THE PMO CARBURETOR: By Richard Parr. President PMO

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 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.