Vapour lock

Vapour lock happens when liquid fuel turns to vapour in the fuel line, pump or carburettor because of heat. Bubbles of vapour block the flow of liquid fuel, so the engine starves: it may splutter, lose power, stall or refuse to restart until it cools down.

It is most common on carburetted bikes in hot weather, in slow traffic or after a short stop with a hot engine, especially where fuel lines run close to the exhaust or cylinder. Modern fuel-injected systems keep fuel under pressure, which makes vapour lock much rarer.

Remedies: route fuel lines away from heat, fit heat shields or insulating sleeves, keep the cooling system working and avoid letting the bike idle for long periods in the heat. Some summer fuel blends evaporate more readily.