I've had sort of similar results. Seems like once a carb is set up for the induction and exhaust system, it's a matter of keeping it clean and checking sync once in a while. The CV carb is a supremely elegant machine.
My wife's Toyota had a CV (variable venturi) carb in it, but it had a lot of trouble. The slide was mounted horizontally, ran on dozens and dozens (literally) of tiny little bearings, and it never worked right after a certain shop gave it a "tune-up." It also wasn't designed to be serviced, only replaced.
I still haven't forgiven Toyota for that little bit of cheapskatery.
Redyaris:
I believe a standard (narrowband) O2 sensor can be had for $50-75 in generic form. From what I understand, a wideband sensor will detect and accurately report a much wider range of fuel/air ratios, which is particularly valuable for the early stages of tuning. The narrow band sensors are made to work as part of a system that's been engineered in detail, so they don't need to be able to detect big deviations. When it senses a condition outside the known parameters, it pitches a CEL/MIL and you dutifully take it to the dealer to get it fixed. Right?
Output from an oxygen sensor on a carburetted engine will probably look different than from the same sensor on a fuel-injected engine. I don't think the carb'd motor would produce the voltage swings characteristic of a FI system in closed-loop mode, but I could be wrong.
It'll be interesting to see if the cross-pipe has enough flow/heat to make the sensor work. Individual bungs on each pipe might work better.
I stand by, ready to learn from your experience!
