Please note: These pages are part of my RS 125 Workshop Page on my web blog and they are not related to the manufacturer Aprilia/Piaggio. These are self-written repair instructions, hints, tips and tricks for the old Aprilia RS 125 with two-stroke engine until year 2012. This blog is operated by a private individual without commercial background and without any benefits. There is absolutely no warranty. Any liability for any kind of damage that may be related to information from this website is totally denied. Use at your own risk.

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This Jetting Setup Chart lists some available factory configurations for the Dell Orto VHSB 34

RS 125 95-97 (23 kW) ​[1]​AF1 125 90 ​[2]​AF1 125 91 ​[2]​RS 125 92/93 ​[2]​Polini 154-cm³-Kit ​[3]​Polini 154-cm³-Kit
with Superval-Reeds ​[3]​
Arrow/Giannelli Exhaust (hom.)RS 125 99 with Arrow Challenge ​[4]​
Main Jet160148158155155 … 158178 … 180160162
NeedleK 57K 56K 57K 57K 56K 21K 57
AtomiserDP 266DP 266DP 266DP 266orig.orig.
Needle Clip Position2nd Notch3rd Notch2nd Notch1st Notch1st Notch1st Notch
Idle JetB 36 (Emulsion Tube)40 (Jet)40 (Jet)40 (Jet)3636
Choke Jet60 (Bleeder Jet)
Throttle Slide4040404050-55*
Float9,0 gr., single
Carburetor Jetting Setup Chart

*: According to instruction sheet, removal of 3 to 4 mm from the stock valve is recommended, this will be the same as fitting a 50 or 55 throttle slide.

A “-” in the table means that no information was given for the respective part in this variant.

The 34 mm carburetor was only installed in the old GS models, these were not offered with reduced power like the later versions ex works, also there were no catalytic converters or other measures for exhaust gas cleaning yet. For this reason there are no different setups for the 34 mm carburetor for throttled and open versions.

Jets and parts on the VHSB 34 LD

Main Jet

Restricted bikes (usually restricted with an electronic speed limiter) are often operated with a smaller main jet, sometimes down to 145.

By experience, even a 155 main nozzle may still be too rich on a derestricted machine, it is possible that one has to reduce to 150 here. In general, a lot of patience is needed when setting up the 34 carburetor, this is always underestimated. It is recommended to have a whole set of main jets at hand for tuning.

Jet Needle

The needle clip indicates the position at which the clip is placed on the jet needle counted from the top position.

Usually the RS 125 is equipped with a jet needle of the type K57. The K56 can be found in a setup for the AF1 125. The differences of the needles are shown on the following picture.

Differences between K57, K56, K21 and K25

The K25 needle was recommended in some Italian forums for the RS 125, it has a leaner mixture at higher throttle positions in high speed ranges and at full throttle through the small tip it quickly produces a richer mixture. The K21 is recommended for the Polini kit with modified reed valve block, but according to a comment from “Paul” on this page it also helps with problems with stuttering between 6.800 and 8.000 rpm.

Idle Jet

The VHSB34 LD is not an exclusive product, it is available in different versions also on other vehicles. For this reason there are different variants on how and which idle jet is installed. The version installed in the latest RS 125 GS normally had a screwed-in emulsion tube jet for idle, this is approx. 2 cm long and is screwed several millimeters deep into the threaded hole. No additional idle jet is then screwed on top of the idle jet emulsion tube, as shown in the picture above.

There is the possibility to fit an idle jet in addition to the mixing tube nozzle or just the conventional idle jet without the emulsion tube, in this case jet size 40 ​[5]​ would fit.

Idle setting

The idle adjustment screw (knurled screw with spring, adjustable without tools) sets the minimum position of the throttle slider, the idle engine rpm can be roughly adjusted that way

The idle air adjustment screw (adjustable with a screwdriver) reduces an air channel to the idle jet. If the screw is screwed in, the air volume is reduced and the idle mixture becomes richer, if it is screwed out, the mixture becomes leaner.

To get the default setting the idle air adjustment screw is screwed in completely and then unscrewed by 1.5 turns.

References

  1. [1]
    Sahner, “technische Daten Motorräder u. Scooter 125 bis 1000 cm3 .xls,” RS125, Aug. 2000.
  2. [2]
    F. Stefani, “Come Potenziare le Aprilia 125 Strada,” Elaborazioni, pp. 6–18, 1993.
  3. [3]
    Polini, “Instruzioni per il montaggio kit Rotax mod. 123 Aprilia 125 – sintesi-futura tuareg wind,” Polini Motori, 2002.
  4. [4]
    “Marmitta e silenziatore per Aprilia RS 125 ’99,” Arrow Special Parts S.r.l., 1999.
  5. [5]
    Persilia, “Set-up Aprilia RS 125 – Modelle,” Aprilia Forum, Jul. 11, 2006. http://www.apriliaforum.de/faq-s-rs-125/10737-set-up-aprilia-rs-125-und-auch-andere-modelle-wie-rx-sx.html (accessed Apr. 23, 2018).

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4 Comments

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  1. Hi has any one used vhsb 34 to honda nsr jc20 with malossi 180 kit? I have problems to setup this carb. My inlet collectore is a bit longer than in rs about 5 cm longer. I had to use dq atomizer instead dp because mixture was instantly to rich. Can atomizer be used and produce to rich mixture? Throttle slider is covered with some silver coat if it goes off and i can se black spots can that throttle produce to rich mixture? I am running on main jets from 180 to 190 on dq 266 atomizer and idle jet 36 needle k21 2nd notch. Engine compression is a bit over 10 bar when I had 11 bar I used 185 main jet but I had to put thicker hed gasket and again problems with regulation started . I wonder what elements should I replace to have stock carb because I am getting tired of regulating it every month. Fuel level is correct and floater has no wholes. If any one is running nsr with malossi 180 i will be very great full for any tips.

  2. We got new 34mm dellorto carb set up k21 needle and 40idle slow jet 160 main jet just full standard settings with different needle it clears that splutter from 6800 rpm to 8000rpm

    we took alot of money and time doing these dyno runs to get it wright what you’re neew shape set up in parts mods euro 3

    1. Hi Paul does the k21 actually work to get rid off the splutter I have been using a k52 and that seemed better but still alittle rich in the middle on my new shape 2009 what needle setting what that I have it on the top notch atm any information would be appreciated