Fayetteville RC Club

MK-X Carbs (Two Needle carbs)

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Fox MK-X Series Carburetors
Operation and modifications



The MKX series was introduced in 1980, beginning with the flange-mount type for engines .40 size and larger. These remained compatible with earlier engines with three sizes originally available: the MKX-A for the .40 and .45, the MKX-B for Eagle and Hawk .60's, and the MKX-C which was intended for use with a Robart. pump. These were all built in the same casting but had different bore sizes. Additional carburetors were soon added to this series with bore sizes suited to applications such as Q500, piped engines, etc.. MKX series carburetors eventually appeared on the smaller Fox engines as well and, finally, those strange old 2-jet carburetors faded into history.


TN Carburetor Concept:

This second-generation carburetor represents a departure in tradition in that the concept did not originate with Fox. In fact, the MK-X series follows the TN (two-needle) design practice most commonly used today, although many manufacturers still use simple air-bleed carburetors on their economy models. While it may not be unique in concept, the MK-X series is still unmistakably Fox in appearance.

The TN design concept employs only a single fuel jet, yet has two needle valves to control high and low-speed mixture. The high-speed needle assembly is attached to the carburetor body in the usual manner, but the low-speed needle is attached to the throttle barrel and moves in and out of the carburetor body in a helical fashion as the throttle arm is moved. As the throttle is closed the low-speed needle travels inward, along with the barrel, gradually entering the jet and reducing the fuel flow. The fuel/air mixture is thus metered properly throughout the entire speed range.

A point of interest is the unusually long helical movement of the throttle barrel. This allowed greater control over the taper of the idle needle for more precise fuel metering. Fox even included instructions on how to modify the taper to customize performance, but we would recommend this only to those very experienced with model engines. The idle needles are generally acceptable in stock form, and the average modeler is likely to do more harm than good attempting modifications this delicate.


MK-X Adjustment Procedure:

If the carburetor is far out of adjustment and the engine will not run, begin by screwing the high-speed needle all the way in, then turn it out 3 turns. Next close the throttle barrel until only a small slit is showing, then connect a length of tubing to the fuel nipple and blow through it. Adjust the idle needle until air is just able to pass through the tubing. As the throttle is opened there should be a noticeable increase in air-flow. This should give you a ballpark setting to begin with. Start the engine, advance to full throttle, then adjust the high-speed needle until the engine holds a steady 2-stroke setting. Next, reduce the throttle until the engine is at a fast idle, then, after a few seconds, advance the throttle smoothly. The engine should quickly, and smoothly, return to full throttle.

If there is a significant hesitation, the idle needle requires adjustment. At this point many modelers have trouble determining which way to go on the adjustment, rich or lean? If you observe and listen carefully, the nature of the of the hesitation will give you a clue. If, when the throttle is advanced, the engine sputters and clears very slowly while spewing lots of smoke and raw fuel out the exhaust, this is an indication that the idle mixture is too rich, so turn the idle needle in to lean it. If the hesitation is very short followed by a very abrupt return to full throttle, or the engine idles only briefly then quits, the idle mixture is too lean, so turn the idle needle out to richen it. If you find that the engine will idle for long periods of time but will not speed up, or even chokes and slows when the throttle is advanced, this also indicates a slightly lean idle mixture.

Continue to cycle the throttle from idle to full throttle while adjusting the idle needle in small 1/8 turn steps until you achieve a smooth transition. If the idle needle is far out of adjustment you will find that the small adjustment steps seem to have little effect at first. As the needle approaches the proper setting however, it becomes more and more sensitive until just a few degrees of adjustment makes a significant difference. If you do not proceed slowly, and carefully, you can easily go too far and pass right over the optimum setting! As you approach the correct setting you will also notice that the idle speed, from which the engine can transition smoothly, becomes lower and lower. The best overall carburetor performance is generally achieved when the "idle mixture needle" is set as lean as possible without significant hesitation. For safety, stop the engine when adjusting the idle needle!

! Important Note

Some modelers attempt to use the idle needle to adjust the idle "speed". This is not correct! Once the correct idle mixture has been achieved, as determined by good throttle transition, open or close the "throttle barrel" as required to adjust the idle speed.

When you are happy with the idle and transition, go back and fine-tune the high-speed needle. Adjust for maximum speed then, richen the mixture slightly until there is an audible decrease in rpm. Always change the needle setting very slowly allowing the engine time to adjust to the new mixture. When the nose of the model is held vertical, there should be an audible increase in rpm. If the engine sags, even slightly, the mixture is too lean! Get in the habit of performing a nose high, full throttle check before every flight. Don't assume all is well just because the last flight was ok. Model engines are very sensitive to changes in temperature, humidity etc. and may require some small adjustment of the high-speed needle as the day progresses.


MK-X Problems and Solutions:

Poor throttle response, frequent "flame-outs" and difficulty finding a consistent high-speed needle setting are common complaints, however, the carburetor is not always the main problem. Rather than the carburetor, these problems are more often due to the design of the head button and/or, in the case of ABC engines, a poor piston-liner fit.

The MKX series often tends to run quite rich in the midrange, even with the low speed needle set as lean as possible. This is due to the high fuel draw characteristics of this series, along with inconsistent manufacture of the low-speed needle. Most modelers will tolerate this, but if you want better performance we suggest installing a Perry carburetor (or the new Fox improved TN when available). You can also try the later Fox "EZ" series carburetor, but be aware that a small modification may be required.

Fuel Leakage

Leakage is a problem that may be encountered on some MKX series carburetors. Fuel leaking from the threads of the high-speed needle, and fuel nipple, not only makes for less consistent needle settings, it is also sprayed back onto the engine by the propeller blast. The oil in the fuel bakes on and, before long, the engine becomes an unsightly black mess! Fuel leakage can be substantially reduced by employing the modifications indicated in the picture below:


Installing a short piece of fuel tubing on the high-speed needle will stop fuel from leaking back through the threads. Some carburetor models use a slotted nut to retain the spring clip. This nut must be installed with the slot facing in, or fuel may still leak past the slot. The fuel nipple can be sealed by applying a small amount of thinned epoxy where it enters the jet assembly. Apply with a small brush or toothpick. Note that the idle needle does not require sealing because the fuel is not under pressure at that point. Although earlier carbs. did not have one, Fox now has gaskets to provide a better seal between the jet assembly and carburetor body. The gasket not only prevents fuel leakage, it also prevents drawing in extra air which can upset the fuel metering process. You can contact Fox Mfg. to obtain a gasket, or alternately, you can carefully apply a thin layer of RTV silicone around the edge of the jet assembly where it mates to the carburetor body.

Inconsistent Idle due to Cam Screw Fit and Wear

The cam screw retains the throttle barrel in the carburetor casting and rides in slot that is milled into the barrel. The angle of this slot determines the in-out motion of the barrel as the throttle is opened and closed. The problem is the excess clearance between the cam screw and slot.

Not only does the cam screw have some clearance in the slot even when new, it also tends to wear rather quickly resulting in significant in and out movement of the barrel even though the throttle arm has not been moved. This can result in an inconsistent low-speed mixture that is particularly annoying at lower idle speeds. You may find that, for the same throttle setting, the idle may be perfect one time but too lean or too rich the next. No matter how much you fiddle with the low-speed needle the problem remains because the position of the barrel, and thus the idle needle, constantly varies.

There is an easy cure for this, however, and it is the same one commonly employed by other makers of TN type carburetors. The fix involves simply installing a spring at the end of the throttle barrel to provide constant tension against the cam screw. In flange-mount MKX carburetors the barrel already has a suitable recess machined into the end that, seemingly, was intended for this very purpose but never employed by Fox. The spring from an inexpensive ball point pen can be modified for this purpose. The procedure is simple: Remove the cam screw, slip out the barrel, slip the *pre-cut spring over the spray bar, reinstall the barrel and cam screw.

*You'll have to cut the spring to a suitable length. We have found that 4 or 5 turns is about right for a typical ball point pen spring but you may have to do some experimenting depending upon what spring you actually have. You want enough spring to keep the barrel nicely tensioned against the cam screw, but not so much that the throttle action becomes too heavy at low throttle settings. A light pressure is all that is really required. Also, if you make the spring too long you may not be able to close the throttle barrel far enough. It may take a little experimenting to get it just right, but it's really not a difficult task on the flange-mount MKX carburetors.


Add a barrel tensioning spring to make idle and transition more consistent

For the square-neck type carburetors, like those used on the small frame .40, the spring fix is more difficult, although not impossible. The added difficulty is due to the fact that the barrel on these carburetors does not have the recess in the end, which means there is very little extra space for a spring. To allow the throttle to close fully there is only enough room for a couple of coils. The typical ball point pen spring may no longer be suitable as the spring wire is often too thick to come up with a suitable spring rate with so few turns. We have had success using finer springs with the coils suitably spread. Although is only possible to get tension at low throttle settings, this is where it is required the most anyway. Finding a source of small springs to play with will be up to you. Of course, if you have the capability, it is also possible to machine a recess in the end of the barrel which will give you a lot more latitude in spring selection.

Fayetteville RC Club
AMA Silver Leader
#680