🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Zenith 35/40 INAT float needle valve failure
💰 Estimated Repair Cost
$15 – $55 USD
Parts and labour estimate
🔧 Tools Required
Flat screwdrivers, vernier calipers, fuel-safe tray
📋 Overview
The float chamber needle valve on the Zenith 35/40 INAT wears or sticks open with age, allowing the float bowl to overfill. This causes an over-rich mixture, difficult starting, black smoke, and in severe cases raw fuel dripping from the carburetors.
🩺 Symptoms
- Over-rich running
- black smoke from exhaust
- strong fuel smell
- hard to start
- spark plugs black and oily
🔍 Possible Causes
- Worn needle valve tip
- debris on valve seat
- incorrect float level
- needle valve seat corrosion
🛠️ Diagnosis Steps
Remove carburetor top. Inspect float level — should be 14-15mm below gasket face. Test needle valve by blowing gently — should seal completely. Check float for fuel absorption by shaking.
✅ Most Confirmed Fix
Replace needle valve and seat as a matched pair. Reset float level to 14-15mm.
82% of users confirm this fix worked (28 reports)
🏷️ Parts Needed
Zenith 35/40 INAT needle valve and seat kit
🔗 Other Mercedes-Benz W115 Faults
Own a Mercedes-Benz W115?
Share your experience with this fault — or report a different issue. Your knowledge helps other classic car owners worldwide.