The stock mufflers and stock ventilation systems used with most GY6 engines restirct air flow, and therefore result in significantly less power than the motor is capable of producing.  You can easily add performance by removing the stock air box and adding this UNI Filter and velocity stack (intake).

Please note that the carburetor must be fine-tuned using the appropriate jets to make sure the air/fuel mixture is balanced.  
We provide a 128mm Main Jet with this upgrade.

The velocity stack (intake) will only attach to a stock 24mm carburetor on a GY6 engine OR any carburetor that has a 1.5" diameter opening on the air intake side. 
(Note: This velocity stack uses a 2" opening UNI filter.) 

When adding this modification, you will need to rejet your carburetor to re-balance the fuel/air mixture due to the increased airflow. What size jet you need to upgrade to will depend on the local elevation and the season (outside temperature) that you will be riding in.  We provide a size 128 main jet.

***This dual-layer UNI air filter MUST be oiled with foam filter oil and cleaned regularly. A properly oiled filter prevents dust particles and debris from getting into the carburetor and your engine. This is essential for proper operation and engine longevity. You WILL eventually destroy your engine if this filter is not oiled with the proper product. This kit will mount as-is to most 125cc and 150cc GY6-based engines***