Carbon build-up on spark plug      Carbon build-up on piston

Pre-ignition description : Engine keeps on turning over after the key-switch is turned to the off position.

There are two main reasons :

  1. Faulty carburetor – when the hours  are very high [ + – 14000+ ]. The butterfly in the carburetor wears away and it keeps on sucking air and fuel into the engine, after the engine is switched of. This is evident when the idling is too high.
  2. Carbon build-up – this is caused by a couple of reasons to name three :

2.1 The most common reason is excessive sump compression that causes carbon build- up in the intake and cylinders. This is caused by low compression because of worn pistons and rings.

2.2 Over-fueling – This causes a very rich mixture that does not burn clean and therefor form carbon-build up.

2.3 Leaking head gasket – If the gasket leaks, oil can seep through into the cylinders that causes carbon build-up because the ignition mixture is oil rich and therefore the oil is not burning away.

Leaking head gasket                                                    Worn out cylinders & pistons