While Eagle series engines are powerful, and generally well made, they may also exhibit some frustrating
running qualities that prevent many users from enjoying their full potential. By applying the following modifications you
can, in many cases, significantly improve the running and handling qualities of your Fox Eagle .60 engine.
Why are modifications necessary?
Our experience with Fox Eagle series engines
over the years has revealed two primary components which may cause running or reliability problems. These are: the head button,
and the EZ series carburetor (on those engines so equipped). We'll summarize each of these briefly:
Problem #1: Head Button:
When the Eagle II was introduced, Duke Fox was determined
to try the get the best performance from FAI (no-nitro) fuel. This resulted in a rather high compression ratio. While he did
manage to get quite a bit of power from FAI type fuel, in our opinion, the running and handling qualities of the engines suffered.
Many engines tended to run hot, were finicky to adjust, and would often "flameout" during throttle transitions. This was particularly
true when the nitro content exceeded 5%. In the Eagle series, II through the present version IV, numerous head button variations
were tried over the years in an attempt to reach a suitable compromise, however, we feel this was never fully achieved. While
the current head button is a big improvement over earlier versions, some problems still remain. We'll show you how to solve
Problem #2: EZ Series Carburetor:
While the EZ series carburetor appears
to be just a simple air-bleed type, internally it is actually more complex than that. The throttle barrel also contains a
metering system to control the mixture through the mid-range as well. It is the metering system that we have found to be a
problem. Because it is fixed, (non-adjustable) metering problems can occur in some cases. In particular, we have found the
upper-midrange (just below full throttle) is often too lean. The engine may run fine at full throttle, and idle well, but
as the throttle is reduced slightly from full, the engine may run too lean and sag, surge or even quit. Numerous, unexplained
deadsticks may occur. When the aircraft is retrieved the pilot often finds the engine to be hot even though the high-speed
mixture seemed fine on the ground. The engine is started again, checks out ok, but the same thing happens on the next flight,
and the next. When this is combined with the head button problem, a very unfriendly engine can result. The carburetor
problem can be fixed, however, and we'll show you how to do it.
A Fix for the Head Button Problem:
The basic fix involves installing the very latest head button (from the current Eagle .60 IV) and, possibly,
up to two .010" head shims. The current head button part number is #60125. The shims, which are just the head gasket from
the old Eagle I series, have no listed part number but are still available from Fox. Note: If you purchased your Eagle .60
within the last few years, it likely has the current button, so you will only need the shims. First try the new button without
any shims. If you still experience flameouts or overheating then add the shims as shown in the following diagram. The number
of shims required depends upon such things as nitro content, and the elevation of the flying site. Near sea level, using typical
10% sport fuel, both shims are generally required. At higher elevations the engine may run fine with only one, or even none.
Generally, more nitro = more shims. Don't use more than two shims, however, as the power may drop significantly, and/or you
may again experience running problems.
|Installing Head Button and Shims|
Note: Head Shim Side Effects
The number of shims required depends upon such things as head button version, propeller
size and nitro content. The altitude of the flying site also plays a part. The closer you are to sea level, the more head
shims are required. Engines operated at higher altitude sites may run acceptably with no shims at all. We recommend adding
no more than two .01" shims. If too many shims are added, power may be significantly reduced, and/or the engine may not be
able to attain a proper two-stroke setting. As you try to lean the needle it may just quit abruptly. Adding shims also has
the undesirable effect of moving the glow plug higher up in the combustion chamber and, if carried too far, can again bring
about unexpected "flame-outs".
A Custom Head Button
While the basic fix is generally acceptable, if you have the capability to do a bit of simple
machining, the "current" 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 a super-smooth 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 may or may not be still be required
after the button modification. Start without them and add one or both as required.
|Head Button Modification|
Note: The button modification does not apply to the .74. The .74 button is quite different in
design and requires a custom made button.
Head Clamp Modification:
Another little modification we like to do involves machining the surface of the head clamp where it contacts
the head button. The head clamp casting tends to be quite rough and/or porous which reduces the surface area contacting the
head button. We have seen quite a number of engines with a layer of baked on castor in between the top of the head button
and the head clamp. Machining this surface allows full contact to further aid cooling. Although only a very small amount of
material will be removed, we also recommend removing an equal amount from the bottom of the head clamp. The head clamp comes
very close to the cylinder casting and this will ensure that it does not contact the cylinder before the head button is properly
A Fix for the EZ Carburetor:
An overly lean upper-midrange may be exhibited by some EZ series carburetors, particularly on larger Fox
engines. Here is one way to help determine if you have this problem: Adjust the needle valve for proper full throttle operation.
Adjust for maximum speed, then back off (richen) a bit so the engine slows down a few hundred rpm. When the nose of the model
is held up, the engine should not sag. Now, with a friend holding the nose up, slowly reduce the throttle from full.
If there is a range in which the engine surges, sags, or quits, you have the carb. problem. Note: Do not hold the nose up
for extended periods as this will increase the operating temperature and could result in a false indication. If the problem
is present, it can be cured with a modification to the throttle barrel. Remove the barrel, then use a small round "needle"
file to elongate the fuel jet hole as shown in the picture below. The modified fuel jet provides a more gradual transition
to the thin metering slot, richening the upper mid-range. File a little at a time then reassemble the carb. and test run.
Continue to elongate until the problem is gone. Be sure to clean the barrel thoroughly before re-installing! If you go a little
too far, the engine will just be a bit rich in the upper midrange, which is actually desirable anyway. The picture illustrates
the typical amount the fuel jet hole must be modified on the .60 size engine.
|EZ Carb. Throttle Barrel Modification|
If you would prefer not to play around with modifications, another option would be to install a Perry carburetor.
Some recent problems:
There are 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 some 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 may not have the adjustment range to compensate for this, resulting in the inability to achieve a proper mixture
adjustment. Often the engine will run poorly in the low to mid-throttle range although some engines suck in enough air that
they run poorly over the entire throttle range. The engine need not be disassembled to check for the presence of the slot,
just remove the carburetor and the end of the slot is visible as shown below.
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. The engine must be disassembled, then 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
An Important Final Note:
It is important that your engine still be in
good condition before employing these modifications. The tendency to run hot can result in an engine that is "fried" in very
few flights as the frustrated pilot tries over and over again to get a complete flight without a flameout. Your Eagle .60
should have strong, snappy, compression when you flip it over. If this is no longer the case, you will likely need a rebuild
first. The modified head button, or shims, cannot be expected to cure a worn or damaged engine.