Since I recently got my Thatcher CX4 flying I have been tinkering and tuning. The Zenith carb was dripping a lot of fuel after shutdown so I replaced the needle and seat. I had to take the airbox off to get the carb off and that exposed the bottom of the engine so I changed oil while I was at it. Plus I had some oil leaks to fix, etc. The list kept getting longer and the whole thing turned into a 3 week project. Partly inspired by the March Kitplanes article on the Ballenger A/F monitor and partly because I always wanted to try an 02 sensor, I installed an NGK/NTK wide band A/F ratio monitor while I was at it. https://www.ngk.com/product.aspx?zpid=41326 My Zenith carb has the mixture adjustment on the bottom of the carb and I also made an apparatus to allow the mixture to be adjusted from the cockpit. I have made one flight with the 02 sensor and the cockpit adjustable mixture control. NGK recommends recalibrating the sensor for every 3000' change in altitude. Calibration requires powering up the sensor and letting it hang in free air for a few seconds. My field elevation is 5200' MSL and the local mountains top out at a little over 11000'. Before I installed the sensor, I took it up the mountain in the car and calibrated it at 9000' since that should give good readings from 6000' to 12000'. The first results are interesting. My engine is a Hummel 2180 with the Zenith 14992 carb. The ideal air fuel ratio is 14.7 to 1, where all the fuel and all the oxygen are combined. Maximum power comes at about 12.5 to 1. My engine idled the best at about 10.5 which is a little on the rich side but it was cold. Before I flew, I set the mixture on the ground at 2400 RPM to 12.5. Throttling back, the mixture went rich below 2000 RPM to about 9.5. After take off, the mixture turned out to be slightly rich and I readjusted it to 12.5 during the climbout to 8000'. I played with it a bit at 8000' and found that the engine did not want to run much leaner than 12.5. Peak RPM occurred at 12.5 and RPM would drop around 50 RPM as the mixture got richer but it would run happily on the rich side. I then climbed to 12,500' and was amazed to see the mixture held at 12.5 all the way up. When I throttled back to descend the mixture richened below 2000 RPM to about 9.5 just as it had on the ground. On the ground, at idle with a hot engine, mixture went to a little over 11. I didn't know what to make of the results. Everybody knows that mixtures get richer as you climb and you need to lean at altitude. It's in all the books! Mine didn't! I don't know whether to believe that gauge or not. I told another pilot who flys a Sonex with the Aerovee engine about it today and he said that he had read somewhere, that years ago, many people installed Zenith carbs on their airplanes because they hold a constant mixture through changes in altitude. So, my preliminary results indicate that you don't need a mixture control on a Zenith carb! Set it and forget it! Wouldn't that be great! Further testing to follow!