Specifications for Fox .40

Bore: .840
Stroke: .715
Disp: .396
Weight: 9.5oz
RPM: 12,800 w 10-6

ABC, BB and Bushing models all have same specs

User Manual for engines with MKX carburetor

User Manual for engines with EZ carburetor

Evolution History of the C-Frame .40

Introduced in the mid-80's, this "C-frame" series engine has its origins the .36 combat U-control engine. The .36 crankcase was modified to accept a muffler, and lengthened to allow the installation of a carburetor. With the addition of a new carburetor designed to fit the unusual square intake stack, the compact C-frame series was born.

Original "Compact Series" engines were introduced with a single rear main ball bearing. The front of the crankshaft ran directly in the aluminum casting. Durability problems quickly became evident with this configuration and a ball bearing was soon added to the front as well. (The current .40 sport bushing version incorporates a sturdy bronze bushing and has no such durability problems.)

Engines in this series originally included the compact .29, .36, and .40 bushing series, as well as the .40BB standard and .40BB Deluxe. Bushing and standard BB models all use steel liners and mehanite pistons, while the deluxe version employs ABC technology. The .29 and .36 sizes are no longer produced, but during their relatively short production period exhibited few running problems. The .40 size engines in this series continue to be produced and we will address some problems that have occurred with these. Test reports on early models praised the high power, excellent idle and friendly running qualities of this engine series (.40 size) and our experience confirms this. Some later examples, however, proved to be much less user friendly.

Although they have remained virtually unchanged in external appearance, over the years internal changes have been made which significantly affected performance and handling qualities. The single greatest change involved the design of the cylinder liner. Very early examples, including ABC types, employed cylinders that were not continuously tapered as you might expect in a non-ringed design, rather they were abruptly "stepped" down .002" in diameter just above the exhaust port. Only the upper portion of the cylinder was slightly tapered. Later models employed cylinder liners with a more conventional constant taper. The diagram below is exaggerated to better illustrate this:

Since the fit was critical, and the original stepped ABC liner proved difficult to hone after being chrome plated, pistons were individually precision ground to fit each liner. Obviously, it was difficult to keep production costs down with this approach. The conventional configuration, however, apparently allowed more latitude in fit. Now with minimal liner honing, and only a few standard piston sizes, a good fit was quickly achieved. Those models employing steel liners were likely also changed to keep production procedures consistent, but it seems only Duke really knew for sure. Unfortunately, this change also negatively affected performance and running qualities. (Particularly the ABC version). Needle valves became more difficult to adjust, and engines were prone to quitting, particularly when operating in the midrange. Intolerance to nitro was another symptom, and anything over 5% often aggravated the problem. Maximum rpm was also reduced noticeably. Older versions, including bushing models, often turned a 10X6 propeller at 14,000 rpm, while many later examples ran closer to 13,000 rpm. (In fairness, it should be noted that Fox only ever claimed 13,000 for these engines, which is still very good performance.)

Perhaps the most significant of these problems was the tendency to quit when operating in the lower mid-range. After only a few minutes running at about half throttle or less, the engine would quit when the throttle was advanced, and no amount of adjusting could eliminate this. Since the original MKX carburetor was slightly rich in the midrange, it allowed the crankcase to slowly load up with fuel. When the throttle was advanced, the excess fuel simply put out the plug. Unlike earlier versions, these engines had become quite sensitive to the fuel mixture, and would quit rather than clear. While the introduction of the EZ series carburetor offered some relief, the problem was not entirely solved. Lowering the compression provided further improvement and this was achieved with a new head button design. The following picture illustrates the difference between early and current model head buttons.

The combination of the head button modification, a modified piston and some fine tuning of the liner taper, have substantially restored the running qualities displayed by early versions. Previous carburetor woes can also be overcome with the new "improved" two-needle design, however, for some strange reason, Fox continues to supply the .40 with the troublesome EZ series carb. The new two-needle carburetor is available from Fox at the same price as the EZ series. Power still lags slightly behind early models, but is still very good, particularly when you consider the compact size and light weight of this engine series. Even the economical bushing version easily out-powers most imports of similar size and configuration

Some Solutions:
If you own one of the friendly early versions, and it is still in good condition, treat it well and be sure to use fuel that contains some castor oil. The original cylinder liner and piston set are no longer available, so a rebuild will involve a new series liner, piston and a new head button. One of the new improved TN carburetors is also highly recommended. Ball bearing equipped engines containing the original stepped liner can be identified by the front of the crankshaft. On early versions the crankshaft had eight small studs, and the thrust washer was keyed to fit these. Later versions employ a taper locked thrust washer and the crankshaft has no studs. Bushing engines, however, continue to use the crankshaft with the studs so cannot be identified in the same way.

If you own an "unfriendly" engine that was built during the troublesome design transition period, there are a number of possible options that provide a solution, depending upon the cost and/or level of performance you consider acceptable.

OPTION 1: Our first recommendation is to upgrade to the latest standards. Assuming the piston-liner fit is still good, this is easily accomplished by just installing a new head button and one of Fox's new improved TN carburetors. This new carburetor (part # 2600) replaces both the MKX and EZ series, and works very well.

OPTION 2: If you are not overly concerned with the somewhat rich midrange exhibited by some two needle MKX carburetors, you may find that just installing the latest head button is an acceptable second best option. Needle settings will be more friendly and, while the engine may still stumble a little in the midrange, it should no longer quit when the throttle is advanced.

If you like to experiment, an MKX carburetor with an excessively rich midrange can be improved somewhat by slightly enlarging the jet in the spray bar. Remove the jet assembly from the carburetor, remove the high-speed needle, then drill out the spray bar with a #55 drill.

NOTE: This modification is for older .40 size "MKX" series carburetors only. Don't try this on the new "improved" two-needle carburetor!

This modification allows the idle mixture needle to be screwed farther into the jet to lean the midrange without causing an overly lean idle. Because it will now travel further into the jet, in some cases it is necessary to shorten the idle mixture needle slightly to prevent it from contacting the high-speed needle when the throttle barrel is fully closed. This is accomplished by filing a small amount off the tip.

OPTION 3: The simplest, and least expensive solution involves simply adding two .01" shims between the head button and the top of the cylinder liner to reduce the compression. While not as effective as the first two options, it may still be acceptable, particularly if the nitro content in your fuel does not exceed 10%. You can also apply the spray bar modification shown in option 2 to further improve midrange performance, if required. While not as effective as the first two options, it may still be acceptable, particularly if the nitro content in your fuel does not exceed 10%. You can also apply the spray bar modification described in option 2 to further improve midrange performance, if required.

NOTE: Many engines (particularly the ABC version) were prematurely "cooked" as their owners tried in vain to overcome the rich midrange, and flameout problem, by setting the high-speed needle overly lean. Some earlier ABC engines were also not fit properly. Before applying any of the suggested improvement options check the piston-liner fit! (Project #6 Determining Engine Condition describes how to check this).

Carburetor Air Leak:
When first introduced, the unusual square-neck carburetor was retained with a single setscrew and sealed to the crankcase with RTV silicone. On later engines a rubber gasket is used along with two setscrews. The RTV method was a better method for sealing these carburetors. The fit of the carburetor in the crankcase casting is often such that tightening the setscrew/s tends to push the base of the carburetor away from the crankcase. The very thin rubber gasket provided may not seal properly. We have even encountered situations where the carburetor will jam in the crankcase before it fully seats against the gasket. The fit is quite variable! We recommend deleting the gasket and applying silicone as done previously. If the carburetor is not completely sealed to the crankcase, the engine will run very poorly!

Some recent problems:
There have been new problems showing up in more recent engines. These are related to a combination of the EZ carburetor and a modification to the crankcase. The crankcase has been modified internally to include a thin slot that runs between the carb. intake and front bearing. The intent was for the intake vacuum to prevent fuel from leaking past the front bearing while the engine is running. Unfortunately, in many cases, too much air is also drawn in through the slot upsetting the mixture. (This is basically an air leak!). The air-bleed EZ series carburetor often does not have the adjustment range to compensate for this, resulting in the inability to achieve a proper mixture adjustment.The engine may run poorly in the low to mid-throttle range. Some engines suck in enough air that they run poorly over the entire throttle range. The engine must be disassembled to check for the presence of the slot. With the crankshaft removed, look through the rear cover toward the front bearing. You should be able to see the slot between the carb intake hole and front bearing. We have been filling the slot with JB weld to fix the air leak problem, but this must be done carefully so that the operation of the crankshaft is not impeded. Remove the front bearing and clean the crankcase thoroughly with solvent. Carefully fill the slot (using a toothpick, etc.) trying not to get too much excess. Allow the JB weld to set for a while, but not fully cure, then insert the crankshaft. The front edge of the crankshaft will neatly scrape away the excess JB weld for a perfect fit. Allow the JB weld to cure fully before installing the front bearing and re-assembling the engine.

Other Suggestions:
We recommend you read the page on EZ series carburetors , if your engine is so equipped. There have been some problems with these, apart from the slot/air-leak problem described above.