Zenith-Stromberg Carburetor Rebuild
Article by Mark Trotta
Up until the mid 1970's, Zenith-Stromberg carburetors were original equipment on Jaguar, Saab, Volvo, and others. American-bound Triumph and MG sports cars were also fitted with them.
Carb Identification
The basic 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.
Exact OE applications are sometimes difficult to identify. Very often, the original small square metal tag that was attached under one of the top cover screws is missing.
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.
Leaking Carb
After a car sits unused for many months, a carburetor gas leak may develop. Fuel sitting in the bowl 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.
When overhauling, you'll want to check both the float chamber gasket and the O-ring around the sealing plug in the bottom of the chamber.
NOTE: Use caution when installing fasteners to the aluminium body. Do not over tighten or force anything.
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Jaguar XKE Carbs
Model years 1967 through 1975 Jaguar E-types were factory-fitted with Z-S carbs. These were emission-calibrated to meet U.S. air-quality standards.
Note: 1971-1975 Jaguar V12 engines used two pairs of Z-Stroms, which flanked both sides of the engine.
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Rebuilding A Zenith-Stromberg Carb
No special tools are required to remove and disassemble Z-S carbs, but there are special tools needed to tune them. A factory service manual for your make and model is a worthwhile investment.
Here's a quick overview of the two carbs from my old '68 Jag:
Once disassembled, the pieces were cleaned and inspected for reuse.
Soak the Carburetor
If you need to rebuild an old and dirty carburetor, consider investing 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.
Because this car had sat in storage for many years, the fuel in the carbs actually turned to varnish.
Right side of float bowl cleaned, left side not yet cleaned
A little gentle rubbing with a nylon brush helped clean things up. Parts were then rinsed with water and dried with compressed air.
Even after soaking, you'll still need to run something through the small passages to clean out the stubborn stuff.
Carburetor cleaning brushes work best.
You may be surprised at how much dirt and grime come out of the ports!
More cleaning, and then the carb rebuild followed. New gaskets, diaphragms, seals and o-rings were installed.
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.
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.
Diaphragm and Alignment
The diaphragm replacement rubber should be soft and light in texture, not stiff rubber.
Upon re-assembly, note that the piston has an alignment tab and groove. The inside rim of the diaphragm has the matching tab.
Make sure you don't have it upside-down. Line up the indents to correctly seat the diaphragm.
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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 (20W50 is common) may also be used.
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