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 hardware below is for the Rotax two-strokes in this record. A full-power bike in the brief is around 21–23 kW.

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 and PYA are restricted codes the brief names inside these years, 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.
3. Exhaust. From 2003 there is a catalyst in the expansion chamber. 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. Haynes 4298 lists a Dell’Orto PHBH28BD for both columns: main jet 115 at 11 kW and 120 at 20 kW, pilot jet 62, starter jet 70, needle BN 264 restricted and BN 266 unrestricted. The plug in that manual is an NGK BR10EG, gap 0.5–0.7 mm, torque 20 Nm. A derestriction blog in the brief says the main moves up from 115 and that you leave a BR8EG for a colder BR10EG. The two mains line up. The manual extract does not list a BR8EG. Both stay. Do not put the PHBH jets in a 34 mm carburettor. The full stack is on the carb specs page. Change the fuelling when you change the pipe. A catalyst-delete on the old jets is how these engines seize. The manual’s open column is labelled 20 kW. Homologation figures already on this site stay about 21–23 kW.

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. Do not write a wheel figure on a receipt unless you have a dyno sheet. There is a blank dyno log on the tuning page.