Not to be confused with modern
two-"needle" (TN) carburetors, these two-"jet"
designs appeared on the earliest Fox RC engines. It is important
to note that these carburetors DO NOT operate the same as the
TN carburetors most of us are familiar with today. Although seldom
encountered now days, these early carburetors were so unique
and innovative in design we feel they are still worthy of discussion.
While other manufacturers were
still offering only simple air-bleed types, Fox was manufacturing
more advanced fuel metered carburetors. Referred to as "two-jet"
carburetors, they were actually more sophisticated than many
of today's current designs. Unfortunately, they were also more
difficult to operate and proved too much for many modelers. (Especially
those who did not read the instructions!) Properly adjusted,
however, they were capable of performance superior to the simple
Even today, carburetors used
on model engines are typically of the single-jet type. The jet
is the small hole through which fuel enters the carburetor venturi.
(Most modelers refer to it as the spray-bar). The amount of fuel
flowing through the jet is adjusted by a needle valve to achieve
the correct fuel/air mixture. Early Fox carburetors, however,
employed two jets, each with its own needle valve. One was intended
to adjust the idle mixture, the other to adjust the high-speed
mixture. As the throttle valve was closed, the fuel flow from
the high-speed jet was also gradually reduced to provide proper
metering through the midrange as well. This was accomplished
by either a tapered slot in the throttle barrel, or a protrusion
in the carburetor casting that covered the jet as the throttle
was closed (depending upon the carburetor model). The low and
high-speed needle valves did not operate exclusively from one
another however, which resulted in significant interaction between
adjustments. Understanding this interaction was the key to successful
It was important to understand
that fuel always flowed through the "idle" jet, whether
at full throttle or idle! Unlike the high-speed jet, fuel flow
from the idle jet was not reduced as the throttle valve was closed.
With the throttle almost fully closed, the high-speed jet was
completely restricted, leaving only the idle needle valve in
complete control of the fuel mixture. As the throttle was increased,
more and more fuel gradually entered through the high speed jet
as well, and the total flow became the "sum" of the
high and low-speed jets. If the correct adjustment sequence was
not followed, success would be unlikely.
When operating the familiar air-bleed
carburetor it was common practice to adjust the high-speed needle
first, then fine tune the idle mixture with the air-bleed screw.
Attempting to apply this procedure to an early two-jet Fox carburetor
would not be successful, yet many tried, and then blamed the
carburetor! Although the correct procedure was essentially the
exact opposite to the one familiar to most modelers, even the
original Fox instructions did not always make this clear! The
following quote from an early Fox .40BB and .45 BB owners manual
...For normal tank installations and flight
conditions, we recommend that the low speed mixture adjustments
be made for maximum RPM and then slowly back the needle out until
the motor speed slows down 500 RPM. The high speed is the same
way: screw the high speed needle out until the motor slows down
Unfortunately, the importance
of adjusting the low-speed mixture first (then leaving it alone)
was not stressed and, while decreasing the speed 500RPM from
maximum may have been correct, most modelers would not have had
a tachometer. Determining this by ear was not too likely either.
Besides, adjusting the idle needle with the engine running could
be dangerous because of the close proximity of the propeller
and hot muffler. It would have also been helpful to include some
method of pre-setting the needle valves to a good starting point
for those times when things got really messed up.
Many early Fox engines have been
collecting dust for years simply because their owners could not
operate the "strange" carburetor. Admittedly, even
knowing the correct procedure, it still took a real feel for
model engines to achieve a good setting. Furthermore, the precision
with which these old carburetors were made was not always the
best making it even more difficult, and sometimes impossible,
to achieve a really good setting. Fortunately, many older Fox
engines will accept the current, more conventional, carburetor
versions without modification. We strongly recommend doing this
to vastly improve reliability and user friendliness.
Since new Fox carburetors are
very reasonably priced, updating an older engine is still economically
viable. Early .40 and .45 schnuerle versions were powerful even
by today's standards, and engines such as the .19 and .25 bushing
or Eagle I .60 would still make great sport engines. The old
.78 would serve well in a larger scale project. By simply updating
the carburetor, these early engines can be put back into service
with very pleasing results.