I ran one single wire Bosch O2 sensor & Nordskog A/F meter on a previous I4 bike. The O2 sensor was mounted on the mid-pipe & performed just fine. There are heated O2 sensors if you are concerned about sensor temperature, however IMO as a practical matter it's much ado.
The ideal setup would arguably be a separate sensor & meter for each cylinder, however as a practical matter a single sensor in the crossover will work just fine.
What many don't realize is that the A/F ratio is going to vary in different conditions & sometimes from moment to moment, no matter how well tuned the carbs are.
