12.12.12(a) . over-rich mixture settings; 

over rich mixture can cause:
loss of power
rough running
fouling of spark plugs
bore washing
increased fuel consumption

12.12.12. Explain the consequences of operating with:

12.12.10. Explain the correct operational use of a manual mixture control and idle cut-off.

For take-off and landing mixture is normally kept full-rich unless operating at high elevations.
To lean the mixture the lever is moved slowly towards the lean position. With a fixed pitch propeller the RPM will increase as the correct mixture is obtained, and then decrease if leaned further. With constant speed propeller exhaust gas temp. will increase then start to decrease. Normally a setting to the rich side of the peak is selected.
For detailed information on leaning refer to individual aircraft flight manuals.

12.12.8. Explain the function of a manual mixture control and idle cut-off.

A manual mixture control allows the pilot to adjust the fuel/ air ratio in order to compensate for changes in the operating environment. (eg increased altitude)
The idle cut-off system allows the motor to be stopped by cutting off the fuel at the carburetor. This systems ensures that there is on combustable mixture left in the cylinders, but the carburetor is left full of fuel for the next start.

12.12.6(f) . idle cut-off. 

A valve in the carburetor which closes the fuel off between the float chamber and the venturi. Used to shut down the motor because it ensures that all fuel in the manifold is burned before the motor stops

12.12.6(e) . mixture control; 

Function – regulating the amount of fuel being metered into the air at any given throttle setting

Operational use of mixture control
– FULL RICH for take-off unless at very high elevation airfield
– left in FULL RICH for the climb ( unless climbing to a high altitude)
– place on FULL RICH at the top of a descent (unless descending from a high altidude in which case it would be progressively enriched until full rich at lower altitude)
-Idle cut-off is normal means of shutting the engine down
–

12.12.6(d) . enrichment at high power settings; 

Enrichment system
– provides extra fuel when the throttle apporaches and reaches its wide open position
-his excess fuel does not burn but provides cooling through evaporation at high power settings
– helps to prevent overheating and detonation

12.12.6(c) . acceleration enrichment; 

Is used to ensure that sufficient fuel is available when a sudden opening of the throttle causes a rapid increase in airflow through the carburettor

12.12.6(b) . idling circuit; 

Aircraft carburetors will normally have an idling circuit to provide fuel to the engine when the butterfly is closed and the main fuel jet delivery becomes less reliable.

12.12.6(a) . atomisation and diffusion; 

Atomisation and Diffusion
-In order for the fuel to mix properly with the air in the intake manifold it needs to be atomised and diffused among the air. To assist with this an atomisation and diffusion unit is positioned at the main jet outlet to improve the vapourisation of the fuel.