Triumph · 1970–1977The Stag Archive

How-To & Repairs / Fuel/Carburettors

Balancing and tuning the twin Strombergs

How to balance and tune the twin Zenith-Stromberg CD-series carburettors fitted to the Stag: setting a synchronised idle, matching airflow between the two carbs, and adjusting mixture. A well-balanced pair transforms low-speed running, throttle response and fuel economy.

Tools needed

  • Carburettor balancing meter (airflow gauge)
  • Long thin screwdrivers for idle and throttle-stop screws
  • A short length of clean fuel hose used as a stethoscope
  • Fresh air filter (or clean existing element)
  • The correct mixture-adjustment tool for Zenith-Stromberg CD carburettors

Parts needed

  • Any consumables specified by the workshop manual (e.g. dashpot oil, gaskets if disturbed)

Warnings

  • Do not force the mixture-adjustment tool — the wrong tool damages the metering needle carrier.
  • Keep the workspace tidy and free of ignition sources; carburettors mean petrol vapour.

Steps

  1. 01. Warm the engine and set the ignition side first

    There is no point tuning carburettors around a poor ignition system. Confirm plugs, HT leads and timing are correct before touching the carbs.

  2. 02. Top up the dashpots with the correct oil

    Check and top up the dashpots to the correct level with the oil specified in the workshop manual. Wrong oil weight changes how the pistons rise and directly affects throttle response.

  3. 03. Measure and equalise airflow at idle

    With the engine at idle, place the balancer over each carb intake in turn. Adjust the throttle-stop screws so both carbs draw the same airflow at your target idle speed.

  4. Owner-uploaded photo slot
    Balancer in use on one carb. The airflow balancer seated on one carburettor intake with the engine at idle.

  5. 04. Set mixture on each carb in turn

    Adjust the mixture on one carb until it runs at its smoothest and highest idle, then repeat on the other. Re-check airflow balance afterwards — mixture and idle interact.

  6. Owner-uploaded photo slot
    Mixture adjustment tool in place. The correct mixture-adjustment tool engaged in the carburettor.

  7. 05. Blip the throttle and re-check

    Snap the throttle open and closed a few times, then re-measure airflow and idle. Small final adjustments after the throttle linkage has settled are normal.

Related faults

No directly linked fault-finding entries yet.

Further reading

Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.

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