The IV series carburetters are of downdraught design and a further development of the successful "V" type instruments we have manufactured over a number of years. Improvements embodied are twin floats in a float chamber set close to the throttle bore to give a high flooding angle, effectively controlling fuel level so that performance is not affected by angles of incline, fast acceleration, harsh braking, or surging on bends. The emulsion block, or jet carrier, has an outlet in the centre of a lead-in to the choke restriction that is cast in the barrel portion of the instrument.
All jets and the accelerator pump piston are in the emulsion block which can be readily removed with a screwdriver and a 1 inch spanner. The float chamber has no screws or plugs on the exterior and is cast in one piece with the throttle barrel, thus eliminating any joint below the fuel level from which petrol can leak outside the carburettor.
The design and construction ensure a very free-breathing instrument that will pass maximum volume or weight of mixture relative to the bore diameter.
Operation
The petrol inlet 21 at the top of the float chamber is a parallel tube to accommodate plastic fuel pipe. From here, fuel enters the float chamber via the needle seating 20, where the flow is controlled by the needle 19 and the float 18. As the petrol level rises, the float lifts and, by means of the arm connecting the twin floats, closes the needle on its seating when the correct level has been attained. When the engine is running, petrol is drawn from the float chamber; the float descends and more fuel is then admitted through the needle seating. By this means, the correct level is automatically maintained the whole time the carburettor is in action.
Starting from Cold
When the choke control is pulled out it operates a lever 7 at the side of the air intake. This allows the spring-loaded strangler (or choke spindle), to which the strangler flap 8 is affixed, to rotate and close the air intake. Simultaneously, the interconnection rod 1 between the strangler lever and a loose lever 3 on the throttle spindle opens the throttle beyond the normal idle position to provide fast-idle.
On switching on the ignition and rotating the engine, it will fire and run. The increased depression in the carburettor when the engine starts will open the strangler flap against the spring action that held it closed, thus weakening the mixture to avoid over-choking once the engine is running. As the engine warms up, the choke control should be pushed in to open the strangler flap and reduce the fast-idle speed.
The Main Circuit
As the instrument has embodied a mechanically operated accelerating pump, one can usually start without using the strangler or choke by depressing the accelerator pedal once or twice before, or during, the period the starter is rotating the engine. Do not use the choke and pump the accelerator, as this could cause delay in starting by over-choking.
When the throttle 9 is in the idling position, the mixture is supplied by the slow-running jet 22, which obtains its fuel from the metered side of the main jet 17 (at the base of the emulsion block) through a calibrated restriction. The petrol from the jet is emulsified by air bleeding into the vertical channel through the orifice 23. This opens into the air intake, and the resulting mixture is then drawn down the channel to the idle hole 26 where the tapered end of the volume control screw 25 enters.
In certain applications of the "IV" carburettor the slow-running jet 22 has incorporated a ball valve. The purpose of this valve is to eliminate what is termed 'back bleeding' of air into the main carburettor jet circuit. Adjustments to the idling are made by both the throttle stop screw 5 and the volume control screw 25. Turning the throttle stop screw clockwise increases the speed of idling, and turning the volume control screw clockwise weakens the idling mixture.
Economy Device
On the float chamber cover adjacent to the air intake is a small casting attached by three screws. Under the cover is a diaphragm valve 30 which is normally held in a flexed condition by a compression spring. The chamber above the diaphragm communicates via a hole in the cover and a channel in the body to an outlet 28 in the throttle bore on the engine side.
Under part-throttle cruising conditions the manifold depression is high. This depression (or "suction") is imposed on the spring-loaded side of the diaphragm, thereby lifting the valve from its seat to admit further air from the passage 33 in communication with the air intake, increasing ventilation to the jets and weakening the mixture to ensure the best possible consumption figures. When the valve is off its seating under high manifold depression, the extent of ventilation for cruising is controlled by the ventilation screw 29. At full throttle or when manifold depression is low, the diaphragm 30 will be closed on its seating by the spring 31 and the air bleed is limited to the small full-throttle air bleed orifice 32. Increasing the air bleeding by fitting a larger ventilation screw will weaken the mixture by reducing the depression on the main and compensating jets.
Accelerating Pump
The purpose of the accelerating pump is to prevent any hesitation in accelerating when a carburettor is adjusted to provide the leanest mixture at part-throttle cruising. To obtain economical running at such speeds, a controlled and metered supply of fuel is required when the throttle is opened quickly, and this is supplied by the accelerating pump. When the pump piston 12 is at the top of its stroke, the cylinder is charged with fuel admitted from the float chamber through the non-return inlet valve 11. Upon the throttle being opened, the piston rod 13 is forced down, compressing both the inner and outer springs; the expansion of the inner spring, bearing on the top surface of the piston, forces it down to discharge a stream of petrol through the non-return valve 14 and the pump jet 1. The outer spring ensures the return of the pump piston and rod to the top of its stroke, recharging the cylinder with fuel. The travel of the piston may be varied as required, with a short stroke for summer and a long stroke for winter – the change being effected in the linkage (4) from the throttle to the pump operating lever (6).
Automatic Ignition-Advance Control
A connection (2) is provided in the throttle body for suction-operated automatic ignition-advance control. The small hole, where it breaks through into the throttle bore, is very carefully drilled both for size and position, and should under no circumstances be tampered with.
General
To gain access to the emulsion block and float mechanism, remove the four screws in the top half of the carburettor and take out the clevis pin on the pump linkage after removing the split pin; the top half can then be lifted off (see fig. 3). The main and compensating jets in the base of the emulsion block have screwdriver slots and can be removed for cleaning without further dismantling. The jet at an angle is the main jet and the compensating jet is vertical. Note that these two jets are cadmium plated to distinguish them from other jets of similar size and shape but with a plain brass finish. Although the two types appear identical, their flow characteristics are entirely different – hence it is important that only plated main and compensating jets are used in this series of carburettors. No washers are needed on these jets.
To remove the emulsion block after taking off the top half, first slide out the float hinge pin in either direction and lift away the float. The fuel inlet needle can then be lifted from its seating. Next, remove two screws with spring washers and the needle seating so that the emulsion block can be lifted clear. See fig. 4. Take care not to lose the soft aluminium washer that ensures a petrol-tight joint for the seating on the under surface of the emulsion block.
On lifting off the block the pump assembly will drop out. The cover gasket between the block and the top half can then be removed and replaced if necessary. The pump piston rod and spring assembly are not designed to be taken apart. The slow-running jet and the pump discharge valve screw in the top face of the emulsion block and the pump discharge jet screws in the side of the block.
At the base of the pump cylinder is the ball inlet valve, retained by a circlip. This valve should not need removal – it can be cleaned with an air line or by syringing with clean petrol to remove any sediment. Note that the choke tube restriction in the IV series is incorporated in the throttle barrel (the lower half of the carburettor). Surrounding the spigotted top end of the choke restriction is a circular rubber "O" ring (10) to provide an effective seal with the emulsion block when the top half is reassembled. When dismantling, check the "O" ring and replace it if necessary.
The brass jet (with screwdriver slot) in the underside of the top half, visible when the emulsion block is removed, is the ventilation screw (29). With the emulsion block removed, several drillings in the cover become exposed – ensure all are clear, particularly those that open into the air intake.
When reassembling the top half, ensure the gasket lines up with the drilled holes in the underside of the cover and that the short cam lever on the pump spindle aligns with the pump assembly in the emulsion block. Position the block on the cover, screw in the needle seating, then fasten the screws with their washers evenly and fully. Next, drop the needle into its seating and install the float by sliding in the axle pin. The float chamber has no fixing for the pin as its end movement is limited by the interior of the float chamber when the top half is in position. While the top half is off, dismantle the economy device by removing the three screws and lifting off the small die-cast cover; a gasket is used on each side of the diaphragm.
ADJUSTMENTS
A number of variables are incorporated in the IV series of carburettors which enable the instrument to be tuned to provide the correct fuel/air ratio at all points in the throttle range and for all speeds and loads. In those instances where the IV carburettor is fitted as initial equipment on a standard engine, the sizes of the jets and other variables have been determined after extensive bench and road tests. In such cases, we recommend that no changes be made to these standard settings without first investigating other engine factors such as ignition, valve tappet adjustment, etc.
Unless otherwise stated, all the jets and air bleeds used in this carburettor are calibrated in hundredths of a millimetre. Main, compensating, slow-running, and pump jets are normally available in steps of five units, and air bleeds in steps of ten. In all cases, a higher number indicates a larger calibration – for example, a jet stamped 105 is the next size smaller than one marked 110. Half-size main and compensating jets can be supplied to order for final tuning; one stamped 97 is midway between 95 and 100.
Altitude
The obvious exception to the above is when a vehicle operates at altitudes in excess of 5,000 feet. Where a vehicle is consistently used at these altitudes, some reduction in jet sizes is necessary to maintain the correct fuel/air ratio in the rarefied atmosphere. Below we detail recommended changes to main and compensating jets for different altitudes:
5,000 to 7,000 ft, 7,000 to 10,000 ft, 10,000 to 15,000 ft.
While no change is specified for altitudes up to 5,000 feet, in cases where an owner is more concerned with economy than performance it may be worthwhile trying a smaller main jet when operating above 3,500 feet. Note that these suggested changes apply only when a vehicle operates consistently at altitude.
It must be made clear that the above suggested changes apply only when the vehicle operates consistently at altitude; no change is needed when the car frequently ascends and descends varying altitudes (for example, in the European Alps). In addition to changes to the main and compensating jets, other adjustments may be made to suit a particular engine, such as those affecting the choke tube, main jet, and compensating jet. In the IV series, the choke tube is cast in the throttle body and cannot be varied.
The main jet, together with the compensating jet, determines the mixture at speeds above idling. The main jet influences power and speed, while the compensating jet supplements the main jet when the fuel level above it is exhausted during acceleration. Any variation to the size of the compensating jet will affect the mixture strength over the entire throttle range, though its influence is less than that of the main jet.
Slow-running jet – This jet, with its calibrated hole, meters petrol to the slow-running outlet and to the two small "progression" holes at the throttle edge. Generally, it is not necessary to change the slow-running jet from the standard size specified for a particular application.
Slow-running air bleed – The drilled hole (23) from the slow-running passage into the air intake bleeds air to the circuit and also acts as an anti-syphon hole.
Ventilation screw – At part-throttle operation, when manifold depression is high, the diaphragm (30) will be off its seating, opening an orifice under the metal insert to admit air from channel (33) to the compensating jet and cross channels above the main jet. The ventilation screw (29) controls the degree of air bleeding. At full throttle, the diaphragm is closed by spring (31) and the air bleed is limited to the full-throttle air bleed orifice (32). Increasing the size of the ventilation screw will weaken the mixture.
Pump jet – This jet governs the amount of petrol injected into the choke tube when the accelerating pump (interconnected with the throttle) is depressed. The stroke of the pump piston can be varied (short stroke for summer, long stroke for winter) via the linkage (4) from the throttle to the pump operating lever (6).
Interconnection for Cold Starting – The degree of throttle opening when the strangler is closed is preset by the length and setting of the interconnection rod (1). If the throttle opening does not meet specifications, the only adjustment is to set or bend the interconnection rod.
Petrol Level – Set at the factory to 15.5 mm down from the top edge of the float chamber. This level is achieved when the top half is assembled and the float holds the needle closed against the fuel line pressure. When the top half is removed and the float is taken out, the level will drop. To check the level, remove the top half, leaving the float in position, reverse the cover so the float arm closes the needle, then measure from the gasket face to the highest point of the float (should be 32–33 mm). The level can be adjusted by carefully bending the float arm tag or by varying the thickness of the washer under the needle seating.
Needle Valves and Seatings – These are calibrated in millimetres, with the size stamped on one of the hexagon flats. The correct size depends on fuel pump pressure and engine capacity. Ensure the correct needle valve and seating are installed.
General – In addition to the jets and air bleeds which can be changed if necessary after service, many drillings in the carburettor can be varied during manufacture for tuning purposes. Once the sizes of these variables are determined, the mixture strength remains constant provided the carburettor is clean and mechanically sound.
In any investigation of suspected carburettor issues, first verify that the specification (including choke tube, jet, and air bleed sizes) is correct for the application. Although it is important to ensure that jets have the correct numbers stamped on them, do not overlook the possibility that a jet or air bleed may have been tampered with or altered by careless cleaning. If any interference is suspected, fit a new part of the correct size.
While a general check is advisable in any complaint, we detail below, under specific headings, points related to particular problems.
Difficult Starting from Cold
Ensure there is petrol in the float chamber and that fuel from the pump flows through the needle seating within a few seconds of starting (or using the hand primer). If sediment or gum causes the needle to stick on its seating, fresh petrol may be prevented from entering, resulting in difficult starting—especially if petrol has evaporated during prolonged shutdown. If a sticking needle is suspected, remove the top half and operate the needle by hand. Rinse the needle and seating in mentholated spirit to dissolve any deposits. Also, ensure that the strangler flap in the air intake closes completely when the choke control is operated.
Difficult Starting when Engine is Hot
This issue is caused by an over-rich mixture. First, verify that the needle and seating are of the correct size. After cleaning, reinstall with the correct washer and ensure a tight fit. Examine the float for damage and ensure it moves freely on its pivot. If necessary, replace the needle and seating. If petrol floods into the manifold from the emulsion block outlet within seconds of shutting off the engine, check and, if needed, reduce the fuel pump pressure. An additional washer under the needle seating may help prevent flooding.
Erratic Slow-Running or Stalling on Deceleration
Ensure the slow-running jet is clean and that the feed hole for the jet is unobstructed. A useful test is to remove the slow-running and main jets and use a petrol-filled syringe on the slow-running tube’s removal hole; petrol should flow out from the main jet’s hole. Also, check that the slow-running air bleed is clear, and that the outlet and progression holes in the throttle barrel are free from carbon deposits. Remove the volume control screw to inspect its tapered end; if it has been worn by excessive tightening, replace it. Ensure the spring under the screw effectively prevents vibration. Adjust the throttle opening and volume control screw to achieve a stable idle speed (typically 500–600 rpm) with smooth acceleration as the throttle opens.
Excessive Fuel Consumption
Verify that the carburettor retains its standard jet and air bleed settings and that all passages are clear. Ensure the strangler flap opens fully when the dash control is released or pushed in. When reassembling the economy diaphragm, check that the valve is clean and moves freely and that the spring is correctly positioned in its recess. Tighten the securing screws evenly to avoid leakage, which would affect the diaphragm’s ability to maintain the proper depression.
Poor Acceleration
Check that the pump piston moves freely and returns to its upper position by its spring. Remove the pump jet to ensure it provides a continuous stream of petrol and verify that the rubber "O" ring sealing the jet is intact. If hesitation occurs during throttle opening, adjust the slow-running mixture after ensuring that the progression holes are clear. Over time, wear may affect acceleration; in such cases, consider replacing relevant components.
Loss of Power
Ensure all jets and passages in the emulsion block are clear and that the throttle opens fully. In engines modified for increased power, alternative settings may be recommended based on prior experience with similar installations. This applies to conversions to twin or multiple carburettors as well.
Twin Carburettors
When twin carburettors are fitted, ensure that the petrol levels and settings are identical in both units. The throttles must be synchronized so that both carburettors open equally from the dead-shut position, allowing the progression holes to operate simultaneously for smooth acceleration.
If idling speed or mixture varies and requires frequent adjustment, check for air leakage at manifold joints.
In dismantling the carburettor for cleaning and jet inspection, always have a set of new gaskets available. A genuine Zenith Gasket Pack (which includes all washers and gaskets for your model) is recommended.
While no change is specified for lower altitudes, if a vehicle operates consistently at high altitude (above 5,000 ft), a smaller main jet may be tried for improved economy. However, if the vehicle frequently moves between high and low altitudes, no adjustment is necessary.
It is essential to follow all recommended procedures and use genuine Zenith parts for best performa
