Fox old style 2-jet carburetors Operation and Setup

Fox 2 - Jet Carburetors
Theory of operation

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 air-bleed types.

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 operation.

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 is typical:

" ...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 500 RPM...."

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.