125cc Air filter

This is the section of foam/paper in your air box. It keeps debris and moisture out of the engine. But in doing so, it also restricts the airflow.

K&N and Pipercross filters are popular, as they still provide the much-needed protection to the engine, but they restrict the airflow less than the standard filter. A common mod is to remove the snorkel from the air box, as this helps the flow of air into the engine. Also, drilling holes in the top and sides of the box (ensuring the holes are above the filter of course) can help airflow and increase power. Do not be tempted to run your bike without the filter. Although some will argue it will not harm the engine, it will. It opens it up to dust, moisture and a host of other crap to get into your engine, and this will decrease reliability.

K&N and pipercross are the top filters in the industry.