Again, regardless of density altitude, if the idle mixture setting is adjusted properly, there is no need to lean. The problem is that too many owners, schools, and rental facilities see idle mixture adjustment as something that's done when the carburetor is installed or reinstalled, and that's it.
If the base elevation of the aircraft is changed, or the average temperature for a given elevation is changed (such as occurs seasonally), then the idle mixture should be readjusted as a maintenance item.
You can check the idle mixture by yourself, and should always do so as part of your post flight engine checks (just prior to shutdown). An idle check, and an idle mixture check should be part of the shutdown proceedure (and if you'll read the engine manufacturer guide, you'll probably find that for the aircraft you're flying, it is).
Run the engine up to runup RPM (typically about 1700, though it may be governed by manifold pressure instead; some engine manufacturers and aircraft manufacturers dictate using barometric pressure as the guide, rather than engine RPM...this is especially true of larger recip engines), and lean it to ensure it's clear and not loaded. Run it like that for a minute or so, leaned out, and then retard the power to idle, Return the mixture control to rich. Note the resting RPM with the throttle closed. Your airplane will have a range within which it should be idling, generally 600-800 rpm.
Check to see that it does idle, and that it isn't too erratic. Begin moving the mixture slowly to cutoff. At some point, you should get a RPM rise of no more than 25-50 RPM (check your maintenance manual for specifics), before the engine dies. As the engine begins to die, return the mixture to rich and try it again; you should get the same result.
You've just checked the idle mixture. As you moved the mixture to cutoff, if the idle mixture was adjusted properly, you saw a very slight increase in the idle rpm, often almost imperceptible. If there was no increase, the idle mixture was already too lean. If there was any more than about a 25 rpm increase, it was too rich.
Note that the point on the mixture travel where this occurs is the only place that the mixture control will have any effect on an idling engine. You can move the control where ever you like the rest of the time, to no effect. Try it next time you fly, and you'll quickly see that there is only a very small point on the mixture control, a couple of milimeters of travel often times, that has any effect on that idle mixture.
Bear in mind that other things can cause loading up during taxi. A leaking primer can do so, and can cause a number of other problems, including engine failure. A plunger type primer with a bad packing/o-ring can lead to the engine drawing fuel, or air, and can alter the mixture at the individual cylinder. This is sometimes a little harder to detect, as usually only a few cylinders receive primer; the effect isn't as perceptible.
Before looking at ground leaning practices (which may be important), look at the maintenance the airplane is receiving in the first place. That's always a good place to start.
If the base elevation of the aircraft is changed, or the average temperature for a given elevation is changed (such as occurs seasonally), then the idle mixture should be readjusted as a maintenance item.
You can check the idle mixture by yourself, and should always do so as part of your post flight engine checks (just prior to shutdown). An idle check, and an idle mixture check should be part of the shutdown proceedure (and if you'll read the engine manufacturer guide, you'll probably find that for the aircraft you're flying, it is).
Run the engine up to runup RPM (typically about 1700, though it may be governed by manifold pressure instead; some engine manufacturers and aircraft manufacturers dictate using barometric pressure as the guide, rather than engine RPM...this is especially true of larger recip engines), and lean it to ensure it's clear and not loaded. Run it like that for a minute or so, leaned out, and then retard the power to idle, Return the mixture control to rich. Note the resting RPM with the throttle closed. Your airplane will have a range within which it should be idling, generally 600-800 rpm.
Check to see that it does idle, and that it isn't too erratic. Begin moving the mixture slowly to cutoff. At some point, you should get a RPM rise of no more than 25-50 RPM (check your maintenance manual for specifics), before the engine dies. As the engine begins to die, return the mixture to rich and try it again; you should get the same result.
You've just checked the idle mixture. As you moved the mixture to cutoff, if the idle mixture was adjusted properly, you saw a very slight increase in the idle rpm, often almost imperceptible. If there was no increase, the idle mixture was already too lean. If there was any more than about a 25 rpm increase, it was too rich.
Note that the point on the mixture travel where this occurs is the only place that the mixture control will have any effect on an idling engine. You can move the control where ever you like the rest of the time, to no effect. Try it next time you fly, and you'll quickly see that there is only a very small point on the mixture control, a couple of milimeters of travel often times, that has any effect on that idle mixture.
Bear in mind that other things can cause loading up during taxi. A leaking primer can do so, and can cause a number of other problems, including engine failure. A plunger type primer with a bad packing/o-ring can lead to the engine drawing fuel, or air, and can alter the mixture at the individual cylinder. This is sometimes a little harder to detect, as usually only a few cylinders receive primer; the effect isn't as perceptible.
Before looking at ground leaning practices (which may be important), look at the maintenance the airplane is receiving in the first place. That's always a good place to start.
