I'm gonna guess that you meant a model airplane engine on having a mixture control, in actual real aircraft carbed engines, the mixture is controlled, as you suggest by a needle valve arrangement, but it meters the fuel coming out of the main jet. In order to be able to adjust the mixture, you would need a method to measure cylinder head temperature for all cylinders, to insure one cylinder doesn't run too lean and hole a piston. That is one of the cautions/warnings in all carbed aircraft air-cooled engines. As far a using an Exhaust Gas Analyzer, I borrowed one from a friend many years ago when they were available from Sears. I adjusted all my motorcycle engines to 16:1 air-fuel ratio and never had a problem. All the analyzers I've seen lately are permanently attached to the exhaust and not easily moved from car to car. It appears that the only way to get a mobile unit is to purchase a very large analyzer that runs into the thousands of dollars. Plus, the ones I've seen available for permanent installation only go to a max of 15:1, which, IMHO, doesn't have the range I would like. Here's a little story told by my A & P engine instructor. The US Navy was going for a record for non-stop flight in a four turbo-engined aircraft, Leaving Austrailia, they started leaning out the carbs. Keeping an eye on the cylinder head temp gages, they increased the turbo boost pressure as they leaned out the carbs and eventually had the excess turbo air cool down the cylinder heads. They did set a record flying from Austrailia to the States, landing somewhere in mid-continent. The air-fuel ratio was somewhere in the 20:1 range, if I remember correctly. Todays cars are set up for 14.7:1 ratio which makes the EPA happy, but, from what I was told many years ago is too rich, optimum being supposedly at 16:1. Now if all the cars were set up using 16:1, that would be an across the board savings in fuel of 3%, world wide, plus the same percentage of emissions not going out the tailpipe. Of course that would be compounded by the numbers of MPG increase too.
Again, many years ago, and experiment was made with full belly pans of cars and vans. Cars had a modest increase in MPG but the vans had a substantial increase in MPG. Probably because of all the disrupted air flow going under the vans creating turbulance and holding them back. Smooth airflow over, under, around, and through was the key back then. The key for an aerodynamic antenna shaft is to have it teardrop shaped. This was used, again, in the aircraft sector on fixed landing gear aircraft. There was a very noticeable gain is speed and distance using a teardrop shaped airfoil over the round landing gear struts used at the time.