Synchronizing dual Zenith Stromberg carburetors ensures both carbs deliver equal airflow and fuel to the engine for smooth performance, especially at idle and part throttle. This procedure is common on British cars like the Triumph TR6, MGs, Volvos, and Jaguars. Here's a step-by-step guide to synchronizing dual Zenith Strombergs:
Tools You’ll Need:
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Carburetor synchronizer tool (Uni-Syn or similar)
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Tachometer (optional but helpful)
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Flathead screwdriver
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Vacuum gauge (optional)
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Patience and a warm engine
Step-by-Step Synchronization Procedure
1. Warm Up the Engine
Start the engine and let it reach full operating temperature. Make sure the choke is fully off.
2. Initial Inspection
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Confirm both carburetors are mechanically sound (no vacuum leaks, worn throttle shafts, or misaligned linkage).
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Inspect the throttle linkage to ensure it's free and returns properly.
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Make sure both mixture screws and idle screws are in a neutral baseline position (usually 1.5 turns out on mixture screws).
3. Disconnect the Linkage
Disconnect or loosen the throttle interlinkage between the two carburetors so each carb can be adjusted independently. This is critical—do not try to sync them while still mechanically connected.
4. Balance the Airflow (Idle Synchronization)
Use a Uni-Syn or vacuum gauge to compare airflow between both carburetors at idle:
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Place the Uni-Syn tool over the throat of the first carburetor and adjust the idle screw until the airflow reading is stable.
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Move to the second carburetor and adjust its idle speed screw until the airflow reading matches the first.
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Go back and forth until both read the same.
Without a Uni-Syn? Use a short piece of hose as a stethoscope. Place one end near your ear and the other near each carb's intake. Try to match the sound pitch and intensity between both carbs.
5. Adjust the Mixture Screws
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Turn each carburetor’s mixture screw in slowly until the engine begins to stumble (lean condition).
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Then back it out just until the engine smooths and reaches maximum idle RPM.
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Do this one carb at a time, switching back and forth until idle is smooth and even.
6. Reconnect the Linkage
Carefully reconnect the throttle linkage so both throttle shafts open at the exact same time. Adjust the linkage arms if necessary. Recheck synchronization briefly after this step to ensure the linkage didn’t disturb the balance.
7. Final Idle Speed Adjustment
Use the primary throttle stop screw to set final idle speed to the manufacturer’s spec (typically around 800–1000 RPM for most applications). Only adjust one carburetor’s idle stop screw to control total idle speed.
8. Test Drive and Fine-Tune
Take the vehicle for a drive. If there’s hesitation, surging, or bogging under light acceleration, revisit the mixture screws and idle balance. Slight adjustments often make a big difference.
Common Synchronization Issues
| Problem | Likely Cause |
|---|---|
| Surging at idle | Air leak or carbs out of sync |
| Engine stalls under load | One carb too lean or out of sync |
| Uneven idle or vibration | Misadjusted mixture screws or imbalance |
| Poor throttle response | Linkage misaligned or uneven fuel delivery |
Final Thoughts
Properly synchronized Zenith Stromberg carbs result in smoother idle, better throttle response, and optimal fuel economy. Take your time, make small adjustments, and always recheck your results. If the carbs still won’t sync, consider a vacuum leak test or rebuild.
