Fox EZ Series CarburetorsOperation and modifications
The EZ series is currently used on .40 and larger size engines. Smaller engines continue to use the MKX type. While outwardly
these appear to be just simple air-bleed types, they are actually a bit more sophisticated than that.
EZ carburetors are simple to operate, just like any other air-bleed type, but the throttle barrel also contains a metering
slot that reduces fuel flow as the throttle is closed. This provides metering through the midrange, something not usually
seen on air-bleed carburetors. The method employed is neither new or unique however, being virtually identical to that used
on certain 70's vintage Enya carburetors. In some respects, the metering system is similar to that employed in the Perry carburetor,
although it is not adjustable like the Perry, and this has sometimes proven to be problem. More on that later.
EZ Series Adjustment Procedure:
Begin by presetting the air-bleed screw so that it covers about half of the air bleed hole. Start the engine, advance to
full throttle, adjust the high-speed needle for maximum speed then richen the mixture slightly until there is an audible decrease
in rpm. When the nose of the model is held vertical, there should be a noticeable increase in rpm. If the engine sags, even
slightly, the mixture is too lean.
IMPORTANT: Make sure the throttle barrel is "fully" open when adjusting the high-speed needle. The design of the
metering system makes this particularly important on EZ series carburetors. If the barrel is even slightly closed, the upper
midrange may become excessively lean causing the engine to overheat, sag or quit as the throttle is reduced.
When satisfied with the high-speed adjustment, allow the engine to idle for a few seconds, then advance smoothly to full
throttle. If the engine hesitates significantly, the air-bleed screw requires adjustment.
If the engine sputters and slowly increases to maximum rpm when the throttle is advanced, the idle mixture is too rich,
so turn the air-bleed screw out. A somewhat shorter hesitation, followed by an abrupt jump to full speed indicates a lean
idle mixture, so turn the air-bleed screw in. Continue cycling the engine from idle to full throttle, adjusting the air-bleed
screw ¼ turn at a time until the engine accelerates smoothly from idle to full throttle. For safety, stop the engine when
adjusting the air-bleed screw!
NOTE: The air-bleed screw functions in the opposite manner to the high-speed needle. Turning in will richen the
mixture, turning out will lean it. The air-bleed screw is used to adjust idle mixture not idle speed! To alter the idle speed
open or close the "throttle barrel" as required.
EZ Series Problems and Solutions:
Lean upper midrange (.45-.74)
Due to manufacturing tolerances, and the fact that the midrange metering system is fixed (non-adjustable), some EZ series
carburetors may not meter properly through the mid-range. Some carburetors may exhibit a "lean spot" when operating in the
"upper" mid-range. Since the same carburetor is used on all engines in this size range, the presence, and severity of, the
problem tends to increase with displacement, being most prevalent on the .60 and .74. When the barrel is closed slightly from
the full throttle position, the mixture may become too lean and, if allowed to operate in this range, the engine could overheat
and sag, or even quit. In a typical scenario the engine starts easily, runs well at full throttle and idle, and after a successful
takeoff the throttle is reduced slightly for a comfortable cruising speed. Soon, however, the engine surges, sags or even
A few simple tests can help verify if the problem is the carburetor. Adjust the high-speed needle properly at full throttle,
then hold the nose of the model vertically to verify that the engine does not sag. With a friend holding the nose vertically,
slowly reduce the throttle. If there is a range in which the engine seems to sag, surge or quit, a carburetor problem is indicated.
NOTE: Do not hold the nose vertically for extended periods as this will increase engine temperature and could result
in a false indication.
Yet another test involves setting the high-speed needle slightly rich, then slowly reducing the throttle from full speed
with the nose level. The engine speed should gradually decrease as the throttle is reduced. If there is a spot where the engine
speed increases again, this indicates the mixture has become leaner (in this spot), and verifies that the problem is carburetor
If you are experiencing this problem, it can be corrected with a small modification to the throttle barrel. Remove the
barrel from the carburetor body, then use a round needle file to slightly elongate (radius) the edge of the fuel jet hole
as shown in the diagram and pictures below. Modify only the barrel, not the carburetor body! File only a few strokes at a
time, re-assembling and test running the carburetor each time until the problem is eliminated.
This picture shows the typical amount the fuel jet hole must be modified for a Fox .74. The .60 is similar.
Smaller engines may require only slight modification. Many require none.
Lean idle (.40)
EZ carburetors on .40 size engines may exhibit a "lean spot" between idle and ¼ throttle. In this case, the engine also
runs well at full throttle and idle, but exhibits a lean hesitation when advancing the throttle off idle. Continued slow cruising,
or an extended landing approach, may also result in the engine surging or quitting. You may find this condition cannot be
corrected by adjusting the air-bleed screw.
If you are experiencing this problem, it can also be corrected by modifying the throttle barrel. Remove the barrel and
use a small round file to remove material from the bottom edge of the hole as shown in the diagram below. Note that this modification
is done to the bottom side only. Do not alter the shape of the hole on the top side! Now, when the throttle barrel is just
fully closed when viewed from the top, there should still be a small slit showing on the bottom side. This modification will
increase the fuel draw to richen the mixture at low throttle settings, and the air-bleed screw can now be adjusted normally.
File only a few strokes at a time, re-assembling and testing the carburetor each time until the problem is corrected. Be careful
not to damage the fuel jet!
NOTE: Carburetors on earlier Fox small frame .40 engines were sealed with RTV silicone to prevent air leakage where
the carburetor fits into the crankcase casting. Every time the carburetor is removed you must apply a new coating of silicone
when you re-install it, and allow sufficient drying time. The engine will not run properly if you do not! If you have previously
removed the carburetor and did not re-seal it properly, this alone could be your problem. While current engines do employ
a rubber gasket, it is very thin, and may not always seal properly. When the set screw used to attach the carburetor is tightened
it may cause the carburetor to tilt slightly pushing it away from the rubber seal on one side. We still prefer the RTV method
for a reliable seal.
Air Leak due to Crankcase Vacuum Slot
The crankcase on many recent engines (with ball bearings) has been modified to include a slot to the front bearing. The
intent is for the vacuum, created by the intake, to prevent fuel from leaking out the front bearing. The problem is, air is
also draw in through the slot upsetting the mixture, (particularly at idle), and the idle mixture cannot be made rich enough,
even with the air-bleed screw fully turned in. The result is a lean hesitation, or flameout, when transitioning from idle
to full throttle. In some cases, the engine may not run reliably at all below about 1/3 throttle! Even employing the throttle
barrel modification to richen the idle mixture may not be enough. We find this recent modification to the crankcase curious
since the crankshaft previously employed a machined spiral groove that served the same purpose without upsetting the idle.
Perhaps it was a cost savings measure?
If you are experiencing this problem, first make sure your crankcase is the type containing the slot. To determine this,
the engine must be disassembled, and the front bearing removed. The slot will be visible running from the carburetor intake
hole to the front bearing. If a slot is present, you may consider the following options:
1) The slot may be filled
with JB weld. This will certainly cure the problem but must be done carefully so that the crankshaft still fits properly and
rotates freely. Clean the crankcase thoroughly with solvent before applying the JB weld! (Note: JB Weld is a two component
epoxy type material useful in repairing metal articles. It is generally available in hardware or automotive supply outlets.
JB Weld Co., P.O. Box 483, Sulphur Springs Texas 75482.)
2) If you have, or can find one, try installing one of the older MKX series carburetors, or alternately, a Perry carburetor.
These carburetors "sometimes" have enough adjustment range to compensate for the crankcase slot.
Problems with External Fuel Pumps
You should also be aware that EZ series carburetors may not work well with an external fuel pump. Generally, using an
external fuel pump, such as a Perry, is not recommended on engines employing an air-bleed type idle circuit. You may find
that the engine will load up at low throttle settings, transition poorly, or quit, when the throttle is advanced. If you must
a use pump, because of tank location etc., adjust it for minimum acceptable fuel pressure. Of course, installing a Perry carburetor
will also solve the problem.