THE PMO CARBURETOR
During the fifteen years we sold
three barrel Weber carburetors we noticed certain
shortcomings such as short life span caused by
sleeve bushings and axial float of the throttle
shafts, float bowl slosh caused by obsolete float
design, fuel starvation in the turns caused by
offset fuel pick-up, wobbly auxiliary venturis
caused by ineffective support, lack of adjustability
of the idle circuit, poor atomization of the
acceleration circuit caused by the fuel stream
hitting the manifold walls, upsizing a smaller
casting by boring larger (the 46 IDA Weber carb is a
bored out 40mm carburetor).
The PMO carburetor was designed
from the beginning to be a state-of-the art
carburetor. We start off with a casting designed
around a 50mm throttle plate and downsized to 46 and
40mm to achieve three different sizes. We use an
excellent material and process: A-356 aircraft
aluminum and permanent mold casting. Permanent mold
casting is a preferred method in the aerospace
industry. This compares favorably with conventional
carburetors, including Weber, which are die cast pot
metal.
Since we designed a larger
carburetor from scratch the air entry and flow is
much greater than the three barrel Weber. This is
accomplished by having larger main barrels in the
throttle body, tapering the top covers and then
installing larger diameter velocity stacks of
corresponding taper. This maintains the boost from
the velocity stacks down to the exact point where it
operates on the auxiliary venturis.
Flow through the carburetor is
also optimized by many additional details such as
having the accelerator nozzles barely protrude out
in the air stream. The wings on the auxiliary
venturis are CAD designed using a mathematical
formula for an aerodynamic ellipse. Even the
throttle plate screws are specially designed with a
small Torx head and the exact length to engage all
the threads in the throttle shaft but not protrude
out into the airflow.
The stainless steel throttle
shafts are suspended radially by sealed ball
bearings which are preloaded by Belleville springs
to prevent axial float.
Slots are wire cut by a state of
the art computer controlled EDM machine. This keeps
the slots in perfect alignment as opposed to
conventionally machined or sawed slots found in
Weber and all other carburetors.
Float bowl slosh in the PMO
carburetor is controlled by moving the emulsion tube
pylons close together which effectively divides the
float chamber into two halves. To accomplish this we
use the IDF float which is composed of two solid
foam pontoons suspended by a stamped metal frame.
The main jets, emulsion tubes and
air jets are in one stack in the center of the
casting. This allows fuel to be drawn from the
center eliminating fuel starvation in the turns.
Our auxiliary venturis are
investment cast with a round base giving 360°
support to permanently prevent any wobble. The
auxiliary venturis come in two sizes: 4.5 and 5.0.
The 5.0 size being for race engines in the 3.6-3.8
liter range.
The PMO carburetor uses standard
Weber main and air jets found in the Weber IDF, DCOE
and 48 IDA. The idle jets are specific to the PMO
and IDF. The emulsion tubes are common to all three
styles of Weber and to the PMO. We've added a
replaceable idle air jet to more finely tune the
idle circuit.
We've redesigned the accelerator
circuit to cause less air flow turbulence and still
be more efficient. The accelerator nozzles barely
protrude out into the airflow and are aimed at the
curved surface of the auxiliary venturi tube. This
shatters the fuel stream allowing complete
atomization before the air-fuel mixture even leaves
the carburetor. The increased efficiency allows less
fuel to give the same effect. We have a complete
selection of float bowl check valves to fine tune
the accelerator circuit output to the demands of the
engine. We even use a space-age rubber in the
diaphragms which outlasts the material used by Weber
ten to one in today’s gasoline.
To achieve aerospace quality we
machine the throttle body, top cover and accelerator
pump castings on our own four axis CNC milling
machines and rotary tables. By using this equipment
we are able to improve on the techniques used to
manufacture Webers. For instance, the drill runs on
the accelerator and idle circuits are straight shots
eliminating bifurcation or trifurcation. We stress
relieve our throttle bodies after rough machining so
we can maintain precision tolerances in the final
machining. We were able to eliminate the set screws
holding the main venturis and the springs holding
the auxiliary venturis by closely watching the
tolerances on these two items.
The heat treatable aircraft
aluminum we use is stronger, more stable and allows
much improved finish and tolerances over the
material found in ordinary carburetors.
It should also be noted that all
parts that comprise PMO carburetors are machined to
PMO blueprint specifications and individually
inspected to aircraft standards. All assembly is
done in our facility with every stage documented. We
install jetting according to each individual order
from each customer. This customer information is
also entered in our database. We offer technical
support and encourage feedback from customers.
In design, construction and
performance, PMO carburetors are a truly unique
product on the market.
