One idea at a time in focus mode
Stock PHBH28BD jetting stays the Haynes specification. The 34 mm carb people fit to a Type 122 is a separate, sourced upgrade.
Before you start
Your own risk. Wrong jetting or timing can seize the engine. This is information, not advice.
How many steps: 5
Step 1 of 5
Stock is the PHBH. A 34 mm carb is a mod.
Stock, as Haynes 4298 prints it: a Dell’Orto PHBH28BD for both the 11 kW column and the 20 kW column. Main jet 115 restricted and 120 unrestricted, pilot 62, starter 70, needle BN 264 restricted and BN 266 unrestricted. Those jets belong in that body. The research brief pairs a Dell’Orto VHSB 34 with the Rotax 123 and a PHBH 28 with the Rotax 122. Both records stay. Read the stamp and the casting. Popular upgrade, when the Type 122 left with the 28 mm carb: a 34 mm carb. PJME sells a Dell’Orto 34 mm for 1993–2005 and says the standard 28 mm inlet manifold will not take it. Hartrusion says the same change is widely made, usually to the VHSB 34 LD that earlier bikes left the factory with, and that a 34 mm manifold exists for the original reeds.
Step 2 of 5
The usual 34 mm choice is a Dell’Orto VHSB.
Hartrusion names the Dell’Orto VHSB 34 LD as the carb people change to, because the earlier machines used it and it has a cold-start circuit and an oil-pump connection. He also names Mikuni, Keihin and Polini as other makers, without a jet stack for them. One Italkit manifold listing says its carb connection is 40 mm and names Keihin PWK 32–36 mm and Mikuni TM(X) 32–35 mm as bodies that connection suits. That is a seller’s fitment note, not a jet chart. Hartrusion also names a Dell’Orto VHST 28 CD as a standard 28 mm carb from about 1998, beside the PHBH 28 BD. Haynes does not list the VHST. It is not the 34 mm upgrade. Hartrusion says bikes up to about 1997 with the Rotax 123, and a few with the Rotax 122, had the VHSB 34 as stock. On those bikes it is not an upgrade. He also says a rumour that the 34 mm carb meant 25 kW is wrong: the GS figure he gives is 23 kW, the first MP bikes that still had the 34 mm carb are 20 kW, and the later 28 mm bikes are 22 kW. The 23 kW GS figure matches the research brief. The 20 kW and 22 kW sentences are his.
Step 3 of 5
It needs a larger manifold, and jetting for that bike.
Licence, MOT, insurance or a seizure.
Two independent sellers and workshop pages agree that the 28 mm manifold does not suit the 34 mm Dell’Orto. PJME says you need a new inlet manifold if you are replacing the standard 28 mm carb. Hartrusion shows a 34 mm manifold with the VHSB 34 LD. The reed block is a separate choice. Hartrusion says a manifold exists for the original reeds, and that complete aftermarket blocks also exist. The Italkit W-Force listing supplies a 34 mm manifold and says the original manifold is 28 mm. Details are on the reed page. The RS 125 blog says the standard air filter still fits the 34 mm mouth. Hartrusion says the standard air intake has a smaller cross-section than a 34 mm carb, so opening that intake up is part of the same job. Those two sentences can both be true: the filter can clamp on, and the tract ahead of it is still the smaller hole. Hartrusion says that from 2006, where the fuel tap is vacuum-operated, a carb without a vacuum take-off needs one. He does not give a drilling size. That size is To verify.
Step 4 of 5
Published jet stacks do not agree. Tune the bike.
Licence, MOT, insurance or a seizure.
Do not put the PHBH jets in a 34 mm body. Haynes never gives that stack for a 34 mm carb. Hartrusion’s chart, citing a Sahner data sheet from August 2000, lists one VHSB 34 configuration for an RS 125 of 1995–97 at 23 kW: main 160, needle K57, atomiser DP 266, clip in the 2nd notch from the top, idle emulsion tube B36, choke jet 60, slide 40, float 9.0 g. That is a cited setup for those bikes. It is not a measured setup for a Type 122 with a different pipe. PJME says the 34 mm carb they sell for 1993–2005 is supplied with a 160 main, a pilot jet 40 and a K57 needle, and that this should suit most standard bikes. They also say a derestricted exhaust with no catalyst is how you get the best from it. Pilot 40 and emulsion tube B36 are not the same part. Which idle piece a given carb uses is Disputed/needs checking until you have it in your hand. Pedparts lists a VHSB 34 LD with main 160, needle K57, atomiser 266, slide 40, idle emulsion 40, choke 60, float valve 300 and a 9.0 g float, and calls it an OEM carb for Rotax 122 and 123. The OEM claim does not match Haynes, which lists a PHBH28BD for both power columns. The parts that match the Sahner column are the main, the needle, the atomiser and the slide. The idle piece still disagrees. Hartrusion’s own experience, not a manual: restricted bikes are often run with a smaller main, sometimes down to 145, and a 155 main can still be too rich on a derestricted engine, so 150 is sometimes the next try. He says you need a set of mains and patience. The RS 125 blog’s separate baseline is main 155, idle 36, choke 60, K57 in the top notch, atomiser 266, air screw 1.5 turns out, and a float height of 14 mm against a Dell’Orto racing figure of 11 mm it also quotes. That blog stack, including 14 mm, stays disputed. It is not the specification. Hartrusion’s stated default for the VHSB idle air screw is 1.5 turns out from fully in. That is his default for that carb, not a Haynes figure, and it is not a setting for the PHBH. Haynes does not publish pilot-screw turns, float height or a needle-clip position. Those three stay To verify for the PHBH.
Step 5 of 5
Read the plug on a proper run, not around town.
On the VHSB chart the clip position is counted from the top notch. The needle named for the RS 125 column is a K57. Other needles on that chart belong to other bikes or to a Polini kit, and they are not a starting point for a standard 125. Slide 40 is the slide in the 1995–97 column. Hartrusion says an Arrow and Giannelli sheet treats a 50 or 55 slide as the same idea as taking 3 to 4 mm off the stock slide. That cut is not repeated here as a job. A larger cutaway leans the early throttle. Getting it wrong leans the engine. The plug in Haynes is an NGK BR10EG, gap 0.5–0.7 mm, torque 20 Nm. A derestriction blog also names a BR8EG. The manual extract does not. Both stay. The carb guide already says a plug from town riding is a pilot story, not a main-jet reading. A colour chart for that reading is not in the Haynes extract used here. The colours are To verify.
This guide is done. Your place stays on this device.
Stock, as Haynes 4298 prints it: a Dell’Orto PHBH28BD for both the 11 kW column and the 20 kW column. Main jet 115 restricted and 120 unrestricted, pilot 62, starter 70, needle BN 264 restricted and BN 266 unrestricted. Those jets belong in that body.
The research brief pairs a Dell’Orto VHSB 34 with the Rotax 123 and a PHBH 28 with the Rotax 122. Both records stay. Read the stamp and the casting.
Popular upgrade, when the Type 122 left with the 28 mm carb: a 34 mm carb. PJME sells a Dell’Orto 34 mm for 1993–2005 and says the standard 28 mm inlet manifold will not take it. Hartrusion says the same change is widely made, usually to the VHSB 34 LD that earlier bikes left the factory with, and that a 34 mm manifold exists for the original reeds.
Source: Haynes 4298, PJME 34 mm carb listing, and Hartrusion’s carburettor tuning page
Hartrusion names the Dell’Orto VHSB 34 LD as the carb people change to, because the earlier machines used it and it has a cold-start circuit and an oil-pump connection. He also names Mikuni, Keihin and Polini as other makers, without a jet stack for them.
One Italkit manifold listing says its carb connection is 40 mm and names Keihin PWK 32–36 mm and Mikuni TM(X) 32–35 mm as bodies that connection suits. That is a seller’s fitment note, not a jet chart.
Hartrusion also names a Dell’Orto VHST 28 CD as a standard 28 mm carb from about 1998, beside the PHBH 28 BD. Haynes does not list the VHST. It is not the 34 mm upgrade.
Hartrusion says bikes up to about 1997 with the Rotax 123, and a few with the Rotax 122, had the VHSB 34 as stock. On those bikes it is not an upgrade. He also says a rumour that the 34 mm carb meant 25 kW is wrong: the GS figure he gives is 23 kW, the first MP bikes that still had the 34 mm carb are 20 kW, and the later 28 mm bikes are 22 kW. The 23 kW GS figure matches the research brief. The 20 kW and 22 kW sentences are his.
Source: Hartrusion carburettor tuning page, and an Italkit W-Force manifold listing
Two independent sellers and workshop pages agree that the 28 mm manifold does not suit the 34 mm Dell’Orto. PJME says you need a new inlet manifold if you are replacing the standard 28 mm carb. Hartrusion shows a 34 mm manifold with the VHSB 34 LD.
The reed block is a separate choice. Hartrusion says a manifold exists for the original reeds, and that complete aftermarket blocks also exist. The Italkit W-Force listing supplies a 34 mm manifold and says the original manifold is 28 mm. Details are on the reed page.
The RS 125 blog says the standard air filter still fits the 34 mm mouth. Hartrusion says the standard air intake has a smaller cross-section than a 34 mm carb, so opening that intake up is part of the same job. Those two sentences can both be true: the filter can clamp on, and the tract ahead of it is still the smaller hole.
Hartrusion says that from 2006, where the fuel tap is vacuum-operated, a carb without a vacuum take-off needs one. He does not give a drilling size. That size is To verify.
Legal and safety. Hartrusion writes that an unapproved carburettor change can void homologation. In the UK it can also change the insurance position and what you are legal to ride. The licence limits are on the UK law page.
Source: PJME manifold listing, and Hartrusion’s carburettor tuning page
Do not put the PHBH jets in a 34 mm body. Haynes never gives that stack for a 34 mm carb.
Hartrusion’s chart, citing a Sahner data sheet from August 2000, lists one VHSB 34 configuration for an RS 125 of 1995–97 at 23 kW: main 160, needle K57, atomiser DP 266, clip in the 2nd notch from the top, idle emulsion tube B36, choke jet 60, slide 40, float 9.0 g. That is a cited setup for those bikes. It is not a measured setup for a Type 122 with a different pipe.
PJME says the 34 mm carb they sell for 1993–2005 is supplied with a 160 main, a pilot jet 40 and a K57 needle, and that this should suit most standard bikes. They also say a derestricted exhaust with no catalyst is how you get the best from it. Pilot 40 and emulsion tube B36 are not the same part. Which idle piece a given carb uses is Disputed/needs checking until you have it in your hand.
Pedparts lists a VHSB 34 LD with main 160, needle K57, atomiser 266, slide 40, idle emulsion 40, choke 60, float valve 300 and a 9.0 g float, and calls it an OEM carb for Rotax 122 and 123. The OEM claim does not match Haynes, which lists a PHBH28BD for both power columns. The parts that match the Sahner column are the main, the needle, the atomiser and the slide. The idle piece still disagrees.
Hartrusion’s own experience, not a manual: restricted bikes are often run with a smaller main, sometimes down to 145, and a 155 main can still be too rich on a derestricted engine, so 150 is sometimes the next try. He says you need a set of mains and patience. The RS 125 blog’s separate baseline is main 155, idle 36, choke 60, K57 in the top notch, atomiser 266, air screw 1.5 turns out, and a float height of 14 mm against a Dell’Orto racing figure of 11 mm it also quotes. That blog stack, including 14 mm, stays disputed. It is not the specification.
Hartrusion’s stated default for the VHSB idle air screw is 1.5 turns out from fully in. That is his default for that carb, not a Haynes figure, and it is not a setting for the PHBH. Haynes does not publish pilot-screw turns, float height or a needle-clip position. Those three stay To verify for the PHBH.
Legal and safety. A main jet that is too small seizes the engine. A stack copied from a different pipe, a different day’s temperature, or a different carb body is how that happens. Nothing on this page is a jetting you can type in and ride.
Source: Hartrusion VHSB 34 chart, PJME’s 34 mm listing, and the RS 125 blog
On the VHSB chart the clip position is counted from the top notch. The needle named for the RS 125 column is a K57. Other needles on that chart belong to other bikes or to a Polini kit, and they are not a starting point for a standard 125.
Slide 40 is the slide in the 1995–97 column. Hartrusion says an Arrow and Giannelli sheet treats a 50 or 55 slide as the same idea as taking 3 to 4 mm off the stock slide. That cut is not repeated here as a job. A larger cutaway leans the early throttle. Getting it wrong leans the engine.
The plug in Haynes is an NGK BR10EG, gap 0.5–0.7 mm, torque 20 Nm. A derestriction blog also names a BR8EG. The manual extract does not. Both stay. The carb guide already says a plug from town riding is a pilot story, not a main-jet reading. A colour chart for that reading is not in the Haynes extract used here. The colours are To verify.
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