Rebuilding Zenith Stromberg CD-series Carburettors

Strip-down & re-assembly of this common carburettor
on Monday 12 February 2007
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Zenith Stromberg CD series carburettors were common on european cars of the '60s & '70s. In fact, due to their better emissions performance, they replaced the infamous SU H and HS series on many models. This article is specifically about the 175 CD-2 model, but general principles apply to all.

Strombergs or SUs? That is often the question, and many manufacturers swapped between the two as they are "bolt-on" compatible. Today, companies offer SU conversions to replace Strombergs and there is a general feeling that the SU is better than the Stromberg. My personal feeling is that the Stromberg is a more accurate & efficient carburettor when properly set-up, but can be troublesome if poorly maintained. Whereas the SU is a little more 'bomb-proof' and weekend mechanic friendly, but ultimately a poorer design.

Strip Down
The youngest of these carbs is now ~20+ years old, so their general exterior condition is likely caked in oil and garbage, also components of the linkage may be rusted & worn. However, the best thing to do is get in there and pull the thing to pieces, it all needs cleaning anyway.
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First, remove the dashpot cover & withdraw the piston/diaphragm assembly.
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Take great care with the precision metering needle that extends from the bottom of the piston; don't do anything that might risk bending or damaging this.
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Then, remove the Temperature Compensator (whitish/yellow plastic lid, 2 screws), Bypass Valve (3 normal screws, NOT the 3 Philips screws) and enrichment/'choke' assembly.
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Turning the unit over, the float bowl cover can be removed, then the float unclipped and the needle valve unscrewed. Jets are normally a hard push-fit and should only be touched if a replacement is at-hand. Adjustable-jet versions were made, but I've never seen one.
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Finally, the throttle spindle can be disassembled, try to keep the parts in the right order (e.g. by threading them on a wire) and sketch the orientation of the levers and throttle spindle as you will forget...no matter how obvious it looks now.....
To remove the spindle, remove the 2 screws retaining the butterfly (these are soft copper) and remove the butterfly disc. Then, before pulling out the spindle grind down any burrs around the 2 screwholes in the spindle. Then you can withdraw the spindle with no damage to the bearings & seals in the carb body.

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The Stromberg features throttle spindle seals, this means that spindle wear is far less than you see in unprotected SU HS type carbs, and bearings are normally in good condition, in many cases throttle spindles can be re-used. If you can, replace the seals, these little brass retainers can be levered out of the carb body with a screwdriver- but don't do this unless you have spares at-hand.
Cleaning
All parts need to be cleaned by removing oil, by soaking in kerosene, and then removing dust/corrosion. For the carb body I use phosphoric acid based 'Aluminum Cleaner' - this stuff is powerful so brush it on the carb body, work it in & wash it off within a few minutes trying to avoid it contacting any of the critical inner surfaces of the carburettor. This gets the carb clean in 10 mins- all you need to do in throuroughly wash it and dry it out.
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Rebuild Kits
There are different types of rebuild kit, the basic ones have the diaphragms & gaskets, but don't contain parts for the Bypass Valves or spindle seals.
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The full Zenith rebuild kits contain nearly all the parts you could need, including spinble bearings, jets, screws & washers. The things you'll need to add are needles (make & model-year specific) and potentially the plastic covers for the Temperature Compensator, as this cover is often discoloured and warped.

Temperature Compensators
This part of the carburettor is an air-bleed past the thottle butterfly to weaken the mixture at low throttle openings. It is a temperature-dependant air bleed; the device contains a bimetallic strip to sense the temperature of the engine. It's a precision device; DON'T TOUCH the small nyloc nut part-way down the bimetallic strip, this is the fine adjustment of the device.
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This device malfunctions when the valve doesnt close properly, causing weak mixture at cold idle, and this often "fixed" by tightening the small nyloc nut until the bimetal strip is permantly bent, but this will cause the mixture to be rich when the engine is hot affecting emissions and fuel consumption.
If you suspect the compensator is malfunctioning, you can isolate it on the car by blocking the ~8mm diameter hole on the face of the carburettor where the air cleaner fits (in about the 4 o'clock position). I normally use those squidgy foam ear plugs which seem to fit just fine. Once blocked mixture will richen at idle (cold) if the compensator is defective.
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Service for this item involves cleaning and re-setting of the bimetal strip. There are a number of article on the internet about how to set these up- I set mine to be just opening at 28C.

Bypass Valves
This is another throttle bypass, this time vacuum-actuated. It's designed to limit over-rich mixture in over-run engine braking conditions. It consists of a spring loaded valve, the valve head is brass mounted in a rubber 'gasket'. By unscrewing the 3 philips screws the unit will spring apart under the static spring pressure.\
stromberg_175_CD2_10.jpg
When old, the rubber splits and as a result the bypass valve 'bypasses' all the time. Sometimes the spring load is adustable (allowing the vacuum level at which bypass opens to be set), more often they are pre-set/sealed.
Although the parts are not in the basic rebuild kit, my advice would be to always rebuild these units - you need to be confident that they are functioning correctly.
A separate bypass valve kit is available, it's got the brass/rubber bit plus a few gaskets to sandwich it.
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Enrichment Device
The 'choke' on a Stromberg normally consists of this device on the LHS of the carb body.
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It's basically a throttle bypass tube that draws fuel thru 2 sets of 3 precision-sized holes in an aluminium disc to dump a lot of extra fuel into the manifold, the amount of fuel depending how far the disk is turned.\

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Grit can damage the aluminium face against which the spring-loaded disc rests, and the holes in the disc can get corroded. Maintenance involves cleaning & checking of all parts, careful lubrication and reassembly with new gaskets.

Float Chamber

The bottom cover is held on with 6 screws. For designs without adjustable jets (the majority), there is a plastic plug with a brass cap blocking a hole in this bottom cover. Carefully bend the 4 plastic tabs on the inside to push out this plug, then replace the 'o' Ring and clean-up the sealing areas of the cover.
The float should be checked to ensure that the metal frame is straight and that there's no problem of fuel leaking into the float. It should swing freely on the hinge pin.
The needle valve features an integral gauze filter which you can see once it's unscrewed. I've never known these valves wear out, but the gauze filters do get damaged.
On reassembly, make sure the float is fitted the right way up: the plastic float is attached to one side of the metal frame. With the carb upside down, you should be able to see the majority of the metal frame, if it's obscured by the plastic in some areas then it's upside down. It's critical to get this right or the fuel level in the float chamber will be screwed-up.
stromberg_175_CD2_23.jpg

Piston/Diaphragm
When removed from the carburettor, you have the piston/diaphragm/needle sub-assemby.
stromberg_175_CD2_06.jpg
The piston features the adjustment for the needle, this adjustment is done using a long allen key pushed sown the damper tube in the top of the piston. When adjusting the needle in the fully-assembled carb, be aware that the rubber diaphragm is the only thing resisting rotation of the piston. You must hold the piston still whilst rotating the allen key or the diaphragm will be torn. The correct tool for this has 2 spigots that engage with the 2 cut-outs in the top of the piston damper tube.
If you keep loosing the oil in your damper, then suspect that the "O" ring of the brass plug/adjuster at the bottom of the damper tube is faulty.
The needle is the critical metering element of the carburettor, if it's history is unknown then replace it!
The diaphragm must seal perfectly, as the pressure differential either side of this is what causes the carb to 'respond' to changes in throttle/load. These are part of standard rebuild kits, so do replace & keep the old one as an emergency spare.

Carburettor Body

After cleaning, inspect the throttle spindle bearings.
stromberg_175_CD2_19.jpg
If possible fit new spindle seals - the are vital to stop air leaks and to protect the spindle bearings, they are a push fit:
stromberg_175_CD2_20.jpg
Once the bearings are lubricated, the throttle spindle and butterfly can be fitted:
stromberg_175_CD2_22.jpg
Do make sure the spindle is fitted the right way, as it most likely protrudes further from one side of the body than the other. Also, be aware that in multiple carburettor setups, each spindle may be a different length. The copper screws should be bent-over to prevent any chance of the screws loosening and being ingested by the engine....

Piston & Dashpot Refitting

With the rest of the carburettor rebuilt, the piston should be trial-fitted to the dashpot cover to check it moves freely- it should slide up&down without any tight spots. If there is any interference, carefully clean both components with a mild abrasive (i use those plastic scouring pads to clean-up the surfaces).
stromberg_175_CD2_27.jpg
The dashpot cover is normally fitted wrongly! People fit it with the wording cast in the cover visible from the air cleaner face. This is wrong: there are some small casting marks on both parts that indicate they should be aligned with the lump on the neck of the dashpot facing towards the air cleaner face.
stromberg_175_CD2_30.jpg
Note the aligned casting marks in the middle of the pic, just above the air cleaner flange.

Final Points
Once the carb is rebuilt, check the piston drops onto the bridge inside the carb with a sharp metallic 'click' [pull it to the top of its travel with your finger & let go]: this indicates that the piston is moving freely on the dashpot cover and is not binding in the carb body (or the needle could be binding in the jet). If there's no click, pull the dashpot cover off & find the cause; without the piston moving freely the whole carb's operation is compromised.

In the unlikely event that the float chamber needle valve doesnt seal when you first supply petrol, try hitting the carb body with a soft-headed mallet (!), this is a far quicker solution than tearing down the float chamber & finding nothing wrong...

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