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 three carburettors the brief names, how to read a plug, and which figures are deliberately missing.

The four-strokes are fuel injected. They do not have a jet stack. Stop here if that is your bike.

Which carburettor

The Rotax 123 wears a Dell’Orto VHSB 34. The Rotax 122 wears a Dell’Orto PHBH 28, until the Euro 3 generation, which wears a Dell’Orto VHST 28 with a throttle-position sensor and a heater. Putting the 34 mm body on a 122 is a tuning topic people discuss. It is not a verified conversion in the brief, and this page does not describe one. See tuning for that boundary.

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. On a VHST, do not twist the TPS to “see what it does”. Its position is part of the ECU’s map. The set-up procedure is To verify.

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

The brief’s derestriction steps, taken from one blog, say to re-jet, for example a main jet up from 115 on a 28 mm carburettor, and to fit a colder BR10EG instead of a BR8EG. Both are recorded so you can see what that guide claimed. Neither is printed here as the factory baseline. To verify every jet and the plug grade in the workshop manual for your engine, then write your own baseline on the inside of the seat hump: pilot, clip, main, screw turns, plug, and the day’s temperature.

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.

The gap for either NGK is To verify. Do not assume the two-stroke plugs fit the four-stroke.