Carburetor Tune Up Guide
previous page: Dual Carburetors
This type of carburetion is used as equipment on the 1941 and 1942 Buick. Both Stromberg and Carter carburetors are used for this installation.
Compound Carburetion defined in simple language would be the same as saying "dual dual carburetion." Actually two dual carburetors are used; these are installed on one dual manifold. The front carburetor is a complete unit containing all the circuits. The rear carburetor, however, only contains the float system, low speed, high speed, and throttle valve assembly. Between the rear carburetor and manifold a damper valve assembly is used. This damper assembly has no connection with the throttle linkage. In this assembly a valve is mounted in each throat and fastened to a common valve shaft. One end of the shaft extends through the housing and has an offset counterweight mounted on it. This counterweight has a tendency to keep the valves closed. When the throttle valves of the rear carburetor open, the air flowing through the throats force the valves in the damper assembly open. The more air flowing, the wider the valves open.
This damper assembly has a definite purpose. The intake manifold is large. The one carburetor is located on the front end of the manifold. It has the choke assembly mounted in it. The rear carburetor is mounted on the extreme opposite end of the manifold and has no choke valve. The starter switch is mounted in the front carburetor. This switch is operated by the throttle pedal and trunnion rod, moving the throttle shaft which is connected to the switch. If the operator wishes to start the engine, after turning the ignition key on, he steps on the accelerator pedal and forces it to the floor. This operates the starter switch but it also causes an open throttle position in each carburetor. The rear carburetor, not having a choke valve, and the throttle valves being open, would bleed all the vacuum out of the manifold. However, the damper assembly prevents this condition from arising, because the damper valves remain closed, sealing the manifold.
As stated in the "Dual Carburetor" section of this book, "a divided manifold is used with a dual carburetor." This holds true in "Compound Carburetion" installations also. Half of the front carburetor and half of the rear carburetor, supply the mixture for four cylinders; the other two halves supply the mixture for the remaining four. As an example, cylinders 1,2,7,5 would be fed by the outside section of the intake manifold. Cylinders 3,4,5,6 are supplied by the inside section.
By using this type manifolding and two dual carburetors, it really boils down to one carburetor for every two cylinders.
Because of "Compound Carburetion", the engine automatically becomes more critical and it can be readily understood why the ignition and compression must be kept on the ball" for top engine performance.
The Stromberg "Aerotype" carburetor used for the front compound installation is practically of the same construction as the carburetors used on the 1939 and 1940 Buick. The rear Stromberg carburetor is the same as the front except it has only the float, idle and high speed system in it.
This is not true of the Carter "Compound Carburetor" equipment. These carburetors are an entirely new series identified and named by Carter as the "WCD" type units. This carburetor is quite different from any of the other dual carburetors (WDO), that Carter manufactures.
The front "WCD" carburetor uses a split or "Kidney" type float system and a float chamber that surrounds the carburetor. The idle tubes or low speed jets, fasten into the float chamber cover where the idle system air bleeds are located. The main metering system uses a vacuum operated metering rod assembly. The pump system is unique in construction. It uses a spring loaded plunger operating linkage. The pump system also has a relief valve built into it in addition to the inlet and outlet check valves, to prevent any damage to the pump system, due to plugged or restricted jets. The choke system is practically the same as the other Carter duals.
The WCD carburetor used for the rear installation has only the float, low speed, and main metering system minus the metering rods. Also throttle valves and shaft.
Some difficulty has been experienced in "Tuning-Up" the Buick engines equipped with compound carburetion. Our engineers have conducted some research on this problem and their suggestions for locating some of these troubles are the following: The first step is to check the engine compression, making sure it is even in all cylinders. Remove the cap screw that holds the cover on the vacuum section of the booster pump. Remove cover and check to see if oil is present in the chamber. If oil is in the chamber, it will upset the carburetion as this oil will be drawn into the intake manifold, richening the mixture and causing fouled plugs. Remove and repair or replace the pump. Carefully check the distributor, especially the vacuum advance diaphragm and the breaker plate. Test in a distributor machine if possible or use a "Cam Angle Meter". Inspect spark plugs very closely. If in doubt as to their serviceability, replace them.
Be sure ignition timing is correct. Use a timing light. Have proper valve clearance.
When setting the idle on a compound carburetor equipped engine, have the engine warmed up well. Turn the idle needles in until the engine falters; back them out until the engine idles smoothly.
If, when setting the idle needles, there is no change in the engine R.P.M. as the needles are screwed in or out, remove the carburetors and thoroughly clean and repair them. Connect vacuum gauge (Hygrade CPA115 Vacoscope) to manifold. With engine idling, use a spout can filled with gasoline and squirt the gasoline on the intake manifold where it connects to the engine. Watch the vacuum gauge for a rise in its readings. If the vacuum increases, it indicates leaking manifold gaskets. Another check to make is where the shaft comes through the damper valve body, the side opposite the counterweight. To check for air leaking into the manifold at this point, place thumb over shaft hole in the body.
If the engine smoothes out, remove damper valve shaft. Cut a short piece off the end of the shaft. Place a plug in the hole where shaft extended and this will seal the air leak.
The last but not the least important suggestion, is to make positively sure the heat riser valves under each carburetor are operating freely and correctly.