🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Rough idle after warm-up — Zenith carburetor sync
💰 Estimated Repair Cost
Up to $30 USD
Parts and labour estimate
🔧 Tools Required
Vacuum gauge or Carbtune synchronizer, screwdrivers, tachometer
📋 Overview
After the engine reaches operating temperature, idle becomes rough or hunting between 600-900 rpm. The twin Zenith carburetors on the 230.4 require periodic synchronization. An unsynchronized pair causes one cylinder bank to run rich and the other lean.
🩺 Symptoms
- Hunting or oscillating idle when warm
- rough idle between 600-900 rpm
- occasional stalling at lights
- uneven exhaust note
🔍 Possible Causes
- Twin carburetors out of synchronization
- idle mixture screws incorrectly set
- air leaks at intake manifold gaskets
- worn throttle shaft bushings
- ignition points gap incorrect
🛠️ Diagnosis Steps
- Warm engine fully.
- Use vacuum gauge or synchronizer on each carb.
- Adjust idle speed screws to equalize vacuum.
- Set idle mixture on each carb.
- Check for intake air leaks.
✅ Most Confirmed Fix
Synchronize twin carburetors using vacuum gauge. Reset idle mixture screws. Replace intake manifold gaskets if leaking.
82% of users confirm this fix worked (22 reports)
🩺 Related Symptoms
🔗 Other Mercedes-Benz W115 Faults
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