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What a sourced workshop page actually says about RS125 expansion chambers, approved systems, and the jetting that has to follow.
Before you start
Your own risk. Wrong jetting or timing can seize the engine. This is information, not advice.
How many steps: 3
Step 1 of 3
The chamber sets the power. A silencer swap does not.
Hartrusion treats the expansion chamber as the part that sets the engine’s character, and the silencer as mostly a noise and back-pressure part. He says swapping only the silencer does not make a noticeable power difference. A thinner aftermarket chamber also weighs less than the double-wall standard pipes he describes, and it changes the sound. He names makers that have built systems for this bike: Arrow, Athena, DLR, Figaroli, Giannelli, Jim Lomas, Jolly Moto, MIVV, Scalvini, Scario, Tecnigas, TurboKit and Tyga. A name on that list is not a test result.
Step 2 of 3
Arrow and Giannelli made noise-approved systems.
Licence, MOT, insurance or a seizure.
Hartrusion says Arrow and Giannelli sold complete systems with an EC type-approval for some models, that the Giannelli bodies match the Arrow bodies and share the approval mark, and that those systems stopped being made in 2022. He reads the mark as a noise check under chapter 9 of directive 97/24/EC, not as an emissions approval. The same page says the systems come with a long flange and a short flange. The long flange is for a standard engine. The short flange is for a tuned engine, and it moves the resonance higher because the path is shorter. A UK MOT noise limit in decibels is not printed on this site. It is To verify in the current inspection manual.
Step 3 of 3
No stock-bike gain is published. Jetting must change.
Licence, MOT, insurance or a seizure.
Hartrusion estimates about 2 to 5 PS from fitting a complete aftermarket system. That is his estimate. This site does not hold the dyno sheet, so it is not a promise for a particular bike. He describes one comparison of an Arrow system on the short flange against an MP01 header. The engine in that test was not standard: a shortened and rotated intake sleeve, a VHM combustion chamber he quotes at 13.7:1, the 28 mm carb still fitted, and the ignition wedge not fitted. He says the curve moved up the rev range. He also says fitting the wedge in that test made the engine knock, and he does not give the wedge angle. A gain for a standard engine is To verify. The wedge angle is To verify. The same page says a pipe change usually needs the carburettor set up again: a larger main if the peak load is higher, and a slide, needle or clip change if the engine then runs rich lower down. Which jets, for which pipe, are To verify on that bike. From 2003 the expansion chamber on these bikes can contain a catalyst. Removing it, or fitting a pipe that has none, changes emissions. The derestriction notes are the place that fact already lives. This page does not add a cutting method.
This guide is done. Your place stays on this device.
Hartrusion treats the expansion chamber as the part that sets the engine’s character, and the silencer as mostly a noise and back-pressure part. He says swapping only the silencer does not make a noticeable power difference. A thinner aftermarket chamber also weighs less than the double-wall standard pipes he describes, and it changes the sound.
He names makers that have built systems for this bike: Arrow, Athena, DLR, Figaroli, Giannelli, Jim Lomas, Jolly Moto, MIVV, Scalvini, Scario, Tecnigas, TurboKit and Tyga. A name on that list is not a test result.
Hartrusion says Arrow and Giannelli sold complete systems with an EC type-approval for some models, that the Giannelli bodies match the Arrow bodies and share the approval mark, and that those systems stopped being made in 2022. He reads the mark as a noise check under chapter 9 of directive 97/24/EC, not as an emissions approval.
The same page says the systems come with a long flange and a short flange. The long flange is for a standard engine. The short flange is for a tuned engine, and it moves the resonance higher because the path is shorter.
A UK MOT noise limit in decibels is not printed on this site. It is To verify in the current inspection manual.
Legal and safety. A louder pipe, or a pipe with no approval mark, is a road-legality and insurance question in the UK as well as a tuning one. This page does not describe how to cut a silencer open.
Hartrusion estimates about 2 to 5 PS from fitting a complete aftermarket system. That is his estimate. This site does not hold the dyno sheet, so it is not a promise for a particular bike.
He describes one comparison of an Arrow system on the short flange against an MP01 header. The engine in that test was not standard: a shortened and rotated intake sleeve, a VHM combustion chamber he quotes at 13.7:1, the 28 mm carb still fitted, and the ignition wedge not fitted. He says the curve moved up the rev range. He also says fitting the wedge in that test made the engine knock, and he does not give the wedge angle. A gain for a standard engine is To verify. The wedge angle is To verify.
The same page says a pipe change usually needs the carburettor set up again: a larger main if the peak load is higher, and a slide, needle or clip change if the engine then runs rich lower down. Which jets, for which pipe, are To verify on that bike.
From 2003 the expansion chamber on these bikes can contain a catalyst. Removing it, or fitting a pipe that has none, changes emissions. The derestriction notes are the place that fact already lives. This page does not add a cutting method.
Legal and safety. Removing a catalyst and riding the bike is an emissions, MOT and insurance question. Do not assume a tester or an insurer will treat it as invisible.
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