Carburetor Tune Up Guide
previous page: Choke Circuit
Percolation in carburetors is comparable to a tea kettle full of water being overheated. When the water in the kettle reaches the boiling point, it expands and because the lid seals the top of the kettle, the only way the water can escape to release the pressure is through the spout. Of course, if the lid is removed, the pressure is immediately released and the water no longer pours out of the spout.
In a carburetor the float chamber would be like the tea kettle, the float chamber cover similar to the lid and the main nozzle like the spout.
Now let us assume the car had been driven for a long, fast run on a warm day. The engine is then stopped. The heat radiating from the engine and manifold surrounds the carburetor. This heat causes the gasoline in the float chamber to boil, creating pressure in the float chamber. The only escape for the fuel due to this pressure is out of the main nozzle. The gasoline flows out of the main nozzle, down the carburetor throat past the throttle valve, into the intake manifold loading the manifold with raw gasoline. This causes hard starting because of the flooded condition of the manifold. The mixture ratio is entirely too rich for starting or low speed engine operation.
To eliminate this condition, the various types of carburetors have "anti" percolating devices incorporated in them.
The Stromberg carburetor has a main nozzle, of special design to by-pass the pressure into the dome shaped high speed air bleed where it is released.
Some Carter carburetors use the anti-percolating valve principle. This consists of a passage or channel drilled in the carburetor body. The lower end of the passage opens into the base of the main nozzle passage, the upper end in some units is threaded and the anti-percolating valve screws into it. On other type Carter carburetors the anti-percolator valve seat is cast integral in the fuel chamber cover. The valve that screws into the carburetor body is the poppet type. This valve has two horizontal relief holes drilled in the middle of the body and has a spring loaded plunger running lengthwise in the center of it. On the bottom of the plunger is a disc that has a beveled upper edge. With the plunger in the closed position, this disc is held against the base of the valve body due to the coil spring forcing the plunger up. This prevents any air from passing through the relief holes. The plunger is operated by an ear or projection that is part of the countershaft arm. With the throttle valve closed, the countershaft arm is drawn down by the linkage attached to it and operated by the throttle shaft.
When the countershaft arm is drawn down, the ear or projection presses on the anti-percolating valve plunger and forces it down. When the plunger moves down, the disc, being attached to it also moves. This allows any pressure to travel through the valve and out the relief holes.
The anti-percolating valve that has the seat cast in the float chamber cover, or the one where just the seat screws into the carburetor body is called "The Saxophone Key" type. This type valve is a small round piece of metal, that has a piece of leather or leather backed with felt that fits against the jet like seat or vent. This round piece of metal is attached to a lever that operates on a pivot pin. It is held against the vent or seat by a small coil spring. Its manner of opening and closing is the same as the poppet valve.
The anti-percolating valve should be open only when the throttle valve is closed. If the anti-perc opens too early on deceleration, a charge of air will be taken into the high speed system and upon immediate acceleration a flat spot will result.
If the anti-perc valve is damaged, adjusted or set wrong it will affect the engine operation. If the valve should stick open, it will bleed air across the base of the main nozzle, locking the fuel in the carburetor and holding the engine speed to twenty miles per hour or less.
next: Dual Carburetors