Back to the Basics – Intro to Zenith-Stromberg Carburetors

Fear of the unknown is probably the greatest stumbling block most people seem to have in dealing with Zenith-Stromberg carburetors – they are much maligned largely because they are not understood. The basic Zenith-Stromberg, as used on Triumphs in the mid-’60s. for example, is actually simpler and more reliable than the SU carburetor it replaced. While later Z-S models used a large number and variety of “add on” features to meet our pollution control requirements, the basic “heart” of these carburetors remained relatively simple.

Like the more familiar SU carburetors. Zenith-Stromberg carbs operate on the constant depression principle. (The “CD”in  the Z-S series designation stands for “constant depression) Constant depression, also known as “constant vacuum”, “constant pressure”, “variable venturi”, or “variable choke”, simply means that the effective area of the carburetor venturi varies according to the engine requirements, producing constant air velocity and pressure differential across the jet orifice during normal running. This condition is produced by the air valve (air piston) rising and falling in response to engine vacuum. While “modern” SU carburetors rely on carefully and expensively machined parts to produce a moveable vacuum seal at the top of their pistons, Zenith-Strombergs use a thin flexible diaphragm to effect this seal. (It is interesting to note that the original SU models prior to World War I used thin leather diaphragms for this purpose.) What causes the air valve to rise is engine vacuum acting on the air above the air valve through the holes in the bottom of the air valve. When engine vacuum decreases, the weight of the air valve, aided by its spring, brings the air valve downwards.

Fuel is metered by a tapered needle mounted in the bottom of the air valve, and a fixed jet. As the air valve rises, allowing more air to enter, the tapered needle also rises in the jet, allowing the proper amount of fuel to mix with the air. This amounts to having a carefully calibrated variable jet to match the variable venturi – a classically elegant design. Since both of these elements operate together in direct accordance with the requirements of the engine, expressed by the engine vacuum, a properly balanced air-fuel mixture is maintained throughout the engine’s operating range with an absolute avoidance of complexity.

The extra rich mixture required for acceleration is produced by the upward motion of the air valve being retarded by the oil damper in the piston guide rod. This condition allows engine vacuum to draw proportionally more fuel than normal from the jet. Should there be no oil in the dashpot, only a normal running mixture can be produced, and acceleration will be very poor.

Model designation of Zenith-Stromberg carburetors can be a little confusing, as the cast series designation on the top covers are not always complete and accurate. The only way to accurately identify these carburetors is by the small square metal tag attached under one of the top cover screws. The number stamped on these tags is the unique specification number for a carburetor’s particular application.

First introduced in 1964, the original CD series incorporated no pollution control features, and provided cold start enrichment by means of a “starter bar”, which operated as a true choke. Various types followed, incorporating various pollution-control features. The CDSF series incorporated biased spring-loaded metering needles and cable-operated “starter boxes”, along with temperature compensators and bypass valves. The CDSEV series added a float chamber vent valve, which alters float chamber ventilation according to throttle position. The CDST series used a thermostatically operated “water choke”. CD4T and CDST carbs use the water choke, a “Downstream Discharge Idle Circuit”, which is adjustable independently from the non-idle operation of the carburetor, and internal emulsion jet temperature compensation.

The “add-on” devices found on most Zenith-Stromberg carburetors are probably the main cause of confusion, complaint, and problems, mostly because they are not understood. Careful study of them, and the understanding that they are discreet, sell-contained units makes dealing with them much easier.

The rotary “starter box” used on many of these carburetors has a series of holes of different diameters drilled in a circular plate. As the choke cable is pulled out. more of these holes are progressively lined up with a passage leading into the throttle body between the air valve and the throttle disc. Fuel is drawn from the float chamber through the holes in the disc to provide the required enrichment for starting.

The “water choke” units are very complex in construction and operation. They incorporate a bi-metallic coil which controls the fuel enrichment by means of a tapered needle valve assembly. However, in order to meet pollution control requirements, this action is not direct, but acts through a stepped cam, levers, vacuum piston, and throttle position sensor. All the driver has to do is to momentarily depress the accelerator pedal, and the automatic functions do the rest to establish the correct amount of fuel enrichment required.

The external temperature compensators prevent overly rich mixture when the engine compartment temperature, and therefore the fuel temperature, is high, particularly when idling in traffic, by bleeding additional air into the throat of the carburetor. Bypass valves overcome the mixture-richening effects of sudden deceleration and engine overrun. At a certain high level of manifold vacuum the bypass valve opens, allowing a metered bleed of air/fuel mixture to pass behind the throttle disc. This has the dual effects of maintaining efficient (i.e. less polluting) combustion, and lowering the manifold vacuum which caused the overly-rich condition.

'Back to the Basics – Intro to Zenith-Stromberg Carburetors' have 15 comments

  1. August 24, 2018 @ 4:41 am Barry Prentice

    I have a 1976 M.G. Midget. I bought from a person who never had it running. But when I try to start it Gas runs out of the carb. Float is adjusted correctly. I hope this is enough information.


    • October 26, 2018 @ 10:53 am David

      The float is either stuck or has a hole in it. The float is not closing the needle valve to shut off the fuel from the fuel line., IMHO


  2. January 18, 2019 @ 10:07 pm Gus heredia

    Engine does not run unless I squirt gas into car. Pump works ,float is set correctly . It has gas ? 1978 MG


  3. July 1, 2019 @ 7:14 am Ben

    Stromberg for a Series 3 6cyl. LandRover:
    The engine runs great at idle.
    In Neutral it is possible to obtain high RPM’s
    In 1st and 2nd gear the car runs normally.
    In 3th the and 4th gear the engine hesitates when pushing in the gas pedal. Short shocks.
    Looking forward where to look for
    Cheers Ben


    • September 25, 2022 @ 6:38 pm James P McVey

      I bid Y’all a gracious good day. I am trying to restore and rebuild a 1979 MG Midget. Trying to decide if the Zenith Stromburg CD150 carburetor air value is positioned correctly with the holes in the bottom on the engine side ? Or are they on the air filter side ?


    • November 6, 2022 @ 11:57 pm Sam Chambers

      Land Rover P100 2.6 engine series 3.. CD-175 Zenith carburettor..

      Suspect faulty Coil.. Replace your ignition coil, this is what happened to me. Sporadic faults are ignition related.. points, plugs, leads, coils… check those and your timing first.. get this right first. Point gaps, leads, terminals all good.. timing at BTDC 1° and more for every 1000′ elevation above mean sea level you are driving. Then check your valve tappet gaps. Once this is done, and only once this is done, do you progress with your carburettor.

      Take photos of your carburettor. For positioning.

      Very careful with fasteners – aluminium body carburettor .. do not over tighten or force anything.

      Mark position & Remove dashpot cover.. remove top. Inspect diaphragm for kinks, nicks, tears .. this DOES happen. Don’t be surprised.

      Remove oil dampener, check if any oil was present.
      Remove air piston attached to diaphragm and spring assembly
      Do not stretch or alter the spring. Replace diaphragm anyway.

      Clean every thing.. but don’t use cleaning agents on rubber diaphragm like I did. Stupid.

      The diaphragm replacement rubber must be soft and light in texture, not stiff rubber.

      The spring must be long and light..

      You can remove carb and give it a good clean.. check your butterfly flap, needle and jet for wear. Carb bowl and floats..

      Replace your fuel filter.. CHECK YOUR VACUUM HOSES .. they must fit tight.. there must be no air leaks ..

      Carb balance and tune settings come later. If you find your have a worn o-ring in your jet needle below the dashpot attachment, use a little Lucas stop leak to bring it round again.. but only use a SAE 20 multigrade oil for your dashpot and don’t overfill! This is what allows your car to accelerate nicely.. and give a rich balanced fuel mixture.


  4. September 20, 2019 @ 8:36 am Dave Sandusky

    Is there another possible reason for fuel leaking from front carb vent, besides an issue with the floats and needle valve? I have replaced both and still have the problem. I have also replaced the fuel pump, thinking maybe there is an over-pressure situation, but that didn’t change a thing. Could there be an issue with the tapered needle and jet somehow? I have run out of ideas!


    • November 27, 2019 @ 7:32 pm Rixter

      It would seem that if fuel is coming out that vent hole, the needle valve isn’t closing, or sealing well enough. So you have the integrity of the valve to check as one item. Look for debris in the valve or seat and freedom of movement of the needle. If you can see that it is functioning ok, you look to the float to ensure that it is functioning properly to push the needle valve closed. So obviously if the float doesn’t float, that is an issue. Also verify that it contacts the valve (not restricted in movement, tab not missing or bent, etc). The only other thing I’ve known to cause the fuel leak otherwise is excessive fuel pressure that pushes the valve open regardless of the pressure pushing back from the float. You should be running pretty low pressure on these cars.


      • September 25, 2022 @ 7:45 pm James P McVey

        Is there by chance an option to change the coolant temperature activated fuel enrichment on the 1979 MG Midget’s Zenith Stromburg CD150 carburetor to a manually operated air restriction plate type ? Trying to get back to basics.


    • July 25, 2023 @ 5:12 pm Patrick DiGiorgio

      I have found that when I changed my floats and needle valves, the tab that actually contacts the needle does not exactly push against the needle squarely. Instead, as the float rises in normal operation, it is actually rotating about the pin which holds the float in place. With that, I decided to slightly bend the tab where it will squarely push the pin closed. I had a similar issue with new needle valves as well and it drove me nuts. They would work fine for a few days, then I’d get a flooding condition due to the needle not seating properly. Another observation I had was concerning the tab on lever of the float itself. Over years of use, you may notice an actual “dimple” worn into the contacting surface of the tab. If the float is rising in a rotational manner, it may be causing a binding of the needle, especially if the bottom tip of the needle gets held in the worn dimple on the float lever.


  5. April 25, 2020 @ 10:10 pm Chris Smith

    Check the plug seal on bottom of bowl and bowl gasket integrity.


  6. December 3, 2020 @ 1:43 pm Jeff Sanderson

    My triumph herald 150cd will not drop below about 1100 rpm. Both throttle and cold start stops are well clear.
    Any ideas?


  7. December 6, 2020 @ 11:13 am Jeff

    Sorted. It was the return spring not pushing the cable back up, leaving a mm of open throttle!


  8. February 24, 2021 @ 1:32 pm alex lampasona

    good afternoon,
    My 1980 Triumph tr8 will not start unless I use starting fluid. I have rebuilt the water chokes and even using the chokes manually will not start.


  9. June 22, 2022 @ 9:40 am Adrian

    Nicely explained. I wonder if it would be worth mentionining that some stromberg carburetors don’t have a spring in them as the weight of the piston (if it is mounted vertically) is sufficient to bring it down. The 150CD in the Triumph Herald 13/60 is an example. Various spring lengths and spring rates are otherwise fitted depending on the engine, which assist the falling of the piston under gravity and this in turn leads to different needle taper angles. I haven’t quite got my head around the relationship of these variables but there has to be some basic maths on this somewhere – it’d be nice to know what it is. And are the needles tapered as simple (truncated) cones or are there changes in the profile?


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