Question: Almost every motorcycle I have seen has its air intake mounted such that it is sucking in air from the direction opposing travel.

According to fluid dynamics, if the intake is made to face forwards, the engine would benefit from [ram-air effect](https://en.wikipedia.org/wiki/Ram-air_intake), which can result in higher engine power output.

Now the Wikipedia article does say:

> 
> While ram-air may increase the volumetric efficiency of an engine,
>  they can be difficult to combine with carburetors, which rely on a
>  venturi-engineered pressure drop to draw fuel through the main jet. As
>  the pressurised ram-air may kill this venturi effect, the carburetor
>  will need to be designed to take this into account; or the engine may
>  need fuel-injection.
> 
> 
> 

Surely a carburetor can be reengineered to suit the change in pressure profile across the intake? Plus, even motorcycles with fuel injection have their intakes point backwards.

Answer: 
You guys are looking at it from the wrong angle...

It’s not about the carb or the ram sur effect, it is all about the exhaust.

The exhaust side of the cylinder is the hottest side so it needs to be facing forward so it’s gets a bit of cooling.

Plus if you’d put the exhaust facing backward it could get too warm where the cables and electronic are, under the seat.
In my opinion while designing the vehicle, designers consider the density of air coming into the air box. Because of the vihicle design air boxes are fixed behind the engine. So the air in front of the air box will be coming through the engine cooling fins. Which make it hot and reduces the density. Inordar to get cold and fresh air designers will never provide the air inlet mouth directed towards the engine.