[How to Tutorial] [Bike Mod] 650/250/125 – AiS Block Off Removal (EGR Delete)
Hyosung AIS Removal Guide: EGR Delete for GT650, GT250 & GV125
If you are looking to improve the exhaust note and throttle response of your Hyosung GT650R, GT250, or GV125 Aquila, performing an AIS (Air Injection System) block-off—often referred to by riders as an EGR Delete—is one of the most effective “free” mods available. This process is common among race teams and enthusiasts looking to strip away parasitic emissions equipment.
What is the AIS (Air Injection System)?
The AIS is designed to help motorcycles meet strict Euro emissions standards by injecting fresh air into the exhaust port. This helps burn off unburnt fuel in the header pipes. While great for the environment on paper, it often causes excessive “popping” on deceleration and adds unnecessary clutter to the engine bay. It essentially “fakes” cleaner emissions by diluting exhaust gases, acting like a parasite that steals potential engine efficiency.
Benefits of the AIS Block-Off Mod
- Reduced Exhaust Popping: Eliminates the loud, annoying backfiring often heard with aftermarket stainless steel exhausts.
- Improved Induction Roar: Gives the bike a deeper, more natural groan at idle and under load.
- Smoother Mid-Range: Many riders report a noticeable improvement in the 4,000–7,000 RPM range, particularly on smaller 125cc and 250cc models.
- Simplified Maintenance: Clears up space around the cylinder heads, making spark plug changes and valve clearances easier.
Whether you are running a stock setup or a full custom exhaust system, removing the AIS is the first step toward a true performance tune. Full credit goes to the original community author who compiled this technical manual.
Ready to start the mod? Download the step-by-step technical book below:
Community Note: This mod is highly recommended if you have recently upgraded to a Delkevic or similar aftermarket silencer.