Carburettor Maintenance

The routine procedures for maintaining the carburettors are well described in the workshop manual. These principally involve:

 a)      Adjustment for correct fuel mixture, idle speed and choke operation.

b)      Dealing with general wear and the deterioration of soft components, both of which will interfere with proper tuning for optimum performance and economy.

 The SU Carburettor uses low pressure (or “suck”) in the induction system to draw fuel in small droplets from the main jet and mixes it with air in a venturi (a narrowed tube) in the best proportions for combustion under various driving conditions. The volume of air allowed to enter the engine is governed by the throttle valve, which is a spindle-mounted brass disc in the carburettor body that can move through almost 90 degrees between the fully open and fully closed position. The same low induction pressure is also used to regulate the fuel mixture and it is a key feature of this carburettor that it offers fully variable mixture throughout the throttle range. This is the purpose of the “dashpot” where the suction from the induction manifold pulls a piston upwards, drawing with it the main fuel jet needle. The jet is an orifice with a fixed diameter but the jet needle passing through it is tapered so that, as it is drawn upwards by the opening of the throttle, more fuel is allowed to pass, thereby progressively enriching the mixture. The taper of the needle is not regular and the needle is defined by its various diameters at different points along it length, designed to match fuel flow to the needs of the engine. To avoid too much sudden upward movement of the piston, it has a spring to resist the “suck” and it is damped with oil. Over time this oil is naturally lost into the induction air and requires replenishment. Different engines require a different sequence of mixtures so there is a range of needles available to match these needs. If you change your engine, you will probably need to change your needles.

Allied to these fundamental carburettor functions is a metering valve to ensure that the main jet is fed only with the fuel that it needs to satisfy the demands of combustion. The system also incorporates fuel and air filters to ensure that the air and the fuel are free of contaminants before being mixed. Fuel metering is achieved by the float chamber that functions much like the ball valve in a WC cistern.  As fuel is drawn from the main jet, the level of fuel in the chamber drops and the float falls, allowing the pressure on the fuel inlet valve to relax, thereby opening the valve to allow free passage of more pumped fuel from the tank.

In order to facilitate cold starting, the mixture is enriched by a choke that operates by temporarily moving the main jet downward in relation to the needle. This means that the jet has to move easily in a fuel-tight housing and return accurately afterwards to its normal position.

The purpose of describing all these processes in detail is to emphasise how much is going on in the carburettors at the same time and, therefore,how many opportunities there are for the process of carburation to be disrupted. A clean properly adjusted carburettor with no wear will operate for many miles entirely trouble-free. Regular “tweaking” by the owner is generally counter-productive. The best maintenance regime includes vigilance for specific problems as they occur and systematic periodic checking, adjustment and cleaning. It is the longer-term effects of dirt, wear and poor adjustment that will cause the owner problems ranging from poor running through poor economy to complete failure. It is very unusual for a carburettor to fail suddenly. They are generally only affected by slow deterioration of performance. It is an old maxim of the motor engineer that “90% of carburettor problems are electrical” which eloquently expresses the low probability of a carburettor problem causing unexpected breakdown and the greater probability that the problem is in the ignition system. After high-mileage, dismantling and a rebuild is generally the best way to check for wear and deterioration, with routine replacement of soft parts like seals, washers and gaskets. Metal wear may require machining or component replacement.

Typical problems are:

  •  Air leaks into the manifold caused by loose joints or damaged gaskets, making the mixture too weak
  • Excessive weakness caused when the dashpoint oil chamber is empty or the return spring is worn
  • Air or fuel starvation caused by blocked filters
  • Jet and/or fuel valve blocked by contaminants
  • Poor adjustment of the float forked lever causing excessive richness that cannot be adjusted out
  • Jet not properly centred, causing the needle to stick
  • Choke sticks open because of tightness in the glands, badly centred jet  or failure of the return spring
  • Throttle sticks open because of broken or displaced return spring or stiffness arising from wear
  • Fuel leaks from the main jet caused by deterioration of the gland and packing washers
  • Wear in the main jet and needle, making it impossible to set the mixture properly
  • Wear in the dashpot and piston, causing poor vacuum seal or sticking of the piston
  • Wear in the throttle spindle allowing uncontrolled air to pass into the induction port, disrupting the mixture
  • Wear in the fuel valve or seat, allowing uncontrolled passage of fuel into the float chamber, causing  it to flood.
  • Corrosion or other damage to the float that allows it to take in fuel and prevents it from floating - another cause of flooding.

 As the Magnette has twin carburettors, the above risks are doubled up, and correct operation of the fuel system also requires the two units to be working in perfect balance (See Workshop Manual section B9).

Modern accessories like the Colourtune and balancing devices will make routine adjustments easier. The Colourtune, in particular, makes mixture adjustment far less hit and miss by allowing the owner to see combustion actually happening at different engine speeds. Difficulty in getting the mixture right will also reveal other undetected problems,  e.g. if you cannot tune excessive richness out it may reveal a high fuel level in the float chamber. If you have made extensive changes to the power unit, the carburettors or the state of tune, you are best advised to get it properly set up by a specialist on a rolling-road dynamometer.

Be alert for discharge of fuel from the overflow tubes on the float chambers. This can also signal problems with the fuel level, the float or the valve. The float level may need adjusting by bending the fork lever that operates the needle valve. The workshop manual refers to a “7/16 dia test bar” to check for correct adjustment of the lever but a drill shank or bolt of the right diameter is just as good. There is also the option of fitting after-market valves called Grose valves that provide a more sophisticated design making a better seal.

Fuel leaks from hoses and other joints can generally be seen in the form of wetness or drips. Seepage elsewhere can often be revealed by dirty stains where dirt sticks to the gummy fuel residue.

If you find that your mixture runs persistently weak, the throttle spindle and/or its housing may be worn. This is best detected physically with the carburettor removed but, with the engine running, if a little oil is applied around the points where the spindle emerges from the body, a leak will be temporarily plugged and the engine note will change. You may also see the oil being sucked in via the excessively worn housing. Wear can be corrected with over-sized spindles and the carburettor body reamed out to fit (best done by a specialist: the cost of appropriate reamer tools is high).

Persistent richness may mean that the choke is not returning fully after use and this can be detected by simply pushing upwards on it from below to see if there is any more movement available. Before adjusting anything on the carburettor itself, make sure that the cable moves smoothly and is not catching on a broken internal wire. If the jet itself is stiff, it may be time to replace the glands and seals and relubricate the assembly with vaseline. Persistent richness can also be caused by too high a fuel level in the float chamber, though it may not be bad enough to cause visible overflow (see above paragraph dealing with adjusting the float fork lever).

Problems with the dashpots can sometimes be caused by mismatch of parts. The pot and the piston are a matched pair designed for a close fit. If they are accidentally swapped, leakage or sticking can occur. This will be revealed by difficulty in obtaining smooth up and down movement (See Workshop Manual Section B12).

Later carburettor pairs are fitted with a modified throttle linkage. It was found that for a smoother take-off it was preferable for one throttle to open slightly ahead of the other. To achieve this, there is a coupling on the first carburettor with an enlarged hole that acts on a small lever on the second carburettor. The enlarged hole allows the coupling to be adjusted so that the first spindle is able to move a little way before it bears on the lever and starts the second throttle to open. The free movement is set with the carbs in situ and each separately adjusted for slow running and balance. Once this is achieved, the coupling can be set for maximum free movement (Section B15 in later workshop manuals).

A full carburettor rebuild is not beyond the scope of the amateur. A rebuild kit containing all consumable parts can be obtained from specialists. Cleanliness is vital and for a really impressive finish the body and other alloy parts can be blast cleaned with a soft medium. There are many small parts, so systematic marking and storage are advisable. The rods and brass parts in the choke linkages can show signs of wear, such as ovalling of holes causing general slackness. Steel parts may be corroded, so replacement is advisable for aesthetic reasons. New parts are easily obtainable. See above re wear in the throttle spindles. If you have a non-standard drive-train, take care with items like needles and piston springs because they may not be standard Magnette parts. Springs are colour coded with paint dabs and needles are numbered.

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