SU Carburettor Wear and Tuning Issues (240Z, 260Z)
Symptoms
Poor idle, hesitation on acceleration, rich or lean running, black smoke, poor fuel economy, difficult starting. The engine may run well at some RPMs but poorly at others.
Cause
The twin SU carburettors use a sliding piston (suction chamber) that rises and falls based on engine vacuum. Over decades, the throttle shafts wear in their bores, creating vacuum leaks that cannot be compensated for by adjustment. The needle and jet wear, the float valve leaks, and the damper oil evaporates. SU carburettors are precision instruments that require regular maintenance, and most Z-car SUs haven't had it.
Fix
A full rebuild addresses all wear items: new throttle shaft bushings (if the bodies are not too worn), new needles, new jets, new float valves, new gaskets, and new damper oil (light machine oil, 20W).
Cost: $400-800 in parts for a pair.
Finding someone who can properly tune SU carburettors is increasingly difficult, this is a dying art.
Alternatively, many owners convert to Weber DCOE sidedrafts ($1,500-3,000 for a pair with manifold) or electronic fuel injection (Haltech, Link, or MegaSquirt, $2,000-4,000 including sensors and wiring).
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