|When the motorcycle engine is cold, oil viscosity is high (the oil is "thick") and this results in high internal friction in the engine. And the engine does generally require a richer mixture to run safely before it reaches it's operating temperature. |
So during the warm up phase, the engine will need a slightly richer air/fuel ratio and some motorcycle manufactorers also set the idle level a bit higher to prevent the engine from stalling.
Back when motorcycles had carburettors, the fuel enrichment device was known as the choke, and even though electronic fuel injection systems work in a different way, the term "Choke" remains.
History of how to handle the choke function during the warm up phase.
The consept of mixing air and fuel have changed considerably over the past 20 years, and this makes it necessary to split the explanation about the engine warm up in three different parts.
Most carburettors have a user operated cold start lever (The Choke) to make the air/fuel ratio richer when the engine is cold and during warm up. And this lever will often raise the idle level too depending on how much you pull,twist, or turn the lever.
The choke can make the engine run extremely rich, and it's important to turn off or turn down the choke function as soon as possible. Gasoline is an excellent degreaser, and an extremely rich mixture can wash away the oil film between the cylinder wall and the piston - which will make the engine seize.
Early fuel injected engines.
The fuel injection ECU read the oil temperature sensor or the water temperature sensor to detect a cold engine and make the air/fuel ratio slightly richer until operating temperature is reached, so there is no user input required for this part.
But they may also have a lever to raise the idle level slightly to ensure the engine will not stall. This lever is often referred to as a Choke, but it not change the air/fuel ratio - it just opens the throttle valves a bit to increase idle level during warmup.
Modern fuel injected bikes.
As on the first fuel injection systems, the air/fuel ratio during warm up is determined by the ECU, but the modern fuel injection will also control the idle level, so the user does no longer have to focus on this. There is no longer a choke lever, and the rider just have to start the engine and he is ready to go.