Portable Air/Fuel gauge?

It's not that I couldn't weld in a bung, it's that I had hoped to find a really simple portable method for carb tuning multiple vehicles. I had hoped to avoid having to drill/weld on each car I wanted to tune. But I wasn't sure if I was wasting my time looking for something that didn't exist because it couldn't be designed that way. If I were to 'alligator clip' the probe in the tail pipe, - how likely is it that I won't be able to get a 'good' reading?