Information and help for users of FOX model airplane engines

Some Basic Troubleshooting Tips

Taking a little time to check a few simple, basic things can save you a lot of aggravation at the field. Before you decide your engine is junk, or needs a rebuild, we recommend you first check the following:

Fuel System - Assembly & Installation
It's amazing how little care some modelers take in the assembly and installation of components that can have such an impact on the way the engine runs. Taking a little time here can really save you a lot of aggravation at the flying field. While the assembly of the common, plastic, clunk type fuel tank is relatively simple, it is important to pay attention to the little details. Before doing anything, the new tank should be rinsed out well. Why? Because often there will be small plastic remnants left inside from the manufacturing process and these will invariably find their way into the fuel lines, or carburetor. Even a tiny sliver of plastic can clog the fuel jet in the carburetor, often intermittently, causing setup problems and unexplained deadsticks. (A fuel filter would help, but how many people actually use them?).

The next step in the tank assembly usually involves cutting pieces of brass tubing to length, and inserting them through a rubber plug. Too often, people brutally cut, or hack these to length leaving jagged, sharp edges that can quickly cut through the fuel line causing air leaks, and again, setup and running problems. Often this happens inside the tank where the fuel pickup line connects to the brass tube. The result is an engine that runs ok for the few first minutes, but once the fuel level drops to the point where the break in the pickup line is exposed, the engine sucks in air, leans out, and often quits. So, please take the time to dress up the ends of the brass tubing to remove sharp edges and burrs.

The fuel pickup line, and clunk, are installed next, and must be the correct length. If the line is too long, the clunk will not move freely, and could jam against the back of the tank restricting fuel flow. If too short you will not be able to draw all the fuel out of the tank. The clunk should be located in about 1/8 to 1/4 inch from the back of the tank. Note that the problem of the engine surging (often during a takeoff run) has often been attributed to the clunk being too close to the back of the tank so be sure to leave a little space there.

Now that your tank is assembled and the rubber plug secured, there is one step left, and that is pressure testing. Besides avoiding leaks, the tank must be air-tight to retain the pressure that is supplied by the pressure tap on the muffler. Muffler pressure is a necessity on many modern glow engines because they are typically a bit "over-carburetted" and would run too lean without it. To pressure test your tank, connect a length of fuel line to both the fuel and overflow tubes. Pinch off one line and blow into the other building up pressure in the tank. Then pinch off the other line and hold for a few seconds. Now release the line and you should hear the air rushing back out of the tank. If not, your tank is not airtight. Check that the screw retaining the rubber plug is tight and that the tank is not split somewhere. IMPORTANT NOTE: in all cases, and especially where the tank has already been installed and used - BE SURE to drain all the fuel from the tank before doing the pressure test! If you don't, when you release the fuel line, fuel will come out, under pressure, and could get into your eyes, or mouth, causing serious harm. We recommend you remove the tank (or hatch) first so that you can verify that it is empty.

Finally comes the actual installation of the tank. Ideally, the tank should be mounted as close to the engine as possible and the center of the tank should be in line with the needle valve. The tank should be padded all around with at least 1/4 inch of foam rubber to isolate it from vibration and prevent fuel foaming. Never jam the bare tank tightly into the fuselage! The fuel and overflow line (usually connected to the pressure tap on the muffler) should be kept as short as possible. Be wary when using one of those nifty fuel filler valves in line with the fuel pickup. They are notorious for eventually leaking, then causing the engine to suck air resulting in setup and reliability problems. When using these devices it is also easy to end up with very long fuel lines. By the time you route the line to the filler valve (often on the side of a cowling) then back to the engine it is easy to end up with a foot, or more, of tubing. This can be a source of fuel draw problems.

So, if you didn't take the time to do a careful fuel system assembly and installation and are now experiencing setup or reliability problems, we suggest you remove the tank and fuel lines, disassemble it all, and check the items above. Even if you were careful from the start, time and use will take its toll on the fuel system components and something will eventually fail. The fuel system is a good place to start looking when a faithful engine suddenly starts letting you down. Check the tank inside and out and replace the fuel lines. Even a small pinhole can cause you grief.

You may think that the muffler has little to do with the way your engine runs, but it does. On most modern engines, the muffler is not only there to reduce the noise level, it is also an important part of the fuel system because it supplies pressure to the fuel tank. The carburetors fuel metering system relies on acceptable fuel tank pressure for proper operation, especially at higher throttle settings, although the midrange can be affected too. The use of a non-stock muffler can cause problems if it does not supply proper pressure. Some of the popular "Pitts" style mufflers have proven to be a problem in this regard. If the pressure is too low the high-speed needle must be backed out much farther than usual to achieve a rich enough setting. In some cases the engine may still be too lean even with the needle almost fully backed out. This can result in a lean top end, but also an overly rich bottom end since the low-speed needle may not have enough range to compensate. If you are using a Pitts muffler, and are experiencing such symptoms, you can try restricting the exhaust stack(s) a bit to increase the muffler pressure.

Another problem that is easy to overlook is blockage of the pressure fitting on the muffler. On some Fox mufflers the hole is very small and may become restricted with carbon. This can cause some interesting running problems like: the engine will idle but won't run at full throttle, or behavior much like the inadequate pressure problem described above. Run a piece of wire, or small drill bit, through the hole to clear it.

Excessive vibration is not only hard on the airframe and radio components of your model, it can also cause all sorts of engine reliability problems. Fuel foaming is a common vibration related problem that can be the source of endless aggravation. The engine may be lean one minute, rich the next, run hot and near impossible to set up consistently. Run your engine at various speeds while observing the fuel line. If you see a lot of bubbles in the line you likely have a fuel foaming problem. Making sure your fuel tank is padded, not just jammed into the fuselage, will help, but in severe cases this may not be enough. You may need to think about beefing up your motor mount and/or firewall.

One test that we do to check for excessive engine vibration involves running the engine at full throttle while carefully observing the tip of the glow plug and the end of the muffler. These should be fairly well in focus. If they are just a blur, the vibration is too great and the engine mount must be improved. For example, if you are using a plastic mount, consider trying a metal one. If you're sure the mount is ok, you may need to increase the thickness of the firewall. These days it is becoming popular to stuff oversize engines in smaller models to be able to "hover", etc. The problem is, the airframe may not be built to take such power, and some beefing up is required to prevent vibration. Also beware of mounting systems that use a wood "breakaway" plate. The popular Goldberg Eagle .60 (an excellent trainer by the way) is one model that uses such a method, and often requires "beefing up" to cure excessive vibration.

Besides fuel foaming, vibration can severely shorten the life of glow plugs, so make sure your engine is mounted securely and the prop well balanced.

Beware of the aluminum spinner that was supplied with many Fox engines. These are notorious for being very poorly balanced so we recommend they not be used.

Glow Plug
Fox engines are generally quite tolerant to different types of plugs and heat ranges. Of course, we always recommend you experiment to determine which brand and type of plug best suits your fuel type, prop size, etc. We use both Fox and K&B plugs with good success. Although a glow plug may function for many hours without burning out, performance will degrade with use. Idle and transition are often the most affected, so if it has a few hours on it, try replacing the plug first before fiddling with the engine.

Fox has, traditionally, recommended that only castor oil based fuel be used in all their engines. In this day and age, however, modelers will have some problems with this. First, castor based fuel is harder to find these days with synthetic, or castor/synthetic blends being the most popular. However, even if it were easily available, the thick oil mess and varnish buildup often associated with castor based fuel is just not acceptable to many, especially when they see their flying buddies using other engines that don't require it. Fortunately, except for the really ancient models, our experience has shown that most Fox engines will do just fine on the popular castor/synthetic fuel blends. We do still recommend fuel that has a little castor in it for a number of reasons: First, many Fox engines tend to run a bit on the hot side and that little bit of castor is beneficial in extending lifespan. Next, ABC engines should always have a little castor in the fuel anyway, and finally, even a little bit of castor protects against those who are a little "heavy handed" on the needle valve. Another thing to keep in mind is that many Fox engines prefer lower nitro fuels (10% or less). Many reliability problems can be solved by simply lowering the nitro content.

Needle settings
To be truly successful at running a Fox RC engine, you must develop a "feel" for the proper high-speed needle setting, which is just a little different than what you may have become used to. Using the method, sound or feel you used successfully with other engines will often result in an overly lean run with a Fox engine. In general, the high-speed needle setting on a Fox engine should be just a bit richer than your instincts tell you. You must learn to resist the temptation to get that last little bit out of the engine on the ground. It will unload more in the air, especially near the end of the flight, and run too lean. With the engine running noticeably rich, at full throttle, slowly turn the high-speed needle in until the engine is just holding a steady two-stroke. Although it is still nowhere near maximum rpm, don't go any further. Now hold the nose up and listen carefully. The rpm should increase noticeably. If your model, or flying style, does not require maximum power, just leave the setting there and fly. This is the maximum durability setting. However, if you want, or need, more power continue to turn the high-speed needle in a click or two at a time, pausing each time to hold the nose up and listening for that rpm increase. When you get to the point where the rpm holds steady with either the nose level, or pointed up, you have gone too far. While this may be an acceptable setting for many other engines, it is too lean for a Fox and the engine will become hot at some point during the flight and may even quit. It won't take too many flights like this to toast your Fox engine. The needle must be set so that there is always some increase in rpm when the nose is held up, and a just little more than you might think. The more power you demand from your engine the more important is is to do a nose-up check before every flight if you expect reliability and durability.