I had a Cessna 120 for several yrs. It had a Stromberg carb with a back suction mixture that was safetied to full rich, as most of them are.I've been reading about the Thatcher aircraft, and they run Zenith Carbs without mixture controls.
Can mixture controls be added to a Zenith Carb or would that require a different carb?
I use the mixture control to shut my engine down on my Zenith carb. I also use it to run LOP while on the ground. Doing this my plugs always look just like they do when you take them out of the box.
Turning this needle stops the inflow of fuel into the float bowl. It is used to adjust the high speed mixture. At idle it is not really leaning anything. Even though you set this needle before adjusting the low speed side of this carb. You adjust the high speed side first then the low speed side. All you are doing when closing this needle is shutting of the fuel supply to the carb. It takes a few mins for the float bowl to run dry then for the engine to quit. Shutting of the fuel valve does the same thing. I always let the engine run out of fuel and die before shutting off the mag. I reference this to unloading a gun. I don't like leaving my engine sitting with fuel in the carb for this leaves the engine loaded or ready to run. The way I do it, even if the mag is left on the engine will not start. If someone comes along and turns the prop the engine will never start. I don't leave my engine loaded. I unload it before putting the plane away.So you lean it out until the engine stops with the cockpit adjustable main mixture screw. Wish the Zenith had a fuel mixture shut off like on a Marvel Schebler Carb.
With mine being not cockpit adjustable, I shut the engine down by turning the fuel valve off then the mag.