Adjusting a Stromberg carburettor (2022)

A Stromberg carburettor

The Stromberg CD carburettor, like the SU (See Adjusting an SU carburettor), is a constant-depression carburettor, hence the CD.

The two work in much the same way, but the Stromberg differs in having an air valve - commonly called the piston - surrounded by a rubber diaphragm in the dashpot.

The size and type of a Stromberg are given by numbers and letters. Numbers such as 125, 150 and 175 are sizes: they mean a choke diameter of 1 1/4, 1 1/2 and 1 3/4 in. The size makes no difference to the way you tune the carburettor.

Types include CD, CDS, CD2S and CD3, all used on older cars made before anti-pollution laws demanded emission-control fittings.

Later types with emission control have the letter E, as in CDSE and CD2SE. Tuning methods vary according to type, but Strombergs are not stamped with their type marks.

Information in the car handbook should help you to identify your type.

Air cleaners

Before starting to tune the carburettor, check out all the other systems such as sparkplugs, contact-breaker gap and the ignition timing (See How to drain engine oil and remove filter).

Although valve clearances seldom need resetting between major services, always check them before attempting to tune the carburettor.

Check, too, that the damper tube is topped up with oil to within 1 in. (6 mm) of the end of the rod.

Use ordinary multigrade engine oil or the special SAE 20 oil made for the purpose by Zenith (the makers of Stromberg carburettors). Tune with the engine running at working temperature.

(Video) Tuning and Balancing Zenith-Stromberg and SU Carburetors in Your British Sports Car

Adjusting the gap

Adjusting the gap

The fast-idle stop screw opens the throttle when the choke is pulled out. There must be a gap between its head and the choke cam when the choke is pushed home.

The CD and CDS have different cold-starting devices, but the adjustment works in the same way.

Set the gap according to the car-makers recommendations - usually about 1mm.

Some Strombergs have a two position screw to limit the use of the choke according to the season. It has a spring under its head. Set it with the spring compressed for summer and the tension released for winter (only necessary in freezing weather)

Tuning the CD, CDS and CD2S

There are three adjustments on CD, CDS and CD2S carburettors: a throttle-stop screw, a jet-adjusting screw and a fast-idle stop screw.

Check that the choke is fully shut and that the fast-idle stop screw is clear of the choke linkage. Hook your finger under the edge of the dashpot and press the lifting pin upwards to raise the piston by about 1/32 in. (1 mm).

If there is no pin, take off the air filter and lift the piston 1/32 in. (1 mm) with a thin screwdriver. Listen to the engine note while you do so. If the mixture is correct the engine speed should rise slightly for a moment, then return again to normal.

If it rises and stays fast the mixture is too rich.

If the engine dies when the pin is lifted, it is too weak.

(Video) All About Stromberg Carburettor 175CD 2SE adjustment/road testing - Volvo 164 Rescue Ep43

Switch off the engine before adjusting the mixture, and check that the jet needle is central. Remove the air cleaner, lift the piston and let it fall. If the jet is central, the piston falls with a sharp click.

Keep the jet central while you adjust the mixture by taking the damper rod out of the top of the carburettor and pushing a pencil or soft metal rod firmly down the hole to hold the jet in place. Make sure that the jet remains centralised.

Start the engine and bring it up to working temperature.

The mixture-adjusting screw is set centrally in the base of the carburettor on CD, CDS and CD2S models. It is brass and has a wide slot in it. Although a screwdriver can be used to turn it, a small coin is easier.

Turn only an eighth of a turn at a time, then wait about 15 seconds for the engine speed to settle down. Lift the pin again and see whether the engine speed alters.

Screw the jet upwards (that is, anticlockwise looking down on the carburettor) to weaken the mixture, or down (clockwise) to make it richer.

With the mixture setting correct, the idling speed may now be too fast or slow. For most cars it should be 850-950 rpm - judge it by ear if your car does not have a tachometer (or rev counter). Adjust the idling speed by turning the throttle-stop screw.

If tuning fails to make the engine run properly, the carburettor may need cleaning (See Air filter change), or the air filter renewing.

Centralising the jet

Lift the piston so that the needle is clear of the jet, and screw the jet adjusting screw up until the top of the jet is just above the top of the bridge in the carburettor bone.

Use a spanner to slacken the large nut just above the jet adjusting screw by half a turn. That releases the jet in its housing, but allows it to drop slightly.

Wind the jet adjuster up again until the top of the jet is level with the bridge. Let the piston fall back gently so that the needle centralises the jet.

Remove the piston damper and hold the piston down with a pencil or soft metal rod slipped into the damper tube. Tighten the jet assembly. Check several times that the piston drops with a click.

(Video) Rebuilding Zenith-Stromberg CD-175 Carburetors - Disassembly

Adjusting a Stromberg CD3

The CD3 carburettor has a fixed jet, and the needle is loosely mounted in the air valve, or piston, so that it centres itself.

You need a special tool to reset the mixture, which is done by altering the height of the needle in the piston. The tool is a long L-shaped hexagonal Allen key which goes inside a thick-walled tube. Its maker's part number is B20379.

To use the tool, remove the dashpot damper and insert the tube in its place. Turn the tube until a pin on its side falls into a slot in the air-valve shaft.

Push the Allen key to the bottom of the shaft and fit it into the needle adjustment.

Before you turn the Allen key, hold the outer tube of the tool firmly; otherwise the air valve can turn and tear the rubber diaphragm.

Tune the carburettor with the engine running at working temperature, in the same way as with the CD and CDS.

For this carburettor, turn the Allen key clockwise to lift the needle and thereby enrich the mixture and anticlockwise to lower the needle for a weaker mixture.

Take care not to turn the adjustment too far, or the needle can come loose from the adjuster. The range is only about two full turns.

If you find you have to make more than a tiny adjustment, avoid losing the needle. Take off the cover of the carburettor and lift out the air valve, holding it by its shaft so as not to damage the diaphragm. Then set the needle in a midway position with the Allen key.

(Video) 1975 Triumph TR6 - Part 5 - Tuning The Strombergs

The right position is when depending on the needle fitted - the washer around the needle or the groove in it is flush with the bottom face of the air valve.

Reassemble the carburettor and adjust the needle - starting from this point - no more than one turn either way.

After each adjustment is made, run the engine at about 2,000 rpm for ten seconds or so to clear the extra fuel which tends to get into the inlet manifold when you are making an adjustment.

Adjusting a Stromberg carburettor (17)

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How do I adjust the mixture on my Stromberg carburetor? ›

Turn only an eighth of a turn at a time, then wait about 15 seconds for the engine speed to settle down. Lift the pin again and see whether the engine speed alters. Screw the jet upwards (that is, anti clockwise looking down on the carburettor) to weaken the mixture, or down (clockwise) to make it richer.

How do you lean out a Stromberg carburetor? ›

Four Stromberg carburetors you need a special tool make sure the bottom part is seated. And then

How does a Zenith Stromberg carburetor work? ›

Through this main jet down there which is basically a brass tube that is a straight shot all the way

Who made Stromberg carbs? ›

Background. The Bendix Corporation marketed three types of aircraft fuel systems under the Bendix-Stromberg name: The first type was manufactured for low performance aircraft engines and virtually all aircraft engines produced before 1938.

Where are Stromberg carburetors made? ›

Units built in the Elmira, New York, factory can be identified by a "1-1" casting number on the base.

How do you adjust a zenith updraft carburetor? ›

How to make zenith carburetor adjustment ? - YouTube

How does an SU carburetor work? ›

SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburettor.

How do you identify a Zenith Stromberg carb? ›

Zenith-Stromberg units are identified by the opening's inch equivalent so that a 175 CD carburetor model has a 1 ¾” opening. An SU carburetor is identified by a number that is equivalent to the number of 1/8” of an inch beyond 1 inch so that an HS4 has a 1+(4x1/8“)= 1+4/8” or 1½” opening.

How many CFM is a Stromberg 97 carburetor? ›

While Stromberg produced carburetors for a variety of manufacturers, the two-barrels most hot rodders are familiar with first appeared on 85hp Ford Flatheads in '34 with the model 40; the model 48 was introduced in '35 (the preceding were both rated at 170 cfm); and from '36 to early '38, 97s were installed (155 cfm), ...

How do you pressure a carburetor? ›

The fuel is drawn into the carburetor by the vacuum created on the downstroke of the piston. As air accelerates through the Venturi, it creates a low pressure area and velocity of the intake air increases. This rapid acceleration causes the air and fuel to mix and vaporize.

What are the two types of carburetors used in aviation? ›

Aircraft carburetors are separated into two categories: float- type carburetors and pressure-type carburetors. Float-type carburetors, complete with idling, accelerating, mixture control, idle cutoff, and power enrichment systems, are the most common of the two carburetor types.

What is a pressurized carburetor? ›

A pressure carburetor is a type of fuel metering system manufactured by the Bendix Corporation for piston aircraft engines, starting in the 1940s. It is recognized as an early type of throttle-body fuel injection and was developed to prevent fuel starvation during inverted flight.

What does the manual mixture control on a Bendix PS carburetor regulate to control the mixture ratio? ›

The manual mixture control valve controls the fuel flow. By using proper size jets and regulating the pressure differential across the jets, the right amount of fuel is delivered to the discharge nozzle, giving the desired fuel/air ratio in the various power settings.

How does a Holley 94 work? ›

Like the modern versions, Holley 94s use a power valve (also called an economizer valve) that opens to supply extra fuel to the engine when the vacuum drops to a certain point, usually 7 1/2 inches Hg or less.

How does a updraft carburetor work? ›

During this time, atmospheric pressure is acting on the fuel in the carburetor float bowl and pushing the gasoline (or pulling it, depending on the way you look at it) through the circuits of the carburetor. It is for this reason an updraft carburetor works. It's not magic, it's just a pressure differential.

How do you adjust a Zenith carburetor on a tractor? ›

How to Rebuild a Zenith Carburetor on Allis Chalmers WD, WC, B, C ...

How do you adjust a Bendix carburetor? ›

Shovelhead Carburetor idle mixture adjustment - YouTube

How do I adjust the carburetor air fuel mixture in my car? ›

How To Adjust A Carburetor On Your Car - YouTube

What does SU stand for on a carburettor? ›

S.U. stands for Skinner Union and the first carburettor was made by Herbert Skinner. and his brother, Thomas Carl Skinner, (Fig.3) in 1904, and patented in 1905 (No. 3257/05).

What oil do you put in SU carbs? ›

Penrite SU Dashpot Oil is a mineral based oil containing anti-corrosion and anti-wear additives for the lubrication of pistons and dampers in SU and Stromberg carburettors. SU Dashpot Oil is suitable for use in pre 1940 SU carburettors, which do not incorporate a damper in the dashpot assembly.

Is Zenith carburetor still in business? ›

Today, the company is known as Zenith Fuel Systems LLC, with headquarters and a modern manufacturing facility in Bristol, Virginia and still offers original manufacturing equipment and replacement carburetors and kits.

How do you read a carburetor number? ›

How To Identify Your Holley & Demon Carburetor - YouTube

How do I tell what model my carburetor is? ›

It's like a "VIN" for your carburetor and should be hand-stamped on the front of the choke tower to the right of the vent tube on most typical Holley carbs. The list number typically is four to six digits long and may or may not have a suffix number behind it. The date code will be right below it.

Will WD-40 clean a carburetor? ›

On the other hand, WD-40 is safe for your device and allows you to clean the carburettor, throttle body, as well as unpainted metal parts of the machine. The dual-action cleaning system of the liquid leaves no stone unturned to provide you with a clean carburettor.

Will seafoam help clean a carburetor? ›

Use Sea Foam Spray to safely and effectively clean residues and deposits from intake valves, chambers and compression rings! Sea Foam Spray delivers a high concentration of petroleum cleaning solvency and lubricity to carburetor throttle valves, intake runners and valves, and chamber areas.

What do you soak carburetors in to clean them? ›

How To Clean Your Carburetor With Pine-Sol! | Fix Your Dirt Bike

What is a downdraft carburetor? ›

Carburetors are called updraft or downdraft according to their position. If the carburetor is below the intake manifold's input, it is updraft. If it is above, it is a downdraft.

What are the 3 functions of a carburettor? ›

Functions of Carburetor

It regulates the air-fuel ratio and also mixes the fuels. Controls the engine speed. According to the engine speed and load changing, carburettors increase or decrease the amount of mixture. It vaporizes the fuel and mixes the air to a homogeneous air-fuel mixture.


1. Stromberg 150CDSE carb tuning attempt 1
(Driveway Shop (Hazen W))
2. How to Fix a Zenith-Stromberg Fuel Leak & Re-Tune Quickly!
(Midwest Motoring)
3. How to Tune Zenith Stromberg or SU Carburetors on Your British Sports Car!
(Midwest Motoring)
4. Adjusting the Non-Adjustable Zenith-Stromberg CD Carburetor
(Midwest Motoring)
5. My Zenith Stromberg metering needle adjusting video
(Calvin G. Williams)
6. 1971 Stag Stromberg carbs p3 install and tuning - Parental warning
(Church House Classics)

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