Getting Your SU Running Perfectly
A correctly tuned SU carburettor is a delight — responsive, smooth, and efficient. An incorrectly tuned one can be frustrating to diagnose. The good news is that SU carburettors are designed to be adjusted with simple tools and no specialist equipment. This guide walks through the complete tuning procedure for HS and HIF series carburettors.
Before starting, make sure the carburettor has been fully serviced or rebuilt with new service parts. Attempting to tune a worn carburettor is largely futile — the variables introduced by a worn jet, needle, or throttle spindle make accurate adjustment impossible. See our rebuild guide first if your carburettor is in poor condition.
What You Will Need
- A lifting pin or improvised thin rod (2-3mm diameter)
- A flat-blade screwdriver (for the mixture adjustment)
- A tachometer (rev counter) if not fitted to the vehicle
- A colortune plug or exhaust gas analyser (optional but helpful)
- Patience — allow the engine to reach full operating temperature before making adjustments
Step 1: Pre-Tuning Checks
Before touching the mixture or idle screws, verify the following:
- Engine at operating temperature: Tuning a cold engine is pointless. Run the car until the temperature gauge is stable and the choke (if manual) is fully off.
- Ignition timing correct: If the timing is off, the carburettor cannot compensate. Check and set timing first.
- Valve clearances correct: Incorrect tappet clearances cause rough running that mimics mixture problems.
- Air filter clean: A blocked filter restricts airflow and richens the mixture.
- No air leaks: Check the manifold gasket and all vacuum connections.
Step 2: Set the Idle Speed
The idle speed is controlled by the throttle stop screw (also called the idle speed screw), which limits how far the throttle plate closes.
- With the engine at operating temperature, locate the throttle stop screw. On most SU carburettors it is on the side of the body near the throttle lever, with a locknut.
- Adjust the screw to achieve an idle speed of approximately 800–900 rpm for most vehicles (check the workshop manual for your specific application).
- If the idle speed cannot be reduced to the correct figure, suspect an air leak or a stuck throttle.
Step 3: Set the Mixture
The mixture is adjusted by raising or lowering the jet relative to the needle. On HS carburettors, this is done with the mixture adjustment nut under the float chamber. On HIF carburettors, it is a screw accessible from the side of the body.
The standard adjustment method uses the lifting pin:
- With the engine idling at the correct speed, locate the lifting pin on the side of the carburettor body. It is a small pin that, when pressed upward, lifts the piston approximately 1–2mm.
- Briefly lift the pin (just a momentary lift, not held up).
- Observe the effect on the idle speed:
- Idle speed rises and stays up: Mixture is too rich. Raise the jet (turn the mixture nut clockwise on HS; turn the mixture screw clockwise on HIF) by one flat at a time.
- Idle speed drops immediately: Mixture is too weak. Lower the jet (anti-clockwise) by one flat at a time.
- Idle speed rises slightly then returns to normal: Mixture is correct.
- After each adjustment, allow the engine a few seconds to settle, then repeat the lifting pin test.
- Continue adjusting until the correct response is achieved.
Step 4: Re-Set Idle Speed
Mixture and idle speed interact — adjusting one affects the other. After setting the mixture, re-check the idle speed and adjust the throttle stop screw if necessary. Then re-check the mixture. Repeat until both are stable at the correct settings.
Step 5: Check Acceleration Response
With the mixture and idle set, take the car for a gentle test drive. The engine should pull cleanly from low rpm without hesitation or flat spots. A momentary stumble on acceleration can indicate a slightly weak mixture — lower the jet by one flat and retest.
If the engine is hunting (the idle speed rises and falls rhythmically), this usually indicates an air leak rather than a mixture problem. Re-check all vacuum connections and the manifold gasket.
Twin Carburettor Balancing
On vehicles fitted with twin SU carburettors, both carburettors must be balanced — that is, both must be passing the same volume of air at idle. If one carburettor is drawing more air than the other, the engine will idle roughly and the mixture settings will be inaccurate.
The traditional method uses a length of rubber hose held near each carburettor intake to listen to the airflow hiss — both should sound identical. Purpose-made carburettor balancing tools (such as a Unisyn or Synchrometer) give a much more precise result and are inexpensive to buy.
To balance twin carburettors:
- Set idle speed equally on both carburettors (adjust each throttle stop screw).
- Using the balancing tool, compare airflow through each carburettor at idle.
- Adjust the throttle stop screws until both carburettors show identical airflow.
- Then set the mixture on each carburettor individually using the lifting pin method.
- Re-check balance after adjusting mixture — the process may need to be repeated two or three times.
Damper Oil
Do not neglect the dashpot damper. The damper oil slows piston rise on acceleration, providing a momentary enrichment effect that prevents flat spots. Use SAE 20 engine oil or dedicated SU damper oil. Check the level by unscrewing the damper cap — the oil should be 5–6mm below the top of the hollow piston rod. Top up as needed.
Needle Selection
The needle profile controls the fuel mixture throughout the rev range. The standard needle for your application should give the correct mixture across the board when the jet is correctly set. If you are experiencing a persistent rich or lean condition at a specific part of the rev range that cannot be corrected by jet adjustment alone, the needle profile may need changing.
SU produced a wide range of needle profiles for different applications and states of tune. If you are modifying your engine — fitting a different camshaft, changing the compression ratio, or using different carburettors — you may need to experiment with needle profiles. Contact us for advice if you are unsure which needle to try.
Need Parts?
Browse our full range of genuine Burlen-manufactured needles and jets, Major Repair Kits, and individual components. If you need help with tuning or diagnosis, contact us — we are happy to advise.