The ten checks

1. Read the frame number or VIN before you believe the year in the advert. Use the decoder, then look at the plate yourself.
2. Find the engine casting. A 1998 bike can be a Rotax 122 or a Rotax 123. Cylinders, gearbox, clutch, gaskets and ignition do not swap.
3. Read the carburettor. The research brief puts a VHSB 34 with a 123 and a PHBH 28 with a 122. Haynes 4298 lists a PHBH28BD for both the 11 kW and the 20 kW RS125, and gives no jet stack for a 34 mm body. Read the stamp. The jet sizes are on the carb specs page.
4. Ask whether it is restricted, and look for a powervalve blanking plate rather than a RAVE blade. UK bikes were often sold at 15 bhp. That is not the same sentence as the current 11 kW A1 rule. Both are on the UK law page.
5. On a 2003-on bike, assume a catalyst in the expansion chamber until you know otherwise.
6. On a two-stroke, see whether the powervalve moves and whether the solenoid is seized. That fault is named on the bee-tail.
7. Look for exhaust rot, a worn speedo drive on a pre-2006 bike, crank rumble, and — on an early 123 — a tired starter freewheel. While you are there: pad friction material under 1.5 mm, and chain slack outside 25–35 mm, are workshop limits from Haynes 4298.
8. Clocks. The mechanical tacho drive on the Extrema is no longer available. Old digital clocks fail.
9. Paperwork. A UK V5C, an unfinished Italian import, or a frame number that does not match the document. A “replica” is a decal scheme, not a different engine.
10. Write the asking price in the log below, with the date. The parts directory has no prices on purpose. A scarce 123 is not the same purchase as a 122 with a rotten pipe.

Workshop figures while you look

These are from Haynes manual 4298. They are not a valuation, and they are not the MOT pass limits. Double-check them on the bike. The full lists are on specs.

• Pads: 1.5 mm of friction material minimum, front and rear. Disc thickness service limit 4.5 mm.
• Chain: size 520, slack 25–35 mm.
• Tyres, 1993–1998: front 110/70-17 (ZR), rear 150/60-17 (ZR). From 1999 the manual lists more than one size. The swingarm label decides.
• Fork oil, if a leg is weeping: SAE 10W to 20W, 430 cc a leg. The RS125 oil level is not published.

Questions for the seller

• Can you photograph the frame plate and the engine casting, in focus?
• Has the crank been apart, and were the seals replaced?
• Is the powervalve a blade or a plate, and has the CDI been opened?
• For a 2006 RSV-style bike: does the immobiliser behave, and do you have both keys?
• For a replica: which year’s decal set is on it, and do you have photos of the bike before it was stickered?

Red flags by generation

Prices, in pounds, without a valuation

Asking prices move with the engine, whether the numbers match, whether the pipe is solid, whether the restriction is honest, and whether the V5C matches the frame. None of those has a pound figure in the brief. When you see a bike, add a row. A forum “worth about” is not a valuation.

Decal kits are the one price-ish figure in the brief: Decalmoto at roughly €95–€140. That is euros, it is “roughly”, and it is not what the motorcycle costs.