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.
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 Jet | 160 | 148 | 158 | 155 | 155 … 158 | 178 … 180 | 160 | 162 |
| Needle | K 57 | K 56 | K 57 | K 57 | K 56 | K 21 | – | K 57 |
| Atomiser | DP 266 | DP 266 | DP 266 | DP 266 | orig. | orig. | – | – |
| Needle Clip Position | 2nd Notch | 3rd Notch | 2nd Notch | 1st Notch | – | – | 1st Notch | 1st Notch |
| Idle Jet | B 36 (Emulsion Tube) | 40 (Jet) | 40 (Jet) | 40 (Jet) | 36 | 36 | – | – |
| Choke Jet | 60 (Bleeder Jet) | – | – | – | – | – | – | – |
| Throttle Slide | 40 | 40 | 40 | 40 | – | – | 50-55* | – |
| Float | 9,0 gr., single | – | – | – | – | – | – | – |
*: 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.
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.
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] Sahner, “technische Daten Motorräder u. Scooter 125 bis 1000 cm3 .xls,” RS125, Aug. 2000.
[2] F. Stefani, “Come Potenziare le Aprilia 125 Strada,” Elaborazioni, pp. 6–18, 1993.
[3] Polini, “Instruzioni per il montaggio kit Rotax mod. 123 Aprilia 125 – sintesi-futura tuareg wind,” Polini Motori, 2002.
[4] “Marmitta e silenziatore per Aprilia RS 125 ’99,” Arrow Special Parts S.r.l., 1999.
[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).