First of all by "most production motorcycles" I am assuming commuter level motorcycles used in South and South East Asia
its not true that all production motorcycles have intakes facing backwards, its more or less a ergonomics based design decision , some motorcycles have intakes facing 90 degrees to the side. Some have them facing towards the back as you say.. For example the intake duct location on the TVS Apache Series.
A common point in all of these mass production motorcycles is that they use carburettors
Your point on Ram air effect on carburettor is correct it will cancel out the venturi effect but reingnieering the carb to make use of the Ram air effect only works if there is a specific way air can be channeled to the intake via a duct or port which is not the case In case of a commuter bike..
- secondly Ram air effect will work only when you achieve a specific speed , most production motorcycles are not designed to go at high speeds to use the advantages provided the Ram air effect .
Update: There is literally no point in making a intake face in the forward direction when the engine is at the back of the intake , you will need additional space to bend the intake tube which will create ergonomic issues and frankly there is no need to do the exercise , iterating my previous point,(please dont mind the bad drawing)
- You neeed a specilized channel or duct to force air to the intake which is not there in these commuter segment bikes which are mostly naked.
- These bikes do not have the necessary top speed to achieve the advantage of the RAM air intake effect. Most of these bikes have a top speed of 60MPH.
- Agreeing to the points mentioned by ALLman, having the intake facing in the backwards direction also prevents weather and road debris from entering the engine through the intake.