Air-Fuel Mixture Distribution

The intake manifold of your engine contains the air-fuel mixture. If you give a close look to it, you will see that it would be though enough for this intake manifold to distribute the air-fuel mixture throughout cylinders. Theoretically, your engine cylinders should receive the same amount of air-fuel mixture with same richness for proper functioning. But, practically the mixture of end line cylinders is comparatively richer. 

Just take a look at the air flow passages of your engine’s intake manifold; you will see that all passages are different from each other in both shape and size. You also have to understand that liquid fuel vaporizes completely, when they enter the engine cylinder. If your intake manifold fails to deliver complete vaporization, it will contain heavy droplets. That is why, the air-fuel mixture. Chemical property of these droplets is a bit different, and they travel hazardously before hitting the end of the intake manifold. These droplets are collected in a puddle of fuel at the end of manifold. Some of these puddle fuels are collected by the air-fuel mixture, as they becomes evaporated. 

Comparatively, center cylinders of your engine will receive leaner mixture than end cylinders. This distribution becomes finer and smoother, when the intake manifold becomes heated up. After a few strokes, when the engine becomes heated up, the intake manifold of your engine also becomes heated up and it aids to vaporize fuel droplets.