Zenith-Stromberg Carburetor Rebuild
Article by Mark Trotta
Zenith-Stromberg carbs were original equipment on classic cars such as Jaguar, Saab, Volvo, and others. North American-bound Triumph and MG sports cars were also fitted with them.
The size and type of Z-Stroms are given in numbers and letters.
EXAMPLE: numbers CD125, CD150 and CD175 are sizes relating to the choke diameter (1-1/4", 1-1/2", and 1-3/4" respectively). They all look very much alike, but details such as throttle linkage will change from car to car.
One style is the variable-venturi side draft carburetor. Inside the carburetor, there is a piston connected to a needle which slides inside the fuel jet. This is in contrast to the more common fixed-venturi carb, where varying air velocity in the venturi alters the fuel flow.
Constant-Depression (CD) Carburetor
The vacuum-operated piston has a long, tapered, conical metering rod. This rod 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.
A carburetor gas leak generally starts after a car has been sitting for many months. Fuel sitting in the carburetor evaporates, causing the float to drop and the valve to open.
Another cause of gas leaks is the needle valve not closing when the chamber is full, which allows it to overflow. You'll want to check both the float chamber gasket and the O-ring around the sealing plug in the bottom of the chamber.
Jaguar XKE Carbs
The 1967-1975 Jaguar E-type with the 4.2 engine were factory-fitted with a pair of Zenith-Stromberg carbs. These were emission-calibrated to meet U.S. air-quality standards. The loss of horsepower was offset by having a cleaner-running engine. (Note: 1971-1975 Jaguar V12 engines used two pairs of Z-Stroms, which flanked both sides of the engine.)
Because this car had sat in storage for years, the gasoline in the carbs turned to varnish. First, the carbs were removed, all the while taking notes and pictures on how they reassemble.
Rebuilding A Zenith-Stromberg
No special tools are required to remove and disassemble the carbs, but there are special tools needed to tune them. A service manual is essential for Zenith-Stromberg rebuilding and tuning.
Once disassembled, the pieces were chemically cleaned.
Soak the Carburetor
If you need to rebuild a really old and dirty carburetor, invest in a gallon can of Berryman's Chem-Dip. This stuff will dissolve the old dried-up remnants of fuel and remove any sludge. Be careful though, it will also eat plastics, puff up gaskets, and remove paint.
Once the chemicals were done dissolving the dirt and sludge, the fuel bowl was lightly scraped and cleaned. Air passages were blown clear with compressed air.
A little brushing from a nylon brush will help clean things up. Rinse the carburetor and parts with water and dry with compressed air.
After soaking it, you'll still need to run something through the small passages to clean out the stubborn junk. Carburetor cleaning brushes work best.
I was surprised at how much dirt and grime came out of the ports.
More cleaning followed, and then the carb rebuild kit followed. New gaskets, diaphragms, seals and o-rings were installed.
It's nice to have a second carburetor to use as a reference as you're disassembling the other, but keep in mind that front and rear carbs can be different from each other.
Replacing The O-Ring
There is a small star washer and screw that keep the assembly from coming out. Once the nut, washer and metering needle are removed, the piston is empty inside. Clean the inside and outside thoroughly.
The new O-ring sits on the needle adjustment screw. The replacement O-ring may be made of Viton, which holds up better to oil than conventional rubber.
Upon re-assembly, note that the piston has an alignment tab and groove. The inside rim of the diaphragm has the matching tab. Making sure you don't have it upside-down, line up the indents to correctly seat the diaphragm.
Oil The Carburetor
To prevent erratic and sudden movement, a small amount of oil sits in the carburetor dashpot to dampen the piston. If there is no oil in the dashpot, the engine may be difficult to start. The engine will probably not accelerate quickly, because there is no enriching during increased load (accelerator pump function). It may also cause a lean stumble and accelerated wear.
Special lube oil is available, but conventional engine oil may also be used.