From the late 1930s to the advent of fuel injection in the 1980s, American cars used downdraft carburetors, but because of limited under-hood space, most European cars used sidedraft carburetors.
When the Jaguar XKE was introduced in 1961, induction was a trio of sidedraft S.U. carbs. These were used until 1967, when the switch to a pair of Zenith-Stromberg carburetors took place. The Strombergs were emission-calibrated to meet U.S. air-quality standards. The loss of horsepower was offset by having a cleaner-running engine. (1971-1975 Jaguar V12 engines used two pairs of Zenith-Stromberg carburetors, which flanked both sides of the engine.)
Zenith-Stromberg carbs feature a variable venturi controlled by a piston. The piston has a long, tapered, conical metering rod that fits inside an orifice, which allows fuel into the airstream as it passes through the carburetor. The movement of this piston controls the amount of fuel delivered, depending on engine demand. As the tapered needle rises and falls, it opens and closes the opening in the jet, which regulates the passage of fuel. To prevent erratic and sudden movement, there is a small amount of oil in the carburetor dashpot to dampen the piston.
Zenith-Stromberg Carburetor Rebuild
Because the car sat non-running over a decade, I began the Jaguar XKE project by disassembling, cleaning, and rebuilding the carbs. A service manual is a must for this.
The years that this car sat had turned the gasoline in the carbs to hardened varnish. I spent hours carefully dissolving, lightly scraping, and cleaning the fuel bowls.
More cleaning, and then carb rebuild kits followed. It's nice to have a second carburetor to use as a reference as you're disassembling the other, but keep in mind that the Jag's front and rear carbs are slightly different from each other.