Honda Grom Larger Throttle Body & Racing Exhaust Install

Welcome to the second part of our project to enhance our 2018 Honda Grom. This step-by-step guide and video covers how to install a larger KOSO throttle body that ups the size from the stock 30mm to a much better 34mm. 

Also included are steps on how to fit a high flow fuel injector, a KOSO high-flow air filter, and a Yoshimura sport exhaust system. Watch the video above or read on below to see how we upgraded a 2018 Honda Grom 125 with a new aftermarket throttle, exhaust and air filter.

See the parts diagrams:

• Honda Grom throttle body
• Honda Grom air cleaner
• Honda Grom exhaust mount

Tools and Parts – Honda Grom Exhaust &Throttle Body Installation

• Intake tube
• Connecting tube
• Silicone gasket
• Ratchet and sockets
• Torx T25
• Dremel tool
• Foam air filter oil

• KOSO Hurricane air filter
• KOSO Grom throttle body
• KOSO intake manifold OR the intake from the KOSO big bore kit
• KOSO high flow injector
• KOSO injector adaptor kit
• Yoshimura exhaust system

Part 1: Assembling and Installing Honda Grom Throttle Assembly

To mount the throttle body to the cylinder head, you’ll need either a KOSO intake manifold or the larger intake manifold included in the KOSO big bore bit with 4V cylinder head. Having already installed the big bore kit, we’ll use the larger intake manifold.

Step 1: Remove the existing throttle assembly from the Grom. 

Watch the clip below to see how to remove the throttle assembly.

Step 2: Remove the throttle position sensor from the old throttle assembly and install it on the KOSO throttle body.

Step 3: Remove the idle fuel mixture screw from the old throttle assembly and install it on the KOSO throttle body. Tighten the screw into the throttle body until it bottoms out, then back it off by two complete turns.

Step 4: Remove the idle air control valve from the old throttle assembly and install it on the KOSO throttle body.

Watch the clip below to see the throttle body assembly segment of our Honda Grom Intake and Exhaust video.

Step 5: Attach the throttle body to the intake manifold.

Step 6: Install the injector adaptor kit and the high flow injector to the intake manifold.

Step 7: Install the throttle assembly onto the cylinder head. 

NOTE: Don’t forget to include the bracket that holds the airbox as you install the throttle assembly mounting bolts.

Step 8: Attach the electrical connectors, fuel lines and hoses to the throttle body.

Watch the clip below to see the throttle assembly installation segment of our Honda Grom Intake and Exhaust video.

Part 2: Assembling and Installing Honda Grom Yoshimura Exhaust System

The Yoshimura exhaust system is made up of three sections: the header pipe, mid-pipe and muffler assembly. If you installed the KOSO big bore kit, you’ll need to install the two exhaust manifold studs included in the kit into the new cylinder head before you can install the Yoshimura exhaust system.

Step 1: Connect the header pipe to the mid-pipe, and install them along with the spacer to the mounting point on the underside of the Grom’s frame. Leave the bolt loose so you can adjust the exhaust’s position.

Step 2: Make sure the exhaust gasket is in the cylinder head, and attach the header pipe to the manifold studs. Once again, leave the header caps loose.

Step 3: Install the spring between the header pipe and mid-pipe, then install the muffler assembly to the mid-pipe, and attach it to the mounting point on the Grom’s frame. Leave the bolt loose.

Step 4: Tighten the header caps, followed by the bolt in the underside mounting point and the bolt in the muffler assembly mounting point.

Step 5: Install the spring between the mid-pipe and the muffler assembly, then clean the exhaust system with contact/brake cleaner. 

NOTE: If you leave any dirt or fingermarks on the new exhaust, they’ll burn into the metal when the exhaust first heats up as the Grom is running.

Watch the clip below to see the exhaust installation segment of our Honda Grom Intake and Exhaust video.

Part 3: Assembling and Installing Honda Grom Airbox

The larger throttle body requires a larger intake tube and connecting tube, which will require the Grom’s airbox to be modified slightly. 

PRO TIP: Take photos of the orientation of the existing intake and connecting tubes so you can install the new ones in the same way.

Step 1: Remove the airbox lid and air filter, followed by the air temperature sensor from the airbox to prevent it from getting damaged.

Step 2: Remove the tube within the airbox with a Dremel tool (or something similar) to grind away the plastic joints holding the tube’s bracket.

Step 3: Remove the intake tube and connecting tube by prying them off the airbox.

Step 4: Use a similarly-sized socket or other circular object to mark out the larger holes around the existing airbox holes. Cut out the new holes using a Dremel tool (or something similar), then clean out all the plastic shavings from the airbox. The hole in the airbox for the new connecting tube must be 40.5mm, and the hole for the new intake tube must be 50mm.

Step 5: Install the new intake tube and connecting tube, and hold the airbox assembly against the Grom to ensure the tubes are in the correct position.

Step 6: Join the new intake and connecting tubes to the airbox with silicone gasket maker. 

NOTE: Make sure there are no gaps in the seal to prevent airborne contaminants from passing through.

Watch the clip below to see the airbox modification segment of our Honda Grom Intake and Exhaust video.

Step 7: Spray the new air filter’s foam element with foam air filter oil, and install the new air filter into the airbox.

Step 8: Reinstall the air temperature sensor and the airbox cover.

Step 9: Reinstall the airbox to the Grom.

Watch the clip below to see the airbox installation segment of our Honda Grom Intake and Exhaust video.

Part 4: Installing Honda Grom Throttle Cables

Step 1: Put the return side of the throttle cable into the upper slot in the bracket on the throttle body and tighten it all the way down. Next, put the pull side of the throttle cable in the lower slot in the bracket, and adjust it so there is a little bit of slack before the cable begins to pull. 

Step 2: Reinstall the throttle cable cover, and reconnect the crankcase vent hose to the rear of the airbox.

Watch the clip below to see the throttle cable connection segment of our Honda Grom Intake and Exhaust video.

With the KOSO throttle body and Yoshimura exhaust installed on our Honda Grom, the difference in performance will be impressive. Next we’re going to install a Yoshimura fender eliminator.

Watch the video above to see how to install a fender eliminator on a Honda Grom.