Make an after-muffler for your Fox model engine

Project #2 - Make an "After-Muffler"

Reduces sound level by at least 5db

No power loss

Light weight - only 1oz.


Fox engines have always been a little on the noisy side, even with the recommended muffler attached. With today's increased concerns over noise however, it is desirable to keep the sound level as low as possible. Fox does produce effective quiet muffler options for its Eagle, B and C frame engines, but they tend to be a bit large and quite heavy. We prefer to use this lightweight, effective, and very inexpensive after-muffler.

Assembling the After-Muffler:

You can assemble your own effective "after" muffler using a small aerosol shaving cream can. This light weight addition to your stock Fox tilt up, or tilt down, type muffler typically lowers the sound level by at least 5db, and does not reduce power. On some engines, the maximum rpm actually increases slightly! We have found that most Fox engines run smoother, and seem "happier", with the after-muffler installed.

The 50mL (travel size) can, produced by Gillette, works well on engines from .25-.74, and weighs less than 1oz. In addition, you will require some short lengths of rubber hose to attach your after muffler and to provide an exhaust outlet. We recommend automotive heater and/or fuel line hoses because they are sturdy and provide proper support, particularly at the inlet end that attaches to the stock muffler. (Silicone tubing may be too flexible). Since these come in a variety of sizes, you should have little trouble finding suitable diameters for both inlet and exhaust ends.

The inlet hose, that attaches the after muffler to the stock muffler, should have an inside diameter that fits snugly over the "lip" on the end of the stock Fox muffler. The exhaust exit hose should have an inside diameter about the same as the stock exhaust outlet. (It can actually be a bit smaller and still not reduce power, while further lowering the sound level). Brass tubing and JB weld could also be used to fashion the exhaust outlet, however we have found that the rubber hose seems to have additional damping qualities that take some of the "edge" off the exhaust note.

1) First, drain the can completely. Make sure all the pressure has been released from the can or you will likely be wearing the contents in step 2!

Now you must decide on the appearance you prefer. Unless you wish your model to be a shaving cream advertisement, (perhaps you are a Gillette employee?), you have two choices: Either remove the existing finish to expose the natural aluminum, or paint. If you choose to remove the existing finish, it is easier to do it now before drilling holes in the can. Painting is the easiest because the existing finish resists most solvents, so removal generally requires a lot of elbow grease and wet sanding with #400 sandpaper. Painting can be done after completion, but sand lightly with #400 paper and clean thoroughly before application. We have had pretty good luck with Coverite's 21st Century paint.

2) Pull off the spray nozzle, poke the remaining stub into the can, then drill holes (approx. 3/8") in both ends. If you prefer, the exhaust outlet hole may be located on the side rather than the end. The inner workings can now be removed and the empty shell washed out. Continue by using a round file to open each hole to a diameter slightly smaller than the respective inlet and exhaust hoses. You want the hoses to fit into the holes very tightly!

3) Cut a length of hose for each end and force (screw) it into the hole about 1/4". Trim the inlet hose so that about 1/2" extends above the top of the can. Trim the exhaust exit hose so that 1/2" extends beyond the bottom of the can.

4) Fill in the cavity at the inlet end of the can with high temperature RTV silicone to improve the seal and provide some additional support. The cavity should be tightly "packed" with silicone.

5) When the silicone has dried, slip the after muffler over the end of the stock Fox muffler and secure with a nylon zip tie. Since it is so light, no additional support is required.

NOTE: It is very important that the after muffler be installed as close to the end of the stock muffler as possible. Do not increase the length of the hose between the two! The engine will run hot and power will be reduced!

When properly assembled and installed, the after muffler will not reduce power or cause the engine to overheat. On some engines the maximum rpm actually increases! Engines with a two needle MK-X series carburetor may also show an improvement in midrange performance.

After installation, you will notice that the mixture is slightly richer. Readjust the needle valve/s as required and the engine should perform normally.You may also notice a more visible smoke stream in the air. This does not necessarily mean that the engine is too rich. The after muffler seems to act somewhat as a smoke system as well, particularly if your fuel contains a significant amount of castor oil. Do not attempt to run the engine overly lean to eliminate the smoke! Adjust normally and just enjoy the show.