🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Twin Zenith carburetor idle circuit blocked
💰 Estimated Repair Cost
$20 – $60 USD
Parts and labour estimate
🔧 Tools Required
Carb screwdrivers, carb cleaner, soft brushes
📋 Overview
Tiny idle circuit passages in the Zenith 35/40 INAT block with varnish deposits from old fuel. The engine may run at higher speeds but refuses to idle, or idles very roughly and is hard to adjust.
🩺 Symptoms
- Will not idle smoothly
- idle adjustment screw has no effect
- engine dies at traffic lights
- better at higher RPM
🔍 Possible Causes
- Varnished idle jet or passage
- blocked idle mixture screw port
- dirty idle air bleed
🛠️ Diagnosis Steps
Disassemble carburetor. Use carb cleaner spray into idle circuit ports and observe at opposite end.
✅ Most Confirmed Fix
Full carburetor strip and ultrasonic clean. Replace all soft parts from rebuild kit.
79% of users confirm this fix worked (22 reports)
🏷️ Parts Needed
Zenith 35/40 INAT rebuild kit; carburetor cleaner
🔗 Other Mercedes-Benz W115 Faults
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