Tuning and jetting
- Sourced power figures, and the licence boundary.
- A 34 mm carb on a 122 is not a verified conversion.
- No jet chart is printed here.
Power, carefully
What was homologated, and what is still a rumour.
The useful tuning record is a dyno sheet with a date, not a jet size copied from a stranger. This page keeps the power figures the brief actually gives, and it refuses the rest.
Power the brief records
| Generation | Figure in the brief |
|---|---|
| Extrema / GS | 23 kW homologated (31.3 PS). No factory-restricted version, per Hartrusion. |
| Round-light | Open around 22 kW. MPA0 is 11 kW. MPA1 is 5 kW (80 km/h). UK bikes often sold at 15 bhp. |
| Bee-tail | MPB0 22 kW. MPB1 11 kW. SF open 21–21.5 kW. |
| RSV-style | PY0 21 kW. PYA 11 kW. |
| Euro 3 | RM 18 kW homologated, and the brief records a dispute that this may be the exhaust. RD 11 kW. |
| RS4 125 | Around 15 hp (11 kW). A1-legal from new. No full-power version. |
| Current RS 125 | 2021: 15.2 hp at 10,750 rpm. 2025: 15 hp (11 kW) at 10,500 rpm. Whether 15.2 hp sits inside the A1 cap is To verify. |
34 mm carburettor on a 122
The content plan lists “a 34 mm carb on a 122” as a subject people want. The 34 mm Dell'Orto VHSB is the Rotax 123’s carburettor. The 122 left the factory with a 28 mm PHBH, or a VHST on the Euro 3 bike. The brief does not describe the manifold, the reed block, the jetting or the cable geometry of a conversion. To verify the whole job before anyone calls it a bolt-on. This page will not invent that procedure. PJME is named as a supplier of 34 mm carburettors. Their price is not recorded.
Exhausts and big-bore kits
From 2003 the expansion chamber has a catalyst, and the Euro 3 bike has two. A non-catalyst pipe is the step in the derestriction notes, for a bike you are allowed to ride in that state. Brand names and part numbers are not in the brief.
PJME is also named for big-bore kits. Bore sizes, the piston they use, and any change to the transfer ports are To verify in that supplier’s own listing and then on a dyno. Nothing in the four-stroke generation is a big-bore two-stroke kit.
Jetting
One blog, via the brief, mentions a main jet moving up from 115 on a 28 mm carburettor, and a colder NGK BR10EG in place of a BR8EG, after the restriction comes off. That is one account.To verify against the manual, then keep your own baseline: pilot, needle clip, main, screw turns, plug, temperature, and what exhaust was fitted. The carburettor strip is guide 6.
A dyno log with nothing in it yet
| Date | Bike and engine | Exhaust and jetting | Result | Where |
|---|---|---|---|---|
| — | — | — | Not recorded | — |
Sources
- Modellübersicht RS 125 2T (opens in a new tab) — Hartrusion
- Differences between Rotax 122 and 123 (opens in a new tab) — Hartrusion
- RS 125 (1995–2012) review and buying guide (opens in a new tab) — MCN
- Derestriction guide (opens in a new tab) — The RS 125 blog
- Aprilia RS125 parts and engine ID (opens in a new tab) — PJME
- Bike categories, ages and licence requirements (opens in a new tab) — GOV.UK
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.