Please explain carb leaning?

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skipnsb

Filing Flight Plan
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skipnsb
How does carb jetting relate to the mixture control?

Is it that the jetting needs to be close, then the mixture control fine tunes the air fuel ratio?

Thanks Skip
 
You can think of it as the jets controlling just how rich full rich is, and the red knob as a way to eliminate the excess fuel as desired.
 
the mixture control fine tunes the air fuel ratio?
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This is correct in a sense but it misses the main point. A carb is a volumetric device, mixing air and fuel according to a volume ratio. As we climb, though, a fixed volume of air contains less oxygen. Thus, to maintain the right chemical ratio of fuel to oxygen the amount of fuel must be reduced = Leaning.

So it is really more than fine tuning.

Modern engine controls are mass flow devices, so the fuel is automatically reduced as the density (mass) of the air goes down with altitude and increasing temperature. By "modern" I mean late 20th century.
 
Yeah... I believe the real goal is to maintain air/fuel ratio as air density varies with altitude, not to lean it out.
 
jmn444 said:
Yeah... I believe the real goal is to maintain air/fuel ratio as air density varies with altitude, not to lean it out.
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It's actually both. To provide detonation margins and keep CHTs low enough that the aluminum heads don't lose much strength during high power operation, a much richer mixture is needed vs when in cruise flight (a leaner mixture also works but produces less power). So during climb you can lean gradually to maintain the same air/fuel ratio you had at lower altitude and once you level off and reduce power you can lean much further for efficiency (also here operating lean of peak EGT is more practical than during climb with a NA engine).
 
There are also differing means of accomplishing this. The typical Marvel carb uses a metering valve at the base of the discharge nozzle while the Stromberg NAS carbs use a back suction device that restricts the float chamber vent. Bendix PS carbs use an adjustable air bleed to the venturi which, by reducing venturi suction reduces the amount of fuel drawn through the discharge nozzle.

Incidentally the Bendix PS carbs, dating back to the 40's were available with automatic mixture control.
 
skipnsb said:
How does carb jetting relate to the mixture control?

Is it that the jetting needs to be close, then the mixture control fine tunes the air fuel ratio?

Thanks Skip
Click to expand...
When the MA#SPA is on the idle circuit fuel is sucked out of the float bowl by the low pressure at the edge of the throttle valve, that is the only time the idle circuit will be in effect. As the throttle valve is opened the low pressure at the edge of the throttle valve goes away and is replaced by the vacuum created by the venturi.

There are two major metering devices in the cruise circuit, the metering orifice and the mixture valve. The flow across the metering orifice is determined by the flow thru the venturi, more flow, more vacuum, more fuel flow, the metering orifice will deliver a rich mixture at any flow across the venturi.
When it is advantageous to lean the mixture, the mixture valve can be closed to restrict the fuel flow beyond what the metering orifice will flow at that throttle setting.
 
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