Taming the Fox 120 twin

Project #8 - Taming the 1.20 Twin

User Manual

Utilizing cylinder assemblies from the Eagle II .60, the 1.20 twin was introduced in the late 70's with production continuing into the early 80's. Early models were equipped with the ancient 2-jet carburetor while those produced from 1980 and on used the newer MKX series carbs. These engines were known to be finicky to adjust and run hot, with a tendency for a cylinder to flameout during throttle transitions. This should not be too surprising since the Eagle II .60, upon which the 1.20 is based, exhibited the same problems, so with the 1.20 the problems are double! Fortunately, the 1.20 responds just as well to the same modifications that we have used to tame the .60.

So, what's the problem?
The Eagle .60 II was produced at the beginning of an era during which Duke was obsessed with making his engines perform well on FAI (no nitro) fuel. This resulted in unusual head button designs with very small combustion chambers, wide squish bands, and high compression. While Duke did manage to get good power from his engines on FAI fuel, this often came at the expense of poor running qualities when using the typical 10-15% nitro fuel that most modelers preferred to use. Engines were finicky to adjust, ran hot, and would often flameout during throttle transitions. So, naturally, the 1.20, being two Eagle .60's, exhibited the same problems. Having two two carburetors to setup, particularly the older 2-jet types, certainly didn't make running the 1.20 any easier either.

The Fix
The first step is to obtain the parts you will need. An absolute must is to order two new Eagle IV series head buttons, along with four 0.01" shims from Fox Mfg. Don't even bother messing with the engine without these! If your engine has the earlier 2-jet type carburetors, upgrading these to the MKX series, NOT the EZ series, is highly recommended. The ancient 2-jet carb. is difficult enough to operate when there is only one to worry about.

Install the head head buttons and shims as shown in the diagram below:

 Installing Head Button and Shims

While the basic head button fix is generally acceptable, if you have the capability to do a bit of simple machining, the Eagle IV head button can be improved further. By lowering the position of the glow plug by .030" and adding a 5 degree angle, and a slight radius, to the squish band, you can achieve an even smoother running engine. The diagram below shows the details. The squish band angle is not ultra-critical, and the radius can be easily achieved by wrapping some #400 sandpaper around a small dowel, and working the inner edge while spinning it up in a lathe. Don't get too carried away, just round off the sharp edge a bit. Again, not really critical. The shims mentioned above are still required.

Head Button Modification

Adjusting the Carburetors
With the new head buttons and MKX series carbs. installed, the engine is now at least capable of proper operation. All that remains is the challenge of properly adjusting the carburetors. First, you should understand that both cylinders share both carburetors. They are not exclusive! This means there will be considerable interaction between the carburetors during the adjustment procedure. It is possible to have good top end power with one carb set very rich and the other very lean. However, the idle and mid-range will be terrible. Getting the settings properly synchronized is the tough part. What we did was remove the carbs. and install them individually on an Eagle .60 where they were adjusted for similar operating characteristics. Then they were re-installed on the 1.20 and adjusted together, one click at a time until proper settings were achieved. Try to think of the the carbs. as being one large carburetor rather than individual carburetors for each cylinder.When you make an adjustment to one, make the same adjustment to the other. Of course, it helps when both carbs. are pre-set to about the same settings. If you don't have an Eagle .60 on which to adjust the carbs. separately, then pre-set the needle valves as described in our section on MKX carburetors and start from there. This is a procedure that requires a lot of patience, but hang in there and you'll find that proper operation is achievable.

When properly adjusted, the 1.20 will start easily on one cylinder, then a touch of the glow clip will fire up the other. Since the cylinders fire together, the engine is also very smooth with little vibration, and tends to sound more like a large single than a twin.

Propeller Note
Unlike 4-stroke or gas engines, the 1.20 is not happy swinging large propellers. If lugged down much below 10,000 rpm it will quickly overheat. Note the following quote from the owners manual:

"In selecting your propeller remember that the Fox Twin is a high speed machine designed to run in the 11,000 to 14,000 RPM range.. ...In practice, an 11-8 is about as small as practical and a 16-4 is about as large as practical. Leave the huge propellers for motors that are incapable of revving up".