Carburetor Tune Up Guide
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The "dual" or double barrel carburetor is so named because of its construction. Actually it is two single barrel carburetors, built to form one unit. It is so constructed that only one choke system, float circuit, and power system (Stromberg and Zenith) is needed. It contains only one accelerating pump system with the exception of the pump jets of which there are two.
However, all other circuits are double. In other words, two of each of the following: main metering jets (metering rods on Carter), main nozzles, high speed air bleeds, venturi's and complete idle systems. There are two throttle valves, one in each throat of the carburetor. These, however, are mounted on a common throttle shaft.
The basic reason for a carburetor of this construction is its ideal application to larger engines.
In the majority of cases it is used on eight cylinder engines. However, there are a few exceptions where this type carburetor is equipment on six cylinder engines.
When using a dual carburetor, the intake manifold on these particular engines can be divided so that one half the carburetor feeds three or four cylinders, as the case may be, and the other half supplies the mixture for the remaining three or four. Because of this, better carburetion is obtained and consequently, smoother engine operation and fuel economy is realized.
However, because of the dual circuits incorporated in these type carburetors, great care must be exercised in repairing them. Not for the reason of their construction being complicated, but simply that the balance between both circuits must be maintained for good, efficient engine operation. As an example, a dual carburetor having plugged air bleeds on one idle system and the other idle system air bleeds being clear would upset the balance between the idle systems. The mixture ratio flowing from the idle system with restricted air bleeds would be entirely too rich. However, the other idle system would be operating properly. Since the manifold is divided and one half the carburetor is feeding, let us suppose four cylinders, and the remaining half of the carburetor is feeding the other four.
It can be easily understood that four cylinders are receiving too rich a mixture ratio and the other four cylinders are being supplied with a mixture ratio decidedly less rich. The result is an engine that will lope and roll and idle very unevenly. In the majority of cases where this condition exists in a dual carburetor, remove and repair the carburetor.
As previously stated, all downdraft carburetors contain five circuits regardless of size or shape. However, the construction of these circuits will vary according to each manufacturer's choice of design.
The float system in the dual carburetor will be different to some extent than the float system in the single barrel carburetor. Because a greater volume of fuel flows through it, the dual carburetor has a larger float chamber.
Some dual carburetors use a one piece float. This type float is only used however, where the float chamber does not surround the carburetor.
The action of this one piece float is the same as in the single carburetor. The difference between the two is only that the float used in the dual is larger. In the carburetor having the float chamber that entirely surrounds it, a split or "Kidney" type float is used. Its construction consists of two small floats exactly opposite each other that are connected by a "U" shape arm or yoke, which in turn operates the needle valve and seat. The float chamber in some units also contain baffles.
The reason for this is to gain better control over the larger volume of fuel in the float chamber. As an example, traveling at a high rate of speed and then coming to a sudden stop, would cause the fuel to move in a solid body against one side of the float chamber, or going around a sharp bend or turn in the road will cause all the fuel to try to move to one side in a solid mass. In either one of these instances mentioned, if no means of control were incorporated in the float bowls, the fuel would spill out of the carburetor and either stall the engine or flood it.
When setting the split or "Kidney" type float, consult the specifications of the carburetor being worked on for the proper fuel level.
If the float setting is not correct all other circuits will be affected. (For other "Float Circuit" troubles, see earlier part of text).
LOW SPEED CIRCUIT
The low speed system in the dual carburetors, is exactly the same as the single barrel carburetors except there is two of each part.
As there are two throttle valves and two idle adjusting needles or screws, care must be used in setting both the throttle screw and the idle needles. If a smooth idle cannot be obtained by adjusting the needles, which should be set one at a time, sometimes increasing the engine speed by setting the throttle screw will help. Usually a 3/4 turn open on Stromberg is the idle needle setting. Carter in most cases calls for 1 to 1-1/4 turns open.
Using a tachometer when possible to set the idle speed is a wise procedure, as trying to guess the engine R.P.M. is very difficult. Most mechanics watch the fan when setting engine idle speeds. This is a poor policy. The eye cannot distinguish the change in the revolutions of the fan from as little as fifty R.P.M. to as much as one hundred fifty R.P.M. Of course, on some engines this does not matter but on cars equipped with automatic transmissions it will positively make a difference in the automatic shifting of the gears. Also it is impossible to know at what height the vacuum is in the manifold by using the "guess" method.
In using a vacuum gauge (Hygrade CPA115 vacoscope) it will be possible to set a carburetor to get the highest vacuum reading with a smoothly idling engine.
HIGH SPEED CIRCUITS
Having two high speed systems operating together in a dual carburetor, means proper balance must be maintained between them, for all high sped engine operation. To insure this proper balance and operation, main metering jets should be installed in matched sets (also metering rods on Carter) and main nozzles should be carefully examined for wear or damage. On Carter Dual carburetors, the mechanism that operates the metering rods and anti-percolating valves should be thoroughly checked for correct operation. Any changes in the high speed system must be the same for each half of the carburetor. One half cannot be changed without the other and still have correct carburetor and engine operation.
The pump circuit in the dual carburetor is a single system with the exception of the pump jets. Some carburetors have two fuel passages traveling from the outlet check valve to the jets. A separate passage for each jet.
Quite a few dual carburetors use two separate pump jets. The later designed carburetors use a pump jet "cluster." This cluster is in one piece and has two extensions protruding from it. These are the pump jets. The passages from these jets converge in the center of the cluster and open into a passage drill from the bottom of this assembly. The bottom passage is the outlet from the pump system. The cluster fits into the discharge passage of the pump system. A steel ball or brass needle is generally used as a pump check when this cluster assembly is installed. A hollow screw is used to hold the assembly together. Many times these screws have worked loose and instead of the fuel leaving the pump system through the jets, it will leak out around the cluster and screw, causing a "Flat Spot" in the carburetor.
The choke circuits on the dual carburetors are no different than the chokes on the single carburetors. The function and troubles encountered in this part of the carburetor, is covered in the discussion on circuits in the foregoing part of the text.
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