Power the brief records

Haynes 4298 labels its unrestricted carb column 20 kW. The figures in the table above are the research brief’s homologation numbers, about 21–23 kW and different by generation. They are not edited down to 20 kW. 15 bhp, where the brief uses it for UK bikes, is a third sentence.

Stock carb, and the 34 mm upgrade

Stock, in Haynes 4298, is a Dell'Orto PHBH28BD for both the 11 kW column and the 20 kW column. The research brief still pairs a VHSB 34 with the Rotax 123 and a PHBH 28 with the Rotax 122. Those are identification records. They are not a jet stack for a different body.

The upgrade people fit is a 34 mm carb on a Type 122 that left with the 28 mm carb. PJME sells that Dell'Orto for 1993–2005 and says the standard inlet manifold will not take it. Hartrusion says the same change is widely made, usually to the VHSB 34 LD, and that it is harder to jet at low rpm. Published 34 mm stacks do not agree. None of them is typed in here as the setting. The parts, the conflict, and the seizure warning are on mods: carburettor.

Exhausts and big-bore kits

From 2003 the expansion chamber has a catalyst. 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.

Jetting

Haynes 4298 lists a Dell'Orto PHBH28BD for the 11 kW restricted RS125 and again for the 20 kW unrestricted RS125. It does not give a jet stack for the 34 mm Dell'Orto VHSB that the research brief records on the Rotax 123. Do not put these jets in a 34 mm body. Match the stamp on the carburettor. Pilot-screw turns, float height and needle-clip position areNot published. Idle for both columns is 1,150–1,350 rpm. Throttle and choke free play are 2–3 mm.

Spark plug in the same manual: NGK BR10EG, electrode gap 0.5–0.7 mm, torque 20 Nm. A derestriction blog in the brief says to leave a BR8EG for a colder BR10EG. The manual extract does not list a BR8EG. Both stay. Keep your own baseline as well: screw turns, plug colour, temperature, and what exhaust was fitted. The carburettor strip is guide 6. The same table, with the conflict written out, is on carb specs.

A dyno log with nothing in it yet