For some reason, this didn't occur to me in my earlier posting, but
better late than never.
With regard to more vs. less restrictive air cleaners and fuel-air
mixture, there is a significant mitigating factor that I didn't mention:
While the fuel bowls of many older civilian carbs (like most of the cars
I've worked on over the years) are vented directly to the atmosphere,
most military carbs (and a lot of civvie carbs as well) are vented
internally to the filtered air intake. This means that any air pressure
change (such as from a different air cleaner) at the carb intake causes
a parallel pressure change in the fuel bowl and the opening to the air
emulsion tube/air corrector jet. This has the effect of keeping the
pressure differential between the venturi nozzle and the fuel bowl, etc.
much more constant than would be the case if the bowl were vented
directly to the atmosphere.
This is not to say that a more restrictive air cleaner won't enrich the
fuel mixture and vice-versa, but in the case of an internally vented
carburetor, most of the enriching or leaning effect from changing an air
cleaner is due to the change in pressure altitude at the carb intake.
For example, if the air cleaner is highly restrictive, it will cause a
pressure reduction at the carb air intake. This is similar to moving to
a higher elevation, where the air pressure is lower and there are fewer
oxygen molecules per volumetric unit of air, resulting in a richer
mixture.
When you install a lower-restriction air cleaner, it increases the
pressure at the carb air intake, which is like moving to a lower
elevation where the air is denser and there is more oxygen per unit
volume, making the fuel-oxygen ratio leaner.
So one might wonder if the choke works the same way. Well, the choke is
a little different from a clogged air cleaner. The choke plate is
located between the main bowl vent and the venturi, so when you apply
the choke it drops the pressure at the venturi, but not at the bowl
vent, thus vastly increasing the pressure differential between the
venturi and the fuel bowl, allowing much more fuel to be drawn from the
bowl by the venturi.
Regards,
Alan
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