This is what I teach my students to check with respect to the mixture control during the runup. ( for this example I am refering to your typical C150/152/172 1700 RPM runup,not at high altitude airport )
1) Push the mixture knob in. This ensures you are starting from full rich. The knob should stay fully in. If it pops out a little bit it means the mixture cable is too long and is kinking in the sleeve which will result in premature failure. This fault should be snagged.
2) Slowly pull mixture knob out while watching tach. The RPM should initially increase 25 to 50 RPM then slowly decrease. Stop leaning when the RPM has decreased by about 50 to 100 hundred RPM and the engine just starts to stumble. Observe for a rise in EGT indication (if fitted). If the engine does not have an initial rise in EGT then the carb is set too lean or if there is a large RPM rise then the carb is set too rich, and again the fault should be snagged. Obviuosly if the EGT does not rise it should not be used for any inflight leaning. Finally if there is no smooth reduction in RPM and the engine just quits as it is leaned ( As happened to me once ) then the airplane has some serious carb and or mixture control problems and should not be flown.
3) Push mixture control back in and ensure the mixture control moves freely and does not hang up. The mixture knob should return to the full in position and the RPM return to the pre check value.
In practice this procedure takes about 10 seconds and will give you a good indication that the carb is properly set up, The mixture control assembly is in good working order and is in fact selecting full rich and the EGT instrumentation is working.
