By the time Volkswagens were “in the womb”, carburetion was not a new thing. But the German Engineers tasked with creating a viable engine system for Ferdinand Porsche had to create a system that would work with the air-cooled engines which were being demanded. Bit by bit, the VW carburetors were evolving into what would become a virtually fool-proof unit.
With the advent of the 12 Volt Electrical System, Volkswagen was coming of age. Carburetion would be just one of the areas which would benefit—with better starting possibilities. Let’s look at the Choke Mechanism on the VW 105-1 30 Pict-1 Carburetor.
While there are several parts to the Choke Mechanism, it is relatively simple.
But first—what is meant by “choking the engine”? “Choking” might better be termed “restricting”—because that’s what’s happening. When the Engine is dead cold, the Fuel Mixture must be more “rich” in gasoline with less air. The Engine is “hungry”. So, by restricting (or choking) the in-coming air, this need can be met.
Eventually, in a matter of minutes, the Engine will be running well and will need more air in the Fuel Mixture as it begins to reach operational temperature. It will be “starving” for air. Now the restriction on air can be relaxed—the “choke-hold” can be relaxed and the Carburetor can seek its own Fuel Mixture levels as needed.
We will be examining only the Upper Half of a Carburetor during this discussion of the “processes” or elements which comprise the Choking Mechanism.
First, let’s examine the Choke Element: The Choke Element is a simplified heating element consisting of a tiny wire Coil much like the larger coils in a toaster or an electric space heater—but it is tiny. This Coil is housed inside a round metal and ceramic disc which has an Electrical Connector. Onto this Connector is connected a wire which comes from the Positive Pole (#15) Terminal of the Engine Coil. This is the power source for the Choke Element.
The Choke Element is grounded on the Carburetor, which is grounded to the Engine, which is grounded to the Chassis and so forth—completing the necessary electrical circuit.
Inside the Choke Element, is a Bi-metal Flat Spring with a hook on its loose end. Once the Key is turned to the On Position at the Ignition Switch, power flows to the Choke Element’s tiny wire Coil, causing it to glow. The glowing Element slowly heats the Bi-metal Spring, causing it to “relax” its hold on the Choke Plate Shaft Upper Arm—which allows the Choke Plate to open.
Sound complicated? It’s really not. It’s simply one thing causing another to react, doing the intended job. In the earlier years of carburetion, there had been a cable from a carburetor to the dash knob of a vehicle—any vehicle. The driver would pull the knob, activating the manual choke—restricting air flow into the carburetor. As the engine warmed, the driver (when he remembered) would push the knob in, allowing more air to enter the carburetor. At age 14, this was how I did it on the family 1949 Ford V-8. I distinctly recall doing that operation, especially in cold weather.
Auto makers wanted to by-pass human thought in order to have the engine to automatically choke the carburetor. Thus, eventually, had been born the “Automatic Choke”.
That is the principle and that is the chief activation mechanism for the 30 Pict-1 Carburetor Choke.
Let’s look at the parts associated with the over-all Mechanism. I removed a Choke Plate and Shaft from a Carburetor for comparative purposes. In the photo below we can see the bare Shaft and Choke Plate.
Starting inside the Carburetor Choke Mechanism Recess on the Carburetor Housing, we find the end of the Choke Plate Shaft with the Upper Arm which the Bi-metal Spring activates during the heating process.
Working outward, we next see the Bakelite (plastic) Insulating Cup. The Cup has a tab which fits a groove inside the Housing to keep it in place. A slot allows the Choke Plate Shaft Upper Arm to protrude. The Bakelite Cup keeps heat from dissipating from the Heating Coil. Otherwise, heat loss would thwart the Heating Coil’s efforts upon the Choke Plate.
Next comes the Heating Element itself which is inserted into the Carburetor Housing. The Heating Element must be rotated so that the Bi-metal Spring Hook catches on the Choke Plate Shaft Upper Arm.
To secure the Heating Element to the Carburetor a metal Retaining Ring (or Bezel) with 3 holes fits over the Heating Element. Three Screws with 3 Plastic Cylindrical Spacers are used to secure the Bezel to the Carburetor Housing. Thus, the Heating Element can be rotated clockwise or counter-clockwise, as necessary, when adjusting it. Loosen the 3 Screws and the Heating Element can be rotated to the proper position. Then, the 3 Screws are tightened to fix the position of the Heating Element.
While this Process is activated……there is a Second Process at work. It is called the “Choke Pull-off Diaphragm” or the Vacuum Diaphragm. The purpose of this part is to “moderate” the action of the Choke Heating Element.
While the Choke Plate is Closed and the Fuel Mixture is “rich” in gasoline and “poor” in air, the Choke Heating Element is slowly relaxing its hold on the Closed Choke Plate.
During this slow heating-and-relaxing process, the Vacuum Diaphragm comes into play.
As the Accelerator Pedal is activated by the driver of the car, vacuum is achieved as air rushes through the Carburetor. Part of this vacuum “sucks” through various drillings (or passages) in the Body of the Carburetor. One of these drillings connects to the Vacuum Diaphragm. Vacuum “sucks” the Vacuum Diaphragm, pulling its Shaft.
As the Vacuum Diaphragm Shaft pulls, it catches the end of the Choke Plate Shaft Lower Arm so that the Choke Plate is pulled slightly open. Thus a bit more air can be sucked into the Carburetor as the car begins to drive and the Engine needs more air with the Fuel Mixture.
The operation of the Vacuum Diaphragm may seem to be insignificant. However, especially at start-up, it is needed to moderate the choking action exerted upon the Carburetor by the Heating Element.
There was no such apparatus on that afore-mentioned ’49 Ford. You pulled the choke knob and restricted the air to the fuel mixture more or less—as YOU thought necessary. There was no automatic “moderating” counter-force available.
Part II is coming. Part II will examine the Automatic Choke Adjustment Process.
My thanks to David Brown who graciously discussed Carburetor parts and, especially, the Vacuum Diaphragm and its necessary function.
Neva and I worked to get some photos to illustrate how everything goes together. There’s no better way to understanding something than to reduce it to its fundamental parts. Neva listened patiently as I used her as a captive audience while I explained parts and functions. Now—that’s a dedicated wife and VW enthusiast!