Description of the Solex 40 P II - 4 Carburetor

Parts Diagram

solid-shaft This diagram is for the early cars.
split-shaft Later cars.


solex_1 The Type 912 Porsche is equipped with two SOLEX 40 P II - 4 double-throat downdraft carburetors. The induction throats are 40 mm (1.575in) in diameter. Since the carburetors are located very close to the combustion chambers, cold starting enrichment devices are not needed.


solex_2 The carburetor basically consists of the main body and cover, with a gasket separating the two. The main body contains two induction barrels, each having an independent idle and power metering system. The throttle shaft, which passes through both barrels, controls both throttle valves and carries a throttle return stop and throttle arm.
solex_3 The accelerating pump located on the broad side of the carburetor is actuated through an adjustable rod and feeds fuel to both induction throats.
solex_4 The float chamber is located between both induction throats. The fuel level in the float chamber is regulated through the buoyancy of the float, that is, the float tang opens or closes the float needle valve. The float level may be adjusted by means of an externally located screw which adjusts the height of the intermediate swivel joint. This provision makes it possible to easily adjust the float level for the particular grade of fuel used. The fuel level may be checked by removing the plug from the inspection port.

The carburetor cover accommodates the fuel inlet, float chamber vent, and float needle valve (the latter is accessible from within). In addition, two power enrichment nozzles are press-fit into the cover.

Idle Metering

solex_5 The fuel enters through the idle metering jet (g) and mixes with air entering through the idle air bleed (u) forming an emulsion. The emulsion flows to four small discharge ports located near the throttle valve. The amount of emulsion dispensed through the lowest port is controlled by the idle mixture screw (W). The emulsion dispensed into the induction throat through the idle mixture port combines with induction air which is passing through the partly open throttle valve and atomizes into idle mixture.

The idle mixture can be leaned out by turning the adjustment screw in, and enriched by turning it out: both screws should be set approximately same.

The idle speed adjustment controls the engine idle rpm, i. e.. turning the screw clockwise increases the rpm, and turning it counterclockwise decreases rpm.

The idle system employed in this carburetor is an independent system since it draws the fuel from a point before the main jet carrier (Y). As a result, negative pressures prevailing in the induction throat have a continuous effect on the idle metering system and a certain amount of the idle mixture is fed into the induction throats at normal power settings as well.

Intermediate Metering

solex_5a The three discharge ports located above the idle mixture discharge port provide progressive metering at intermediate throttle openings between idling and power. The lowest port, located at the throttle valve level, feeds idle mixture when the throttle is set for idling, while the two upper ports begin to feed the mixture as the throttle begins to open. This metering provision ensures smooth transition from idle speeds to power settings.

Power Metering

solex_6 The fuel flows through the main jet carrier (Y) and the main jet (Gg) into a well which contains the emulsion tube (s) and, directly above it, the air correction jet (a). Vacuum in the induction throat draws the fuel into the preatomizer (X) where it mixes with air and continues to the venturi (K) where it is fully atomized into the combustion mixture. As the vacuum in the induction throat increases, the fuel level in the emulsion tube well decreases and air enters through the air correction jet, mixing with the fuel through orifices in the emulsion tube and effecting a derichment of the fuel air mixture.
solex_7 As long as the engine is operating in the mid-rpm range under partial or full throttle load, only the main metering system supplies the fuel. However, as the air velocity in. the induction throat increases with increasing rpm, the vacuum effect in the throat becomes so intense that it begins to draw supplemental fuel from the power enrichment nozzle (q1); the power enrichment system consists of the discharge nozzle ([1) and the metering jet (q2) and draws fuel directly from the float chamber. The enrichment system comes into action when the engine is running under full throttle at high rpm.

The main metering system together with its supplemental enrichment system ensures a well balanced and metered fuel! air mixture required for combustion, realizing the desired fuel economy and, yet, maximum power output on demand.


solex_8 A mechanically actuated diaphragm-type accelerating pump is employed. The pump receives fuel directly from the float chamber. When the pump is at rest,. the diaphragm (M) is kept outwards by the diaphragm spring (m). A s the throttle valve opens, actuating motion is transmitted to the pump by the pump rod (T) and pump lever (L5), pushing the diaphragm inward against the fuel and forcing it to pass through the pump jet (Gp) and the calibrated injection nozzle (i) into the venturi, enriching the fuel/ air mixture and resulting in smooth engine response at acceleration.

The check valve (Hl), in the pump inlet, prevents the fuel from backing up into the float chamber; a second check valve (HZ), at the base of the injection nozzle, prevents air from entering the pump through the injection nozzle when the pump is on the inlet stroke.

The amount of fuel dispensed by the pump on its pressure stroke at time of acceleration is predetermined by the length of the pump stroke; the stroke can be adjusted through the pump adjustment (t). The pump jet and the calibrated injection nozzle control only the duration of injection.
solex_9 The check valve assembly (HI) has a return flow passage measuring O.36 mm (0.0142 in) in diameter. This port prevents excessive enrichment of the fuel/ air mixture by fuel dispensed by the fuel pump during acceleration, that is, depending upon the speed with which the throttle pedal is depressed, larger or smaller amounts of fuel are permitted to flow back to the pump.
Last modified: Fri, 12 Aug 2005


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