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
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 proper metering through
the midrange, something not commonly seen on air-bleed 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
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.
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!
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 at certain throttle settings. Some carburetors,
particularly on .45-.74 size engines, may exhibit a "lean
spot" when operating in the upper mid-range. The severity
of the problem tends to increase with displacement since the
same carburetor is used on all engines in this size range. 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 quits unexplainably.
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.
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 related.
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.