Do this page in order. The law is the first section because the rest of it is a description of how these bikes were restricted, not an invitation to ride an opened two-stroke on L-plates.

The licence, before the spanners

GOV.UK, as recorded in the brief, sets the A1 category at up to 125 cc, 11 kW and 0.1 kW/kg, from 17. After a CBT, on a provisional licence, you ride with L-plates, you do not carry a passenger, and you do not use a motorway.

A full-power two-stroke in the brief is around 21–23 kW. That needs an A2 or a full A licence. MCN’s wording, also in the brief, is that anything above the restricted 15 bhp is illegal on a provisional or an A1 licence. This site keeps both sentences. It does not convert 15 bhp into 11 kW and declare them the same restriction. If you need the current rule for your own licence, read GOV.UK on the day. The page is linked in the sources.

Tell the insurer if the bike is no longer as restricted as the policy assumes. That is the brief’s advice. The exact wording each insurer wants is To verify with them. “They will not notice” is not a note from the brief.

The RS4 and the current RS 125 are around 15 hp and are described as A1-legal from new, with no full-power version. The 2021 figure of 15.2 hp is discussed on the four-stroke generation page. None of the hardware below exists on those engines.

What the restriction actually is

The brief’s steps, which match the parts named in the generation tables, are:

1. Powervalve. A restricted bike may have a blanking plate instead of the RAVE blade. The open arrangement in the derestriction steps is the blade, taper down and flat side up, plus the solenoid by the battery, the cable, and the RAVE unit. The RAVE guide has the cautions. Which codes left the factory with a plate is To verify on the bike. MPA0, MPA1, MPB1, PYA and RD are the restricted codes the brief does name, and the UK 15 bhp bikes are a further group.
2. CDI. Nippondenso units have an environmental retard around 5–6,000 rpm. The brief says it can be removed and that the detail varies by unit. Verify it for the unit you have. This page does not include a wire colour or a solder map. The Euro 3 bike does not use that CDI. Its ignition is the EFI Technology ECU, and the brief does not describe a “bridge” for it.
3. Exhaust. From 2003 there is a catalyst in the expansion chamber. The Euro 3 bike has two. The brief’s step is to fit a non-catalyst pipe if you are building an open bike you are allowed to ride. Brand names and part numbers are not recorded.
4. Jetting and plug. The same steps say to re-jet — the example given is a main jet up from 115 on a 28 mm carburettor — and to fit a colder BR10EG instead of a BR8EG. That is one blog, preserved so it is not lost and so it is not mistaken for a manual. To verify both figures. Change the fuelling when you change the pipe. A catalyst-delete on the old jets is how these engines seize.

Write down what you changed

Keep a line in the condition report: date, what was fitted, what the plate or the blade was before, and whether the insurer was told. The next owner, and your future self at MOT time, need that more than they need a claimed horsepower. The RM’s 18 kW homologation is disputed in the brief and may describe the exhaust. Do not write “18 kW at the wheel” on a receipt unless you have a dyno sheet. There is a blank dyno log on the tuning page.