle or when the throttle is chopped. the negative pressure in the exhaust causes the reed valves to open and ingest clean air from the airbox.
how do you know your engine is running weak? how weak is it? under what conditions?
I think you're looking for something that isn't there.
in general terms anything less than 15:1 is rich, more than 15:1 is lean although the tendency is to tune for around 13 iirc. so if 13.6 was the weakest what was the richest? where in the rev range was it lean?
12.5 - 13.3 gives best power - richer still. but you will see variation throughout the rev range.
Thanks again, as you say I don't think there is anything to fret about, just I didn't know whether to leave it connected or not. The AFR didn't alter much throughout the rev range, ie from about 13.4 to 13.7 thanks for your help