The mechanical linkage between the speed actuator and the engine fuel injection pump is stiff, corroded, disconnected, or jammed. The actuator moves but the fuel pump does not respond because the connection between them is broken or restricted.

What you'll see

The speed actuator appears to be working (you can see or feel it moving) but the engine speed does not change. The connection between the actuator output and the fuel injection pump speed lever is not transmitting the motion. This linkage can be a direct rod, a steel cable, a bell crank mechanism, or a combination. Any of these can bind, corrode, or disconnect.
Before you assume this is the problem

If the actuator is not moving at all, the problem is the actuator itself or its air/electrical supply, not the linkage. See: Speed Actuator Failure, No Control Air to Speed Actuator.

See all causes of engine won't rev up under load →

How to diagnose

  1. Inspect the linkage visually (engine off)

    Trace the mechanical connection from the speed actuator to the fuel injection pump speed lever. Check for disconnected rod ends, broken pins, seized ball joints, corroded pivots, or a steel cable that has frayed and jammed in its sheath. Try moving the fuel pump speed lever by hand -- it should move smoothly from idle to maximum.
    Result: Binding or disconnection found = repair.

How to fix it

  1. Free, lubricate, or replace the linkage

    Free seized joints with penetrating oil and movement. Lubricate all pivot points with appropriate grease. Replace broken or frayed cables. Reconnect any disconnected rod ends. After repair, check that the fuel pump lever moves smoothly through its full range when the actuator is operated. Verify minimum and maximum speed settings with a tachometer.

Common mistakes

Do not force a stuck linkage with excessive leverage. If a pivot point is seized from corrosion, forcing it can break the fuel pump lever or the actuator mounting. Free it gently with penetrating oil and patience. Also: after any work on the throttle linkage, verify both minimum and maximum engine speed. A linkage that does not reach full travel will limit engine speed and compressor capacity.

Parts & tools

Penetrating oil. Grease. Replacement cable if frayed. Replacement rod ends or pins if broken. Tachometer for speed verification. Basic hand tools.

Review safety precautions before starting →

Safety

Do not work on the throttle linkage while the engine is running. A sudden release of a stuck linkage can cause the engine to race to maximum speed.