Jetting Chart Dell’Orto VHST 28

For the Dell’Orto VHST 28 CD, which was used in the last models (RM and RD), there are several setups available.

On the RD and RM models the carburettor also has a carburettor heating, a TPS (throttle position sensor) and a connection for an additional idle air system.

Model		RS 125 RD/RM without RAVE [1-3]	RS 125 Factory Setting [4]		SX 125 11 kW [5]
	Main Jet	132	132		128
Restricted	Needle	D 49	D 49		D 49
	Atomiser	HN 268	HN 266		–
11 kW	Needle clip position	4th	3rd		–
	Idle Jet	U 36	U 36		36
	Choke Jet	–	–		90
	Throttle Valve	45	45		50
	Idle mixture adjustment screw	1 3/4	1 3/4		–
Model		RD/RM with RAVE [1,2]	Track Upgrade [3]	Recommended by italian drivers [2]	SX 125 full power [5]
	Main Jet	170	185	135 … 175	160
Full Power	Jet needle	D 53	D 53	D 49	D 53
	Atomiser	HN 268	HN 266	HN 268	–
~ 17 kW	Needle clip position	1st	1st	3rd	–
	Idle Jet	S 45	S 45	S 45	38
	Choke Jet	–	–	70	90
	Throttle valve	50	50	45	50
	Idle mixture adjustment screw	2 1/4	2 1/4	4	–

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

Connectors and Jets

These pictures show connectors, components and jets of the VHST 28

Main Jet

The main jet of 185 listed for derestricted bikes is indeed very large [4], for the Euro3 standard in Germany a 170 main jet is used [1]. If the bike stutters and runs too rich, the man jet can be reduced in steps of 5 down to 135 [2].

Compared to the PHBH 28, the VHST has an atomizer nozzle type “HN” with holes for mixing the gasoline with air in the atomizer. This air reaches the atomizer through the air nozzle at the bottom center at the connection to the air filter and is fed directly above the main nozzle. This difference results in the comparatively large diameter of the main nozzle and the large area with high steps 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.

The two nozzle needles D49 and D53 differ significantly in the change of the needle diameter, the D53 is much steeper.

Since the needle clip positions are different in the factory setups, it is not sufficient to compare only the dimensions of the needles. Compared to the D49, the D53 produces a leaner mixture at low gas valve positions and a richer mixture at higher gas valve positions.

Throttle Slide

The throttle slide code number indicates the height of the slider cutaway. This height influences the mixture formation at very low gas slide valve positions by changing or shifting the negative pressure that is formed.

A larger gas slider cutout leads to a leaner mixture, a smaller gas slider cutout leads to a richer mixture at low gas slider positions.

A stutter when starting up due to a mixture that is too rich may indicate that the gas slide opening is too small.

References

[1] Biker4Life<3, “Aprilia RS 125 Euro3 VHST Vergaser-Setups,” Aprilia Forum – Motorrad u. Roller, Feb. 28, 2012. 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).
[2] L. Cavicchini, “VHST 28 (Configurazioni e esploso),” Aprilia 2 Stroke. https://aprilia2stroke.wixsite.com/home/vhst-28-configurazioni-e-esploso (accessed Sep. 05, 2020).
[3] “Changes for competitive (track) use, only,” aprilia, 001–2007, Jul. 2007.
[4] bubu01, “RS 125 Bj. 08. komplett aufmachen (30ps),” 125er-Forum.de, Nov. 27, 2009. https://www.125er-forum.de/threads/78342-rs-125-bj-08-komplett-aufmachen-30ps/page4 (accessed Sep. 05, 2020).
[5] “RX-SX 125 2008-2010,” Piaggio Group, Ersatzteilkatalog AP1098, May 2016.