Project #3 - Improve your Fox
Fox .50BB Specifications:
Weight: 12oz. (without muffler)
RPM: 14,500 with 10-6
Introduced in the mid-80's,
the Fox .50BB came about by increasing
the bore of the .45 to equal that of the Eagle .60. The .50 even
uses the same ring as the .60 although the piston is different.
This was generally a good engine and one of our favorites, the
only real problem being related to the design of the head button.
It was designed during a period
of time when Duke was obsessed with making his engines
run well on FAI (no-nitro) fuel. The design of the head button,
with its extra-wide squish band and small combustion chamber,
reflects this and the compression was often too high for friendly
operation on typical nitro sport fuels. Earlier engines in particular
tended to run hot, could be finicky to adjust and would sometimes
flameout during throttle transitions. Some engines would detonate
severely making a noticeable"rattling" noise that was
often mistaken for airframe vibration. There were three head
button versions, that we know of, and the severity of problems
will depend upon the version you have, nitro content, prop. size
and the elevation of your flying site.
The head button can be modified
to cure these problems and allow the engine to run well on the
typical 10 -15% nitro sport fuels most of us (in North America)
like to use. To accomplish this you must have access to a lathe,
and possess some basic machining skills.
If you would like to try a custom button but don't have the
means to do the modifications, we have a recommended source for
custom machine work.
The degree of the modifications
depends upon which version of head button you have. There are
three that we know of. While they all look the same at a quick
visual glance, the depth which they extend into the cylinder
liner is slightly different. (This is the dimension described
as "variable" in the diagram below). On early engines,
this dimension was 0.160". These are the engines that would
often make the "rattling" noise when run on anything
much above 5% nitro. The diagram below shows the "full"
modification, which is required if you have such a head button.
This modification includes opening the combustion chamber up
until the squish band has been reduced to 0.185". You will
want to maintain about the same radius on the combustion chamber,
and this will require that you make your own cutting tool for
the job. The other modifications shown in the diagram should
be self-explanitory. Just take .030" off the top of the
button, then add a 5 degree angle to the squish band. (The angle
is not super critical, so if you are out slightly it will still
work ok). Just a note on the radius indicated: We have found
that adding a slight radius to the inner edge of the squish band
makes the engine a little more friendly and broadens needle settings.
This is easily accomplished by wrapping some 400 wet/dry sandpaper
around a small dowel and working the inner edge with this while
spinning the button up in the lathe. Again, this is not super
critical, just round the edge a bit. Do not round the outer edge
that contacts the cylinder wall, however, as this will negatively
affect engine performance.
Head Button Modifications
Over the years, Fox realized
that compression was a problem and began lowering it by removing
material from the bottom of the button (ie, increasing the deck
clearance but leaving the width of the squish band the same).
This was done twice, that we know of. The first buttons extended
0.160" into the liner, then this was reduced to 0.150",
and finally on the latest buttons became 0.140". While each
of these modifications helped, the engine could still exhibit
problems depending upon the nitro content, prop size, elevation
of the flying site, etc. We have found that the .0.160: and 0.150"
buttons both work best when modified fully as shown in the diagram.
The 0.140 button, however, may not require opening up the combustion
chamber. However, the other modifications (lowering the plug
and adding squish band angle) should still be applied.
If you've been wondering what
the deal is with lowering the plug, we've found this to be a
contributor to the tendency to flameout during throttle transitions.
The plug is a bit higher up in the combustion chamber than other
engines of similar design and displacement. We assume Duke did
this as part of his original plan for operation on no-nitro fuel.
Oh yes, be sure to continue to use a long-reach plug. We "want"
the plug to be lower and there will still be adequate threads
left after lowering it.
Over the years the .50 has been
supplied with three carburetors. Production began with the same
MKX-B size carburetor used on the .45, later changed to the larger
MKX-C size, which was the same one then used on the Eagle .60,
and finally, an EZ B series carburetor. We feel the engine is
actually a bit over-carbureted with the MKX-C carburetor when
using the most practical size propellers for most applications,
a 10-7 or 11-6. This can result in an overly rich mid-range and
slow, slobbery, throttle response. We feel the slightly smaller
MKX-B size carb. is a better choice for the .50 in most applications.
The midrange is much better, and there is no rpm loss except
when trying to spin smaller props that are really not practical
for the .50 anyway.