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:
- 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
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
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.
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
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.
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