Conventional aero gas engines usually have a mixture control, its the red knob. It is used to control the fuel air ratio in the carburetor and to stop the engine at the end of a flight. Some engines have an automatic mixture control unit in the form of a pressure sensor (Rotax/Bing) or a sophisticated FADEC engine controller (Aerosance et al).
These engines normally do not have a manual mixture control and must be stopped by switching the ignition to the off position. In a carburetor fuel is metered on basis of a certain volume of air and fuel. As altitude is gained during climb, the volume of air the engine intakes remains the same but its density decreases however.
This is the reason we need to use the mixture to lean the fuel as to keep the engine running smoothly. Fuel savings up to 20 - 25% can be reached by leaning the engine properly. Especially during the cruise part where we usually spent most of our time.
Significant fuel savings can be obtained by the pilot by properly leaning the engine, until the day that these engines all will have an engine FADEC. Prior to that time you will need to fondle with the mixture control and exhaust gas indicator.
This is where leaning can really save fuel. And we have a number of leaning methods available: best power, peak EGT, lean of peak (LOP) and with power recovery.
Set the mixture so that the leanest cylinder runs at 100°F rich of peak EGT. This ensures that the exhaust valves stay cool. Maximum power is produced along with the highest airspeed, fuel consumption and CHTs.
This method is known to cause engine trouble over longer periods of time, problems as sticky valves, fouled sparks plugs, carbon deposits to name a few. See mixture too rich in part I of engine leaning.
The way to achieve this without an EGT gauge is to lean the mixture until the engine start to run rough and then enrich until it runs smooth again. Keep in mind to move the mixture knob slowly, you will stop the engine when you pull the knob too far out!
Set the mixture in such a way that the leanest cylinder runs at peak EGT. The EGTs will be 100°F hotter than the best power method and heat damage is possible to exhaust valves. The engine will produce some 4% less power and fuel consumption will be 14% lower than with best power. CHTs are approaching the highest values here.
The engine is now run with a chemically correct mixture, aka stoichiometric, and the fuel burn is at it hottest. EGTs and CHT are both peaking too.
Set the mixture so that the richest cylinder runs at 30-80°F lean of peak EGT. Now the CHTs will be at their coolest and the EGTs will be 20-70°F hotter than the 100°F rich of peak (ROP) method. We now have reasonable cool exhaust valves with the engine producing 7% less power and fuel consumption will be 20% lower than the best power method.
If you want to "recover" the power, increase MAP with an inch or so and make sure that the hottest cylinder does not get hotter than 30°F rich of peak EGT, do maintain LOP operation.
Big radial engines like Pratt&Whitney Wasp / Wasp Major and the Wright Cyclones were all run at LOP, ensuring reliable and long service life and without the problems caused by running rich of peak.
Gradually increase the mixture as you descend, just reduce the throttle (by reducing MAP) to maintain the same power levels. Before entering the circuit set the mixture to fully rich, just in case a go-around is needed.
From the above we can see that the Lean of Peak (LOP) method is the best way to reduce fuel consumption and be assured that the engine has a long and healthy service life.