Solex Accelerator Pump Diaphragm

A Solex carburetor uses a diaphragm-type accelerator pump to prevent a lean stumble when you snap the throttle open. Air speed increases instantly, but fuel flow lags. The pump fills that gap by mechanically injecting extra fuel.

BMW Accelerator Pump Diaphragm

The parts that matter

• Flexible rubber (or Viton) diaphragm – the heart of the system

• Pump chamber – holds fuel behind the diaphragm

• Throttle linkage cam/lever – physically pushes the diaphragm

• Inlet check valve – lets fuel into the pump chamber

• Discharge check valve + pump jet – sends fuel into the carb throat

No vacuum tricks here. This is purely mechanical.

Step-by-step operation

1. Throttle closed (at idle or cruise)

• The diaphragm is relaxed.

• Fuel fills the pump chamber through the inlet check valve.

• The discharge valve is closed, so fuel can’t dribble into the throat.

Think of this as the pump “reloading.”

2. Throttle opens (you hit the gas)

• Throttle linkage pushes directly on the diaphragm.

• The diaphragm flexes inward, reducing chamber volume.

• Fuel pressure rises instantly.

• Inlet valve closes.

• Discharge valve opens.

• Fuel is forced through the accelerator pump jet, spraying into the venturi.

This happens immediately—before airflow can pull fuel normally.

3. Throttle steady or closing

• Linkage pressure is released.

• Diaphragm springs back to its rest position.

• Pump chamber refills through the inlet valve.

• System resets for the next throttle movement.

Why Solex uses a diaphragm (not a plunger)

• Compact – fits small European carb bodies

• Smooth delivery – less “on/off” than piston pumps

• Simple – fewer precision-machined parts

• Reliable – when the diaphragm is fresh

But there’s a catch…

Common failure modes (this is where problems start)

• Hardened or cracked diaphragm ? weak or no pump shot

• Ethanol-damaged rubber ? delayed or inconsistent squirt

• Blocked pump jet ? fuel moves, but never reaches the throat

• Incorrect linkage adjustment ? short or mistimed pump stroke

Classic symptom:

Engine idles fine, but bogs or dies when you hit the throttle.

That’s almost always accelerator pump related on a Solex.

One critical detail people miss

The pump volume and timing are fixed by linkage geometry.
If the diaphragm is correct but:

• the linkage is bent,

• the wrong diaphragm thickness is installed,

• or the pump cover is warped,

…the pump shot will be weak even though “fuel is moving.”