Mechanics Corner- June 12
Understanding Mustang Choke Systems
By Mike Godwin
When the Mustang was introduced the carburetor was the most common form of induction. Granted a few exotic cars had fuel injection including the Corvette. In fact on the Corvette the Fuel Injection system was a rather expensive option and given the reputation for trouble and less than perfect performance resulted in this being an option that was rarely chosen.
In fact most fuel injection systems at that time were finicky and delicate mechanical systems. These systems were a far cry from the injection systems found on every car today. Modern cars have electronically controlled injectors that depend on commands from an on board Engine Control Unit. Rarely do they require adjustment and cleaning at some repair shops is at best a questionable service charge.
Those of us that learned to drive on these carburetor equipped cars learned early that step one of the first start up of the day was to depress the accelerator pedal all the way to the floor several times. No matter how warm it was we had to perform this step on the first start every day. My father did not explain that with the first press of the gas pedal all the way to the floor when the engine is cold that this action is closing the choke plate on the carburetor. The next couple of pumps of the gas pedal will deposit the initial portion of fuel in the intake manifold for the engine to start. Once the engine starts, a choke unloader will open the choke plate about 1/8”. From there, as the engine heats up, the fuel requirements continue to decrease. Within minutes the engine is at full operating temperature and it can run on a leaner mixture. Heat produced either from a heat riser tube or from an electric choke cap will provide the heat to the bi-metal spring inside the choke cap. As the spring gets hot, it unwinds and gradually opens the choke plate to a full vertical position.
Until about mid nineteen fifty every vehicle, except a very few high end luxury cars, came equipped with a manual choke. Then the newfangled automatic choke was introduced and the manual choke was only found on trucks by the early sixties. These manual chokes consist of a knob, typically black with white letters reading CHOKE. Pulling the knob moved a cable for the driver to be able to open and close the choke plate. These certainly were efficient for many years. Today even light trucks have fuel injection systems that do not require a choke system. About the only place we find these manual choke systems is on farm equipment, lawn mowers, leaf blowers, chainsaws and in some race cars. In the sixties manual choke conversion kits hung on the racks of nearly every auto parts store in America. Some people were simply not ready to embrace the new automatic choke systems. These conversions are still floating around today and typically easily spotted because the installer used the supplied “L” bracket under the dash. From that bracket a cable that looks identical to the heater control cables found in early Mustangs passes through the firewall to the carburetor. The only Mustangs that ever came with a manual choke installed by Ford are the original HI-PO equipped cars and of course the early Shelby cars. Most restorers do not like to see them used unless they were original to the vehicle. The conversion kits are hard to find these days as the new induction systems have driven sales toward zero. In fact if you find you have a car with one of these old style conversion kits consider removing the manual choke and buy the parts that will provide the car with a correct automatic choke system.
The sixties found the big three relying almost exclusively on a Hot Air type system. This type of automatic choke action is performed by moving hot air provided by the heat riser tube connected to a sealed passage in the exhaust manifold. These are sealed passages, therefore there are no exhaust gases traveling through the choke tube. It allows cold air to enter one end of the exhaust manifold, get warmed up, and then pass through the insulated tube to choke cap on the carburetor. The hot air will warm the bi-metal spring, which will expand and open the choke plate. This process will take 5-7 minutes which happens to be about how long it takes your engine to reach operating temperature. Pollution control laws were also a factor in the selection of hot air chokes because the full opening of the choke plate corresponds with the time that it takes for an engine to reach operating temperature.