Improved TN carburetor for your Fox 40 engine

Fox "Improved" TN Series Carburetor
for .40 size engines only at this time

This is Fox's most recent carburetor series and was engineered to overcome a few shortcomings that had become apparent in the previous EZ series. These included occasional production inconsistencies, mid-range metering problems, and the inability to work well with external fuel pumps. As well, the fixed (non-adjustable) fuel metering slot employed in the EZ series carburetor was not always able to cope with the wide range of propeller sizes, fuel tank locations, etc. that may be encountered. This "improved" TN carburetor puts an end to all of these problems, and then some! Oddly, Fox continues to supply their .40 size engines with the EZ series carburetor even though this one works much better. Yet another Fox marketing mystery? The "improved" TN carburetor is available however, just contact Fox Mfg. and ask for part # 2600.

While this new carburetor does bear an outward resemblance to its MKX series predecessor, the internal workings are really quite different. The basic theory of operation remains the same as the old MKX series (and TN carburetors in general), however, this carburetor is cleverly designed so that it does not depend upon precisely shaped metering slots, or needle tapers, to achieve good throttle linearity. The idle needle is of constant diameter (no taper), and a fuel-metering slot of constant width is employed in the spray bar assembly. Small variations that may occur during different production runs should have little effect on operation. Another welcome improvement is the addition of a rubber seal on the main (high-speed) needle valve assembly. This will eliminate the fuel leakage that was common on the MKX series, as well as prevent drawing in extra air, which could upset mixture adjustments.

Our experience has shown these carburetors to be pleasant to operate and very cooperative. The idle needle, in particular, seems to have a broader operating range than most two-needle designs making mixture adjustments less critical. Throttle transition and linearity are also much improved over the previous MKX and EZ series. In our opinion, this is probably the best carburetor ever produced by Fox Mfg. and is a definite winner! At this time the improved two-needle carburetor is available only for .40 size engines. Other sizes are under development and, hopefully, will be available soon.

Adjustment Procedure

The adjustment procedure is basically the same as the older MKX series (see MKX page). The only real difference is in pre-setting the needle valves when the settings are really messed up. Turn the high-speed needle fully in, then back it out only 1½ turns. With the throttle fully open, set the low-speed needle so that it "just" begins to enter the spray bar. These settings should provide a good starting point.