If your classic British car runs twin SU carburettors, getting them balanced is one of the most satisfying — and most important — tuning jobs you can do. Unbalanced carbs cause rough idling, uneven throttle response, poor fuel economy and excessive wear. The good news is that with the right approach it's a straightforward afternoon job, even for a relative beginner.
This guide covers the complete process for cars fitted with twin SU carburettors, including the HS, HIF and H-series types fitted to MG, Triumph, Austin Healey, Jaguar and countless other classic British marques.
What You'll Need
Before you start, gather the following:
- Airflow meter (Carbtune or Synchrometer) — the most accurate method. A Gunson Carbtune is the classic choice and widely used by enthusiasts.
- Colortune plug (optional but useful) — lets you see mixture colour through the spark plug hole.
- A length of hose or tube — as a simple hissing-point method if you don't have a carbtune.
- Flat-head and cross-head screwdrivers
- A lifting pin tool or small screwdriver for the piston lift test
- Tachometer or access to a rev counter
The engine must be at full operating temperature before you begin. Take the car for a 10–15 minute drive, or let it idle until the thermostat opens and the temperature gauge settles.
Step 1: Set the Basic Idle Speed
With the engine warm and idling, find the idle speed adjuster on each carburettor — this is the screw that bears against the throttle butterfly spindle. Adjust each one equally so the engine idles at around 800–900 rpm. Don't worry about perfect balance yet; you're just getting both carbs in the right ballpark.
Check that both throttle butterflies are opening equally by observing the linkage. If one carb is doing more work than the other, the car will never idle smoothly regardless of mixture settings.
Step 2: Balance Airflow Between the Two Carbs
This is the core of the job. Hold your Carbtune (or length of hose held near each intake) at the mouth of each carburettor in turn. You're listening for — and measuring — the amount of air being drawn in.
Adjust the idle speed screws until both carbs show the same airflow reading. Work in small increments — a quarter turn at a time — and re-check frequently. The goal is equal airflow at idle, which means both butterflies are open by the same amount and the engine is drawing equally from each carb.
If using a Carbtune, match the columns precisely. If using a hose, listen for the same pitch and strength of hiss from each carb intake.
Step 3: Check and Set the Mixture
With airflow balanced, check the mixture on each carb using the piston lift test:
- With the engine idling, insert a thin screwdriver or lifting pin under the suction piston of the first carburettor and lift it very slightly (about 1mm).
- Observe what happens to the engine speed:
- Engine speed rises slightly and holds — mixture is correct.
- Engine speed rises sharply — mixture is too rich.
- Engine speed drops immediately — mixture is too lean.
- Repeat on the second carb.
Adjust the mixture screw (the large knurled nut or hexagonal adjuster on the bottom of the carb body) to correct the mixture. Turning it clockwise (down) enrichens the mixture; anticlockwise (up) weakens it. Make adjustments in small increments — an eighth of a turn at a time — and re-test.
Aim for the same response from both carbs. Once correct, re-check airflow balance, as mixture adjustments can affect idle speed slightly.
Step 4: Set Final Idle Speed
With both carbs balanced in airflow and mixture, set the final idle speed using the idle screws equally — turn each by the same amount to reach your target idle speed of 800–900 rpm. Adjust both together rather than one at a time to preserve the balance you've achieved.
Step 5: Road Test
Take the car for a short drive to confirm the results. The engine should pull evenly through the rev range with no flat spots, the idle should be smooth and consistent, and there should be no hunting or surging. If any of these symptoms persist, return to Step 2 and recheck airflow balance — small variations can creep back in during the warm-up process.
Common Mistakes to Avoid
- Working on a cold engine. Mixture and airflow settings change significantly with temperature. Always balance warm.
- Adjusting one carb only. Both must be adjusted together. Fixing the mixture on one without re-checking the other will undo your work.
- Over-tightening the mixture adjuster. The needle and jet are precision components. Gentle adjustment is all that's needed.
- Ignoring worn components. Balancing cannot compensate for worn throttle spindles, perished diaphragms or blocked jets. If the carbs won't balance, inspect the components first. We stock a full range of SU carburettor rebuild kits to get your carbs back to factory specification.
How Often Should You Balance Twin SU Carbs?
For a car in regular use, check the balance at least once a season — spring is ideal, before the summer driving months. After any significant work to the fuel system, throttle linkage or carburettor internals, always re-balance. Worn jets and needles are the most common cause of a carburettor that won't hold its balance; if yours keep drifting, it's worth fitting new components. Browse our range of SU needles and service kits to keep your carbs running at their best.
Got a question about your specific carburettor setup? Get in touch — we're always happy to help.