[How to Tutorial] [Bike Mod] 650/250/125 – AiS Block Off Removal (EGR Delete)
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Hyosung AIS Removal Guide: EGR Delete for GT650, GT250 & GV125
⚠ WARNING: This technical guide involves modifying the Air Injection System (AIS). Proceed at your own risk. Incorrect installation can affect engine longevity or emissions compliance. Ensure you have the correct tools before starting.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.
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This works on gt125r 08?
Yes remove the device and block the pipes going to it.
Only 2 things happen = The pops disappear and bike roars a little deeper & the engine/vacuum system has more energy (better throttle responses)
The device is just for emissions – engine doesn’t need it to run lol. If you see a GV250- 2002 it had none 😉
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Where is the ais on a 2006 gt125r??
It is at the rear of the bike next to the FUEL PUMP.
Look for a silver ball with 2 giant pipes.
1 going to rear engine metal pipe = block it.
1 going to UNDERSIDE of airbox = block it
There is also 1 extra mini pipe (about size of 4mm hole) going to intake manifolds = block that.
Now you can remove that fake emissions thing (egr) bike wont miss it!
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Some ECU’s might moan , as the fresh air pumped from the airbox might affect the 02 sensor readings back to the ECU. So i would caution this.
With that being said, I did have a customer 2011 Delphi bike with 2x 02 sensors (i know 2011 and a delphi?) the engine had no AIS system whatsoever not even a plug for it, it was as if Korea saw no need for this particular bike not to have it , then i had a 2008 Sentec ECU bike that had 1 AIS system feeding the rear exhaust where the lambda sensor was.
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Hey everyone, I recently removed the AIS system on my 2005 Hyosung GT650 (carb model), and I’m getting a loud hissing noise after cranking — the bike won’t start.
I’ve attached a photo of my airbox (upside down). Can anyone help confirm:
Which holes on the airbox need to be blocked after AIS removal?
I want to make sure I didn’t block or leave open the wrong one.Do my vacuum/fuel hoses look correctly routed to start the bike?
I suspect I may have missed reconnecting something critical like the vacuum for the petcock.Any feedback would be greatly appreciated. I’m trying to get this running again after the delete. Thanks in advance!
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Yes you can block those 4 holes but if the carb itself has a silver port expecting vacuum from air box, connect it back.
Also I am concerned the fuel filter is facing the wrong way (restricting the flow-speed potentially)
Also make sure the Engine Side is blank, so there is no hissing from the engine cylinders too.
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Thanks for your reply!! The manual in pdf was mentioning that only one hole shouldve be blocked and was unsure. I will try it tomorrow with two holes blocked and coming from engine side I will connect them as well. All the others are disconnected and blocked with blanking kit and fuel filter changrd!
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