Vacuum starter switches were popular in the 40s and 50s, and were used on Buick, Packard and other makes. They worked pretty well. You just turned on the ignition switch, depressed the accelerator pedal, and that activated the switch on the carburetor which started the engine. The problem was, that when the switch failed, mechanics of the day just disconnected them and added a push button starter switch on the dash. We wanted ours to be changed back to original, so we removed the push button and, using our '53 Buick wiring diagram, re-installed the carburetor vacuum starter switch.
Carter carburetors used one type and Stromberg another. Ours is a Stromberg (used by Buick from 1948-1954) and thats the type we will cover in this article. When we installed our used switch, we had a problem. When the accelerator pedal was depressed, the car would start, but it wouldn't release quickly, grinding the starter.
ABOUT THE SWITCH (Figure 1): The accelerator vacuum switch is mounted on the throttle body of the carburetor by two screws, with a gasket between the switch housing and the throttle body seals to prevent vacuum loss. The switch housing has a horizontal cylinder barrel through which vacuum from the engine manifold is applied at one end by passages in the carburetor bodies. This end of the barrel is provided with a washer that forms a seal to prevent leaks when a piston and spring is drawn against it by vacuum. The opposite end of the barrel is vented to outside air through a fine mesh screen and felt washer held in place by a washer and clip. A flat slide actuated by an operating lever engages a Bakelite contact guide. The upward movement is opposed by a heavy contact guide spring. The contact guide carries the U-shaped spring contact that moves up and down to engage contacts for opening and closing the cranking motor control circuit.
Figure 2 shows the accelerator vacuum switch with the engine not running. The throttle is closed and the switch operating lever holds the slide in the upper position, holding the U-shaped spring contact away from the contacts in the terminal cap. When the accelerator pedal is pushed down, this causes the operating lever to move away from the slide, allowing the contact slide and U-shaped spring contact to be in a position to bridge the contacts in the terminal cap, thus closing the circuit. The slide moves into the deeper of the two grooves in the vacuum piston. When the engine starts and the throttle is allowed to close, the slide and U-shaped spring contact is moved upwards by the switch operating lever, opening the circuit .
With the slide in the upposition, manifold vacuum pulls the vacuum piston inward until it seals. When the throttle is opened past the idle range, the operating lever moves away from the slide and is forced downward by the contact guide spring until it strikes the shallow groove in the vacuum piston. This acts as a stop and prevents the switch contacts from engaging while the engine is running. The switch must be set at a specific throttle opening. If contact is made too early, the throttle will not be opened sufficiently to start well. Too late, and the throttle will be opened too far and might cause gear crash, as well as hard starting due to unloading of the choke by the throttle mechanism.
TO SET THE SWITCH TIMING With the engine hot idle speed set at 450 RPM on Dynaflow cars, and engine not running. insert a pin through the center the felt filter. With the throttle closed, push the vacuum piston in and hold it there while opening the throttle. This allows the slide to drop into the shallow groove in the pistons, and will lock it in the inner position and prevent the slide from dropping far enough to complete contact. Hold the throttle open to prevent release of the piston until adjustment is complete. Now, place a 5/64" spacer between the idle stop screw and the fast idle cam while holding the fast idle cam in extreme cold idle position. Turn the ignition on, hold the spacer and open the throttle. The engine should not crank Still holding the throttle open, place a 3/64" spacer between the idle stop screw and the fast idle cam while holding the fast idle cam in extreme cold position. Close the throttle so that the spacer will hold the cam in this position. Open the throttle with the ignition on, and the engine should crank. Bend the tang on the operating lever downward if the engine cranks using a 5/64" spacer. If the 3/64" spacer does not cause the engine to crank, bend the tang upward.
We disassembled our switch and cleaned everything, then discovered that our slide was slightly bent, causing it to hang up. We carefully straightened it and reassembled everything using petroleum jelly as a lubricant. Ordinary lubricants can cause poor switch contact in cold weather. Never lube the slide.
This is just one more original part that has been saved from an aftermarket make-do. Enjoy your cars, and keep 'em driving!