SI 20/20 carburettor: six steps to a clean one

The SI 20/20 on the PX 125 and 150 is simple and tough. When the idle hunts or the Vespa stalls at the lights, nine times out of ten it is dirty.

What you need

• Flat screwdriver and an 8 mm spanner
• Compressed air or a can
• A clean cloth and a tray
• A new gasket for the cover

The six steps

1. Turn the fuel tap off and take off the right side panel.
2. Remove the filter cover and the air filter.
3. Unscrew the bowl and lift out the float without bending its arm.
4. Unscrew the jets. Note the numbers stamped on them first: you need them to refit.
5. Blow the jets and passages through with air. Never use wire: it widens the holes and changes the mixture.
6. Refit, turn the fuel on and set the idle with the engine warm.

When that is not enough

If the engine still will not hold the idle after cleaning, check the intake manifold: a crack lets air in and leans the mixture. In the engine drawing it is the part called Carburettor and filter.