IF you have a stock, unaltered military air cleaner and carburetor
setup, and the mixture is too rich, I would NOT blame the air cleaner.
With the stock setup, a dirty air cleaner (or one where the oil level is
too high) will restrict air flow, which can certainly affect
performance, but it should NOT have a signficant effect upon mixture,
since the carburetor bowl is vented to the area AHEAD of the choke, but
in the enclosed area between the air cleaner and the carb. The main
determining factor in mixture (other than the various orifice sizes and
other design factors) is the pressure differential between the fuel
nozzle and the inside of the carb bowl. When the air cleaner is plugged,
it increases the vacuum in the carb throat, but it ALSO increases the
vacuum in the carb bowl, so the pressure differential really does not
change significantly.
Because the bowl is vented ahead of the choke plate, closing the choke
increases the vacuum on the nozzle, but it does NOT increase the vacuum
in the fuel bowl. That's why the choke enriches the mixture.
On the other hand, IF you had a non-stock carb setup where the bowl is
vented to OUTSIDE air instead of to the enclosed area between the air
cleaner and the carb, then a plugged air cleaner would make a VERY
significant difference in fuel mixture.
In fact, in the troubleshooting sections of the military Carter carb
manuals (TM 9-1826A and TO 19-75CCA-6), a rich mixture (poor fuel
economy) is attributed to a clogged air cleaner ONLY on "outside vented
carburetors."
So, if removing the air cleaner made a significant difference on your
engine's mixture, it may have had something to do with improved air flow
and its effect upon opening a sticking choke plate, or possibly upon
better operation of the air emulsion tube, or a number of other factors
involving carburetor faults. One possibility is a marginal float level
setting or a marginal float valve, in which case the extra vacuum in the
bowl from a plugged air cleaner (or one where the oil level is way too
high) might be just enough to keep the float valve open a bit longer (or
suck some additional fuel into the bowl through a leaky valve) and raise
the fuel level a bit too high. There are several possibilities. It may
have even unplugged an orifice.
Plugged orifices in the air emulsion tube (main vent tube) are a common
cause of an overly rich mixture.
By the way, DO NOT run the vehicle for extended periods without the air
cleaner. The air emulsion tube has some very small holes (one is about
the diameter of a human hair) that can be easily plugged with a bit of
debris. This is probably the main reason why they put the tube on the
top center of the carb bowl, where it can be easily removed for periodic
cleaning without taking the carb apart.
I've seen several instances of an overly rich mixture caused by a bit of
dust/dirt clogging the orifices. It wouldn't hurt to remove the tube and
blow it out. Avoid using a wire to clean the orifices, as they can be
easily damaged and the size is critical. The smallest orifice is near
the top of the narrow tube and is barely visible, so check them all
carefully.
Regards,
Alan
Henry J. Fackovec wrote:
>
> As always, you guys are right on the money, The starting problem which several of you diagnosed as an over richness problem was indeed. I pulled the air filter off the M-37 and she starts right up and runs much more smoothly and with more power. I am going to soak out the filter this weekend and see what happens.
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