Jetting is where copied numbers do the most damage. A main jet that is slightly wrong washes the bore or seizes it, and the number depends on the carburettor, the pipe, the weather and whether the powervalve is actually opening. This guide will show you how to strip the carburettors these bikes use, how to read a plug, and which figures are deliberately missing.

Which carburettor

The Rotax 123 wears a Dell’Orto VHSB 34. The Rotax 122 wears a Dell’Orto PHBH 28. Haynes lists a PHBH28BD for both power columns, and that remains the stock jet chart. Putting a 34 mm body on a 122 that left with the 28 mm carb is a modification, not a third stock specification. The manifold, the disagreeing jet stacks and the seizure warning are on the carburettor mods page. See tuning for the same split in short form.

Photograph the stamp on the body before you buy “an RS125 carb”.

Strip without losing the order

1. Count the mixture screw in, gently, until it seats. Write the turns down. Do not reef it. The seat is soft. The standard setting you return to is To verify if your count is lost.
2. Drain the bowl into a jar. Note any water or varnish.
3. Photograph the needle clip position before the slide comes out. Clip grooves are numbered from one end. The manual says which end. To verify.
4. Lay the pilot, the main, the needle jet and the atomiser in the order they came out. The names differ slightly by Dell’Orto model. Use the exploded view in the manual, not a drawing of a different VHSB.
5. Check the float for fuel inside it. A sunken float runs rich. The height is To verify.
6. Clean the passages and prove them with a short blast of air, with the diagram in front of you so you know which hole should be open.
7. Rebuild from the photographs, in the order the jets came out.

Rebuild with the photographs. A needle clip one groove out is a mid-range change you will not see at idle.

What is not a jetting chart

Haynes 4298 lists a PHBH28BD main jet of 115 on the 11 kW column and 120 on the 20 kW column, with pilot jet 62 in both. The full stack is on the carb specs page. A derestriction blog in the brief says a main jet moving up from 115, which lines up with those two mains and is not a licence to invent a third. Write your own baseline on the inside of the seat hump: pilot, clip, main, screw turns, plug, and the day’s temperature. Pilot-screw turns are Not published.

A 115 main in a different body, or on a bike that still has its catalyst, is not the same engine that blog was describing.

Reading a plug

Use a plug you can still see the colour on. Chop the ignition just after a loaded run in top gear if you want a main-jet reading, and do it somewhere you can stop safely. A plug from ten minutes of urban riding is a pilot-and-needle story, not a main-jet story. The colours people argue about are not standardised in the brief. Treat a wet, black, sooty plug as rich or oil-fouled, and a blistered white insulator as lean or too hot, and then change one thing. Oil from the pump also blackens a plug. Check the oil system in guide 8 before you lean the main jet out of a bike that is simply over-oiling.

Haynes 4298 specifies an NGK BR10EG with a gap of 0.5–0.7 mm. The derestriction blog’s BR8EG is not in that extract. Both accounts stay: the manual’s plug is the BR10EG, and the blog is one person’s step, written up on guide 12.