How to set the mixture on a Carburettor

ow to adjust tockover and fuel mixture on a classic car

This guide captures ‘bygone’ advice on how to set the mixture on a number of carburettors found on popular Classic Cars. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.

Why is it important to get the mixture setting correct

Incorrectly adjusted carburettors are expensive in terms of fuel consumption and possible wear and tear on mechanical parts.

Too fast an idling speed will result in a ‘racing’ engine which will pull against the brakes at low speed and could cause premature wear.

Incorrect mixture, if allowed to persist, can result in internal maladies such as burned valves and pistons.

All conventional carburettors have externally mounted controls so that you can adjust the speed at which the engine ticks over, and on-variable jets, the mixture supplied to the cylinders throughout the whole range of engine speeds.

These individual controls are necessary because manu­facturers’ tolerances in engine construction, plus differing rates of wear, mean that every engine requires slightly different tickover and mixture settings.

How often should you check Carburettor mixture settings

Carburettor settings are normally adjusted every 10,000 km (6,000 miles) as part of normal servicing.

Where to start with mixture setting on a Carburettor

The method of adjusting carburettors varies greatly from one make (or model) to another. So first you need to know what type of carburettor you have – fixed-jet or variable jet, single or multiple, and the manufacturer.

Then you need to identify the idling screw and the mixture screw or screws.

But the actual job of adjustment is quite straightforward. All you have to do is adjust two or more screws in the correct order.

Preliminary checks before mixture setting a Carburettor

Before beginning to adjust the carburettor some other areas of the engine must be checked to see that they are in good working order.

Check the spark-plugs, contact breaker points and air filter.

Ideally, all should be renewed, or at least cleaned thoroughly. This is especially important with the air filter, as a blocked filter leads to the carburettor receiving a richer mixture.

The points gap, plug gaps, ignition timing and valve clearances should also all be checked and if necessary, adjusted.

Next look at the condition of the carburettor itself. Start with the throttle spindle. This is the shaft that runs through the carburettor and is activated by movement of the throttle.

If it is loose or sloppy in its mounts, this indicates that the carburettor has worn badly. The replace­ment of this item is a major task, entirely possible and the chap below might be able to help you.

Check the gasket between the carburettor and the inlet manifold for air leaks by lightly dusting it with french chalk or talcum powder and running the engine.

If the powder is disturbed around the joint, the gasket should be replaced. If it is faulty it will lose air, and thereby weaken the mixture.

Move on now to check the throttle and choke activating mechanisms. Both should operate smoothly and have no appreciable slackness. Where there is a mechanical linkage, this must be frequently lubricated with light oil, while a cable must be lightly greased and free from kinks and frays.

What colour should Spark Plug electrodes be

A guide to the mixture the engine is running and there­fore to the direction in which adjustments should proceed is the colour of the spark-plugs.

Remove a couple and study the electrodes. They should be greyish.

If they are black and sooty the mixture is too rich

If they are white and spotty, the mixture is too weak.

If new plugs have been fitted, study those you have just removed.

Always make sure the engine is thoroughly warmed up before adjusting the carburettor settings and that the choke is fully home.

Never peer directly over the carburettor intake with the air filter removed and the engine running. Any blow-back in the carburettor might result in the expulsion of a jet of petrol which is painful if it gets into the eyes.

Useful Tools for mixture setting

Finally, two devices simplify a great deal of the job of adjusting carburettors. One is a Colortune kit (see pictures and eBay links below), which is recommended by many car manufacturers.

The other is an air-flow meter for use when balancing twin or multiple carburettors. (I am yet to right about these in detail). Both devices remove much of the possibility of incorrect adjustment due to human error or bad judgement. (I am yet to right about these)

The Colortune carburettor adjustment kit in component form
The Colortune carburettor adjustment kit in component form

Gunson Colortune Kits on eBay

Setting the fast idle on a carburettor

There are two main types of carburettor – fixed-jet types such as Solex, Zenith, Weber, Ford, Hitachi, Nikki, Fish, Rochester, Holley and Carter, and variable-jet types such as Stromberg, SU and Hitachi.

The fast idle adjustment is common to both. Other adjustments vary according to type.

The fast idle or choke setting is designed to speed up the normal idle when the engine is cold and to prevent the engine stalling. It is operated by the choke cable.

On most carburettors when this is pulled out a small cam on the cable opens the throttle to give a faster idle by preventing it from closing to the normal idle position. As soon as the engine is warm the choke is pushed back in and the fast idle cam returns to its normal position. The standard idle then takes over.

To identify the fast idle adjustment screw, get an assistant to slowly pull out the choke control inside the car while you study carefully the action of the choke linkage under the bonnet. Although

When you have located the cam and screw see if there is an arrow marked on the cam.

If so this must be aligned with the screw as it is the adjustment point. If there is no arrow pull out the choke until the jet just begins to move ( on variable jet carburettors) or as far as the choke control will go (on fixed-jet carburettors). The fast idle screw is usually held by a locknut and is not spring-loaded.

Start the engine and adjust the screw to give a very fast tickover between 1200rpm and 1500rpm. If no tachometer (rev counter) is fitted this may be difficult to judge but the engine should be running fast enough not to stall when cold and the ignition light should be off.

How do Fixed Jet Carburettors work.

Fixed-choke carburettors are also known as variable vacuum, or depression or fixed-jet or -venturi car­burettors. How confusing is that ?

However all units of this type have a slow-running circuit which is arranged to supply fuel and air mixture to the engine to enable it to tick over at low speed.

This circuit has an outlet below the carburettor for whenever the engine is running slowly, a very high suction exists below the throttle and this can be used to draw the mixture from the slow-running circuit.

When the throttle is opened the suction falls and the circuit goes out of action. The carburettor then runs on the main jet circuit which does not require individual setting. In this way, when the idle is adjusted the mixture supplied when the car is being driven is not altered.

Adjusting Fixed Jet Carburettors

Adjusting Weber 32 DGV 15B Carburettor
The Weber 32 DGV 15B is of the fixed jet design and is fitted with twin venturi
Adjusting Zenith 361V EP Carburettor
The Zenith 361 V EP carburettor is fitted to some Vauxhalls. It is a fixed jet model
Adjusting Zenith 361V Carburetto
This Zenith 361 V model differs slightly, as it is a conversion for fitment to Fords
Adjusting Nikki Carburettors
7The Nikki carburettor is a popular tuning modification, especially on Leyland Minis
Nikki Carburettor throttle stop screw
A second view of the Nikki, showing the complexity of the actuating mechanisms
SOLEX Carburettor adjustment screws
A simple fixed-jet design, the Solex type B30PSEl-2 is fitted to many popular cars

To adjust a fixed-jet carburettor, you must first find its throttle stop (idling) screw. This is a spring-loaded screw which bears on or is mounted on the throttle lever at the carburettor base.

If you have trouble in locating the throttle stop screw the easiest method is to trace the throttle linkage or cable from the bulkhead to the carburettor.

Take care to identify correctly the throttle stop screw as there may be others bearing on the linkage which are not so close to the carburettor base. These are intermediate throttle stops and are held by locking nuts.

With the exception of the fast idle screw they should not be disturbed, as this could upset the entire adjustment of the throttle linkage.

The throttle stop screw simply controls how far the throttle can return towards the fully closed position. In this way it varies the tickover speed.

Turning this screw clockwise will speed up the engine

Turning anti-clockwise rotation will slow it down.

Having sorted out which screw is which the first stage of the actual adjustment is to alter the normal idle to a brisk tickover. To provide a basic setting for this turn the volume control screw in gently until it seats and then unscrew it two turns.

Next start the warmed engine and alter the throttle screw position until a fairly rapid idle results. At this stage the ignition light should be out.

The next step is to adjust the mixture. To do this screw home the volume screw, then unscrew it two turns. Now experimentally turn the volume screw noting the behaviour of the engine. If it slows down and becomes more unstable, reverse the direction of rotation.

If it speeds up and becomes smoother continue until a peak of smoothness is reached. You will note quite easily when the ideal point has been passed as the engine will again begin to slow.

If there are two volume screws each must be screwed into its seat and then screwed out by exactly the same amount. Then equally adjusted until the peak of smoothness is reached.

Now that you have the mixture correct. The final stage is to return the idle to normal.

If the car has a tachometer (rev counter) and the hand­book gives a recommended idling speed alter the stop screw until the correct speed registers.

If the handbook gives no information or the car does not have a tachometer, the idling speed can still be reset with little difficulty.

The throttle stop screw should be adjusted so that the ignition light barely flickers as the engine ticks over. If the light comes on too brightly, the engine will vibrate unsteadily in its mounts and may tend to stall, particularly when you brake the car to a halt on the road.

On cars with automatic transmission the engine may stall when the gears are engaged.

If the idle is set too fast the engine will sound as if it is racing. It may be difficult to engage first gear or, on auto­matics, there may be pronounced lurch when you select drive or reverse. As well as wasting fuel, too fast an idle speed can also wear out the brakes if left unchecked.

Set the screw mid-way between the two extremes. When this is done try a final fine adjustment of the volume control screw in both directions to see if the lower speed requires a slightly different mixture setting. Adjust­ment is then complete.

TOP TIP: On the volume screw there may be a point where at ideal mixture a small amount of rotation has no effect on the idle. If this is the case always turn the screw to the clockwise limit of the leeway. This basic method works with most fixed choke car­burettors but one or two require some additional steps to be taken.

Adjusting Fish Carburettors

The American Fish carburettor (manufactured in Britain by two companies Reece and Minnow) operates on a principle different from other carburettors.

The mixture strength is altered by altering the angle of the butterfly on the throttle spindle. In this way, instead of adjusting the flow of petrol into the carburettor as in other designs, it maintains a steady flow of petrol but meters the amount of air that is allowed to enter.

The main jet is also adjustable. The procedure for Fish carburettors begins by checking the operation of the accelerator pump. This is done by removing the air filter, pumping the throttle with the engine off and confirming that petrol squirts from the butterfly orifices.

If this is not happening the accelerator pump must be checked, which is a job for a specialist.

Next a very fast idle of around 2000rpm must be set.

Seal the air bleed hole in the end of the throttle spindle. If the engine speed remains constant the carburettor adjust­ment is normal.

If the engine speed increases slacken the butterfly clamp screw taking great care not to disturb the position of the spindle. Now turn the butterfly on the spindle to close the throttle slightly.

If the engine speed falls turn the butterfly to open the throttle slightly.

Continue testing by blanking off the air bleed, after first retightening the spindle clamp screw, until there is no change in engine speed while the air bleed is covered.

When this happens re-adjust the idling speed.

If you want to adjust the main jet you will find it under a small plug in the bend in the choke tube and the float chamber. First remove the plug and with the throttle wide open use an Allen key to feel for the jet.

Screw the jet out to enrich the mixture, in to weaken it.

When the performance increase caused by this adjustment reaches its peak set the jet just to the ‘rich’ side of the peak. Replace the plug and then recheck the idling mixture as this may have been upset. Turn the screw adjuster in either direction until the most even idle is achieved.

Adjusting Carburettors with air screws

Some carburettors use an air regulating screw instead of a volume control screw. These are easy to identify as the air screw is always mounted towards the top of the car­burettor rather than the base.

In such cases adjustment is carried out in the same manner as that for the usual volume screw – but whereas on the volume control system, clock­wise rotation of the screw weakens the mixture, on the air regulating system clockwise rotation enriches it.

So, while the volume screw is set as far clockwise as possible, the air regulating screw is set as far anti-clockwise as possible, but not so far, as to upset the idle quality.

Some other carburettors use an air screw in place of the normal throttle stop screw. These can be identified by the absence of any screw adjustment at the carburettor end of the throttle cable or linkage. The air screw meters the air flow to the carburettor and there is a standard volume control screw to meter the fuel flow.

The Solex 26- 32 DIDSA carburettor for example differs from other Solex carburettors in having an air screw. To adjust it first turn the air screw until the engine is at a fast idle of about 700rpm.

Then turn the volume screw, which will be the lower of the two screws, until the engine reaches a peak beyond which it will not rise regardless of the direction in which the fuel screw is rotated .

Next return the idle to 700rpm. Turn the volume screw again until the idle peaks once more and then restore the idle to 700rpm, again with the air screw.

Finally reduce the idle further until the ignition light just comes on as the engine ticks over.

Some Weber carburettors, such as the 32 DIR 8 Mk. 600, also use an air screw, but are fitted with twin volume screws, one for each choke.

Adjusting Twin Choke and Four Choke Carburettors

Some fixed-jet carburettors have two or more chokes .

American carburettors in particular often have four. These types of carburettors are basically two or more joined together. They may be ‘comp ound ‘ or ‘progressive’ and basically are handled in the same way as single chokes .

To see if your own unit is a compound or a progressive type ask a helper to gently depress the accelerator with the engine off. As they do so watch the action of the linkage on the carburettor.

If only one of the two or more throttles opens at all with the engine off the carburettor is a compound type.

If first one throttle opens then the others it is a progressive type . In each case leave all but the first throttle alone, locate the idling and mixture adjustments (which will be on the first throttle only), and adjust as normal.

If both throttles open together, or all four, the unit is a true twin-choke or four-choke carburettor.

In such cases you may find only one idling screw but there will be a volume control screw on each choke or barrel.

Make the usual adjustment but set each volume screw individually to give the smoothest tickover and highest speed. Slow the engine to the desired speed with the throttle screw and then make tiny mixture adjustments for the best idle .

Adjusting Variable Jet Carburettors

Variable-choke carburettors are also known as constant­ vacuum or depression or variable-jet or venturi car­burettors. The most common being Stromberg, SU and Hitachi.

Adjusting Stromberg 150CD Carburettor
All Stromberg carburettors are designed with variable jets. This is the 150CD model
Adjusting SU HS2 Carburettor
SU carburettors are also of the variable jet design. The HS2 model is illustrated

The choke size is varied automatically by the degree of throttle opening and the load against which the engine is working.

Whether or not these carburettors have a separate idle circuit the mixture is adjusted by the raising or lowering of the main jet.

Jet adjusting screw on Stromberg variable jet carburettor
The jet adjusting nut on a Stromberg variable-jet carburettor

The way that this operates is that a tapered needle projects into the main jet and partially obstructs it. Raising the jet moves it to a larger section of the taper and thus weakens the mixture

Jet adjusting nut on SU carburettor
The jet adjusting nut and spring on an SU carburettor

Lowering the jet positions it on a smaller section of the taper and thus causes extra richness.

Correct needle postion on SU Carburettor
The correct (left), and incorrect (right), needle positions

The tapered needle is held in a piston or air valve. The only part of the piston assembly to come into contact with the piston chamber is the actual piston rod. All other parts of the piston assembly have sufficient clearance to prevent metal-to-metal contact. This is essential to the correct functioning of the carburettor.

The SU and Hitachi designs are similar to each other except that where twin Hitachis are fitted the two car­burettors share one idle screw and one mixture screw.

Stromberg carburettors work on the same principle as SUs and Hitachis, but in place of the internal piston they use a rubber diaphragm and an air valve like an SU piston.

Adjusting Stromberg Carburettors

On the Stromberg the main jet is altered by an adjusting screw (see below) which projects from the bottom of the carburettor, through a large hexagonal plug.

Jet adjusting screw on Stromberg variable jet carburettor
The jet adjusting nut on a Stromberg variable-jet carburettor

To adjust the settings begin by removing the air filter and the air valve damper. This is readily identifiable being a large knurled plastic plug on top of the carburettor, which is removed by unscrewing.

Place a pencil down the aperture from which the damper was removed and gently depress the air valve. Next using a small screwdriver or coin screw up the jet adjuster nut until the jet touches the air valve.

Release the pressure on the pencil and allow the air valve to return to its normal position. Now unscrew the jet three full turns.

Set the idle to around 600rpm or until the ignition light is just flickering (the idle screw is just below the vacuum chamber which houses the air valve).

Adjust the mixture screw up or down until the engine idles evenly. Test the mixture by lifting the air valve 1/32in. or 0.8 mm.

If the engine speed increases and stays that way the mixture is too rich .

If the engine speed falls and stays that way the mixture is too lean .

The engine speed should pick up very slightly and then level off. If it does this the mixture is satisfactory.

Recheck the idle and top up the top of the suction chamber, known as the dashpot, with SAE 20 oil. Replace the damper and the air filter.

Adjusting SU Carburettors

The procedure for setting SU and Hitachi carburettors is similar to that for Strombergs.

Variable-jet Hitachis are SUs made under licence . There are some differences however between different SU models, in particular the various kinds of HD and HS carburettors .

Adjusting SU HD Carburettors

On type HD SU carburettors the main jet is totally enclosed in the base of the carburettor. For mixture adjust­ment it is raised or lowered by a lever which contacts the jet within the housing and is pivoted at a central point.

The outer end of this lever projects horizontally from the base of the carburettor and is fitted with a spring- loaded adjusting screw which bears against a fixed lug on the car­burettor.

If this screw is turned clockwise, the jet will be lowered and the mixture enriched .

If turned anti-clockwise, the mixture is weakened.

With this type of carburettor take care not to confuse the mixture control with the cold start adjustment which may be mounted close by. Ensure that the adjusting screw is actually mounted on a lever which passes into the inner part of the carburettor base. Some models have automatic chokes their their own starting carburettors.

  • To adjust the HD remove the air filter, suction chamber, piston and spring. The suction chamber is held on by two or on larger carburettors three set screws.
  • Mark the chamber and its base so it can be replaced in exactly the same position.
  • Check that the needle is correctly mounted in the piston (as shown above). Take care not to damage the needle, and keep the piston in a clean place.
  • Screw out the mixture screw until the top of the jet is flush with the jet bridge, which is visible from the top of the carburettor once the suction chamber and piston have been removed.
  • Turn the screw clockwise by three and a half turns.
  • Now screw in the slow running volume screw or the throttle stop screw to the full extent of its travel. Taking care not to over-tighten.
  • Then unscrew by two and a half turns. This provides the base setting.
  • Replace the piston, spring and suction chamber, and air filter.
  • Top up the damper with SAE 20 oil to within ½in. of the top.
  • Start the engine and adjust the idle to around 600rpm or until the ignition light barely flickers.
  • Check the mixture strength as detailed for the Stromberg car­burettor. Some SUs have a special lifting pin on the side for this purpose (see footer of this section). Adjust the mixture in the same way as on the Stromberg, bearing in mind that screwing the mixture screw clockwise will enrich the mixture, anti­dockwise will weaken it.
  • When the mixture is correct it may be necessary to re-adjust the idle.
  • Where an auxiliary starting carburettor is used as part of an automatic choke it is necessary to energize the starting solenoid.
  • This is done by shorting the electrical terminal of the thermostatic switch located on the inlet manifold by earthing it with a screwdriver.
  • Place the screwdriver across the terminal and one of the retaining screws to make a temporary connection.
  • At the same time open the throttle. The carburettor will be heard to operate with a pronounced hiss. Adjust screw ‘x’ (fig. 10) until the exhaust gas is just black and the idle remains unaffected.
  • Screwing ‘x’ anti-clockwise will enrich the mixture.
Piston lifting pin on SU Carburettor
An SU carburettor showing the special piston-lifting pin

Adjusting SU HS Carburettors

On SU types H and HS the procedure is broadly similar to that for the HD (as described above) but on these carburettors and on Hitachis the main jet itself protrudes from the base of the carburettor.

The jet is tensioned upwards by spring loading but it is held by a jet nut. If this nut is raised, the jet will be moved with it to a larger section of the tapered needle, and the mixture will be weakened.

If the jet is lowered, it causes enrichment.

The base setting can be found in the same way as for the HD model but turn the jet down only two turns when they are flush or as near flush as possible with the jet bridge.

Replace the suction chamber, piston and spring as before, topping up with SAE 20 oil, then run the engine.

Adjust the jet up or down until the fastest idling speed and the most even running are obtained. Again, the idle may re­quire adjustment.

The mixture strength may be tested in the same way as on the HD models.

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