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  Chicagoland MG Club:Tech Tips

Understanding and Dealing With SU Carbs

SU carburetors are some of the simplest "full function" carburetors ever put on modern cars. However, since they work so differently from most other carburetors, few people in the USA really understand them well. Many, if not most, problems attributed to these carbs' being "out of tune" are really faults of the ignition system, valve train, or something else. Before blaming or "fixing" the carburetor, it is essential that all other engine and engine-related systems are in fine order and working well, and that the operation of the SU is understood.

SU carburetors operate on a system variously known as "variable venturi", "variable choke", "constant pressure", or "constant depression". What this means is that the effective area of the carburetor venturi is variable, producing constant air velocity and constant pressure differential across the jet orifice. This is produced by the air piston (also known as the air valve) moving up and down in response to the engine vacuum. Since the tapered fuel metering needle is mounted in this piston, and acts within the fuel jet, the SU has a variable jet which responds directly to engine vacuum. Since engine vacuum is a function of the operating condition and air/fuel requirements of an engine, the SU carb is ideally suited to meeting the requirements of an engine by means of this direct feed-back loop, rather than by the complicated and empirical means employed by almost all other carburetors.

Even for the theoretically "abnormal" conditions of cold starting and acceleration under load, SUs employ a simple means of satisfying an engine's requirements. Cold start is obtained by increasing the fuel flow, without also blocking the incoming air. This is done by either lowering the height of the jet, thereby increasing its effective area (H and HS types), by passing additional fuel into the carb venturi from a separate device on the carb (HIF type), or by starting the engine largely on a very rich mixture from a separate small auxiliary starting carburetor ("thermo-carb", used with HD types). Extra mixture richness for acceleration is provided by a classically elegant means; the upward motion of the air piston is retarded by the oil damper within the piston guide rod. This condition causes greater fuel flow out of the jet in relation to the air flow. This is why the oil in the dashpot is essential. Return of the piston is, in most SUs, aided by a spring, the tension of which is critical to proper functioning of this system.

"If it ain't broke, don't fix it" definitely applies to carburetors-be sure that the problem lies with the carburetors before attempting to fix them! Before trying to tune your carbs, it is advisable to do the following:

  1. Check engine's ignition system for correct adjustment and function. Check valve clearances, compression, and ensure a lack of vacuum leaks in the intake system. Any problems with these will prevent proper tuning of the carburetors.

  2. Make sure that the carbs are clean enough to function correctly, and that their mechanical components are working correctly. Check and/or clean the following:

    • float bowl; clean any sediment and varnish

    • float; check for leaks and adjustment

    • float valve; not binding, and able to reliably shut off fuel flow

    • air chamber and piston; cleanliness and freedom of motion absolutely imperative. Ensure the piston springs have not been stretched or otherwise damaged

    • metering needles; straight and unworn

    • jets; properly centered on metering needles and unworn

    • throttle shafts; unbent and unworn

    • connections; tight and leak-free

    • air and fuel filters; clean, present, and of sufficient flow capacity for your engine.

While specific instructions for adjusting and balancing SU carbs for a particular car are usually found in the appropriate workshop manual, they are not always too clear. The most basic idea to keep in mind is that the air flow and mixture are the same from one carb to the other. A basic "generic" procedure follows:

  1. Warm up engine to normal operating temperature.

  2. Remove air cleaners, and disconnect choke cable from carb linkage. Loosen clamping bolts on the connecting rod between the throttle shafts, so the throttle shafts can be operated independently. Back off the fast idle screw(s) well away from fast idle cam(s).

  3. While maintaining a steady slow idle, use a Unisyn (or the old "tube in the ear" method) to set the carbs to the same air flow by adjusting the slow idle screws.

  4. With the air flow balanced, check the air/fuel mixture of each carb by either pushing up on the piston lifting pin on the side of each carb, or by lifting the piston; if engine speed increases, the mixture is too rich, and the mixture adjustment nut should be screwed up one flat (1/6 of a full turn). If the engine speed decreases, unscrew the nut one flat. (This instruction applies to H and HS types - HIF and HD type carbs have mixture adjustment screws, which operate in similar fashion. ) Between adjustments, "rev" the engine to clear it, and allow it to return to idle before proceeding. Continue this procedure, alternating from one carb to the other, until, when the pistons are lifted, the engine speed either doesn't change, or rises slightly and then falls to an even speed. The mixture settings are then satisfactory.

  5. After the mixture adjustment, the idle speed may be no longer satisfactory. If not, re-adjust the slow idle screws to the desired idle speed, while maintaining a balanced air flow (see step 3 above).

  6. Re-install air filters and re-check the mixture settings, using the piston lifting pins. (Some air filters are restrictive enough to otherwise cause a slightly overly-rich mixture.)

  7. Re-tighten the throttle shaft connectors, re-connect the choke cable, and set the fast idle screws according to the appropriate workshop manual. Do not neglect this last step, as it is vital for correct starting of your engine when using the choke.

"Problems" That Aren't

Some commonly perceived "problems" about SU parts which really aren't problems follow: The cork washers for H type jet glands and sealing ring do not come pre-formed. They are, and always have been, supplied as cylindrical pieces which form their beveled shapes after installation.

Needle dimensions are from the upper shoulder down, in 1/8" increments. If one needle is a bit longer than another of the same type, it doesn't matter.

The number or letter stamping on the shank of metering needles may be raised enough as to prevent entry of the needle into the bottom of the air piston. This is a very common occurrence. Simply smooth off the burrs until the needle fits. Some of the holes in H type jet levers seem to be grossly oversized. They were designed this way to provide for some lost motion in their operation, and to ensure a full return to the "choke off" position.

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