Idle Jet in Pinasco 177+ carb 26/26 si

Hello everybody, I am here again looking for some help, due to my disastrous knowledge in mechanics [?[]

I have bought from SIP and installed in my PX125 (2004) a Pinasco pipe, a Pinasco kit 177cc and a 26/26 SI carb.

Main jet I think is OK, I started with 125 but was too much, I even had to change the sparkplug, finally I am running with a 118, that seems weak, but I think is fine

Idle jet is the problem, I have a 50/120 and a 45/140 . I have tried both, and it is not running properly with any of them, when starting is not going ok, and until I don´t get 1/2 throttle it´s not good ( Is like farting !!!) [:(] .

I think I will try to find other idle jets to try them and see if changing the idle jet it runs properly.

Any advice regarding the size of the idle jet that would be OK for me ??? any other advice ??? As I told before my mechanic skills are more than basic and the problem may be in any other thing…

I am sea-level. I have advanced the ignition to 20ºand sparkplug NGK B9. Thanks in advance

Thank you Curare, I will do this this week and will keep you updated[;)]

Hi when i put 26mm on my T5 with malossi kit it would idle ok as soon as you gave it some throttle it would splutter and stop what ever setting the air screw was set at. i phoned chislespeed had a word with Martin and i had to buy a small drill bit cost £1 and drill out the first hole on the bottom of the idle jet no spluttering now goes realy well hope this helps Gary .chiselspeed 01132811052


the air filter has to be clean and if you are using the 20 filter:
do a coupple of holes on the air filter of about 5mm in the part where the idle jet and air corrector are just above.
Or buy a filter for 24/24.

keep the 50/120 and open the air screw 2 and 1/2 turn out.

Main 118 it is OK.

Of course this problem can be caused by other things but as usual it is better doing one step after another. Try, and keep us updated


I have a set up close to yours. Pinasco 177 &  Pinasco exaust on a PX 150 '05.
I cut back the crank and opened the admission whole in the case as per the kit instructions.

I also welded aluminum on the cases, then grind the case tranfers to get a seamless port matching.

I fit a si 24/24 and grinded the carb case that was obstructing the inflow (have you done this?).

palced the 24/24 air filter wich has wholes over the main and iddle jet systems. 

and upgread the primary and clutch to the px200's 23/65

When I first mounted the carb, it was 4 stroking at constant speeds below 45 mph and really short on gas at full speed (exaust tail was light grey).

I also had an incipient plinking at middle cruising speeds 45-55 mph with 1/3 throotle open

I was running a 116 main, be4 and 190 air.

The answer for me was:

125 main, BE3 and 150 air corrector.



Changing the air corrector jet affects the main jet at wide open throttle. In the mid range the air corrector has little effect and the mix is related to the main jet and mixer tube. At wide open throttle a smaller air corrector will richen the mix (it reduces the air and can also increase suction to pull more fuel) and larger air corrector will weaken the mix

An old dellorto manual recommends running at full throttle in 3rd and using the choke. This will richen the mix, if the motor picks up you are too weak, if it splutters you are too rich. If there is no change then you may be as well going up until some four stroking appears, to be on the safe side. This method only works on the main jet at wide open throttle as the choke will swamp all the other jets.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Si 24 mm Carb. Summary

A carburettor is simply a device for mixing fuel and air and delivering it to the engine. The Si 24 mm Carb. is a very simple carb. however it does differ to other carbs used in other scooters and motorcycles.


There are two 'circuits'. The idle or slow speed circuit and the main jet circuit. Both these two circuits overlap more so than many other carbs, which usually have three circuits ; an idle circuit, a needle jet for mid range and the main jet circuit.


Both the idle jet and the main jet on the Si feature an upper section that lets air in at a fixed rate, a lower section that lets fuel in and both this fuel and air mix together in the central mixer/atomiser tube.


The idle jet is a fixed jet. It either has to be swopped for a richer or leaner idle jet OR the mixture screw on the back of the carb. can be adjusted to control the flow of the idle mixture into the venturi.


The main jet stack can be altered in three ways, seperate air correctors, mixer/atomiser tubes and main jets are available.


The idle circuit works from idle right through to full throttle but works mainly in the 0 to 1/2 throttle range, tapering off between 1/2 throttle to full throttle but never actually stopping delivery of mixture! It's influence on the mixture between half and full throttle is not that great however. When you come off the throttle at high speed/revs, the only fuel and oil your engine is then getting will be from the idle circuit.


The main jet circuit works in the 1/3 to full throttle range. As you can see, there is significant overlap of both circuits.


The air corrector on the main jet stack and the top opening in the idle jet determine the flow of air to be mixed.

The main jet and the bottom part of the idle jet determine the amount of fuel to be mixed with the incoming air.

The mixer tube/atomiser and the middle part of the idle jet determine the amount of mixed fuel and air to go into the venturi of the carb to be burned. In the case of the idle jet, the mixture screw determines the amount of mixture from the idle jet to go to the venturi.

There is significant overlap in where in the throttle position the two mixture sources operate.

The first section describes how to adjust the slow speed or idle circuit and is followed by adjustment of the main jet circuit.


Neither should be adjusted in isolation because of their overlap.


Tuning the standard Si 24 Carb. Setting the mixture screw and idle

Tools required : 7 mm small spanner, or the specially modified NH spanner (see below) or in the case of a Vortex, a flathead screwdriver.


It is too simplistic to say "Set the mixture screw on the back of the carb. to 1.5 turns from closed." In practice it depends on a whole host of factors which include the weather, state of tune etc as to what the settings will be.


The Si E 24mm carb is found on all Vespa P 200s, save for a few models in the <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />USA where the 20mm was used for a short while. The Si G version is found on the T5. The Si G version has different jetting, a different slide profile (larger cut aways allow more air in at idle and hence it can be set for more fuel, for a 'crisper takeoff').


It has a shorter body height that allows a bigger air filter and hence a quicker throttle response! Newer versions are advertised as 26mm, they actually have a 25.5mm venturi. You'll need to match the lower carb. box that sits on the cases to take advantage of the larger venturi! A test a few years ago, Si G 25.5mm V's Si G 24mm in Scootering Magazine by Taffspeed, showed no performance enhancement on a T5. The article was biased in that:


1. The carb box lower wasn't opened out for the 25.5mm carb!


2. The jetting wasn't altered!


3. Had the 25.5mm carb. been compared to the Si E 24mm on the PX200 and not the Si G on the T5 then there would have been a difference I believe!


The idle speed screw is located to the right of the main jet stack in the above photo. You can't miss it, it's very tall and on a standard PX sticks through the top carb box and can be adjusted without taking the carb box top off!


The mixture screw on the back of the carb determines the amount of mixture from the idle jet to go to the venturi. The idle jet supplies fuel 100% of the time the engine is running, however it's contribution to the overall mixture between half and full throttle is small.



Many people have modified this mixture screw that is plainly too long to adjust easily, by cutting it in half and adding a flathead screwdriver grooved top. The Vortex Carb does just that on the left!


The other alternative is to modify the adjusting spanner!


Take a cheap, forged 7mm spanner, heat it up on either a gas stove flame or BBQ flame, holding it with a pair of pliers. When it gets hot, simply use another pair of pliers to bend it into shape, as shown below. Thanks to Nick for the NH Si Carb Adjusting Spanner!


To adjust the mixture screw hold the open end, and use the closed end coming from above, with the downwards section of the spanner pointing from above onto the fuel screw, to adjust. You will find you can adjust the fuel screw 1/4 of a turn at a time. It's much easier than a standard 7mm spanner!!!


The Vortex fuel screw is easy to adjust, it's shorter, has a flathead screwdriver thread and is easy to get to. Use a screwdriver rather than an 7 mm spanner. Also, make sure that you still use the rubber bush on the carb box where the fuel screw slots through the lower carb. box. This rubber bush is not designed to keep air or weather out, it's job is to stop the fuel screw vibrating loose! Make sure it's on!


To set the mixture screw correctly, here's what you need to do :


Start the scooter and go for a 1-2km run to warm the engine up.


1. With the engine running on the stand, take the engine side panel off. Turn the idle speed right up, the long screw with the flathead screwdriver fitting that pokes out of the carb box top, turn clockwise in.


The engine will be racing now!


2. Immediately turn the mixture screw on the back of the carb all the way in, the engine will get choppy and the idle will drop. On PX200 models it will require an 7mm spanner, there's not much room in there.


3. Immediately then turn out the mixture screw from closed in 1/4 turn increments, the idle will increase and the engine will start to smooth out. Take a few seconds wait between each 1/4 turn out. Count the number of turns as you open the fuel screw out.


4. You'll get to a point where the the engine will have smoothed out and the idle stops increasing when you turn out the mixture screw. This is close to where it should be set.


5. Adjust the idle speed back down to an acceptable running level. Then listen to your engine when you blip the throttle.


6. If the engine 'bogs' and feels flat when you blip the throttle it is probably set too rich. If the engine 'hunts' and takes more than 2 seconds to come back to a steady idle after blipping the throttle, it is probably too lean. A lean idle that 'hunts' the revs will make a 'pip, pip, pip sound.'


You should be able to blip the throttle, the engine should rev. clean and it shouldn't either bog, or 'hunt out the setting'. It should rev. and return to a good idle within 1-2 seconds.


7. Make a small adjustment here if neccessary. Then adjust the idle speed slightly.


8. On tuned Vespas, if it takes more than 4 complete turns, then pop in a richer idle jet, and repeat. This is the smaller jet on the left. The PX200 runs a standard 160/55 idle jet. The T5 runs a richer 100/50. The richness of the idle jet is the ratio of the two numbers, the lower the number, the richer the idle jet. The 100/50 is 2.0, the 160/55 is 2.9. The 100/50 is therefore richer.


If it takes less than 2 turns on a tuned Vespa, consider popping in a leaner idle jet and repeat.


On a standard PX200, with an Si E Carb. there should be no reason to change the idle jet of 160/55 unless you have done some tuning work. This could include fitting a genuine expansion chamber system where the 160/55 idle jet may be too lean. Concentrating solely on the main jet may not be good enough because of the huge overlap with the two circuits.


In colder climates you will find that the mixture screw needs to be set at more turns out, ie richer to cope with the dense cold and greater oxygenated air entering the carb. In warmer weather it will need to be set less rich as the air will be less oxygenated.


You may find that a standard PX200 will only need to be set at 1.25 turns from out in Queensland all year round but in a Tasmanian winter it may be at 1.5-1.75 turns from out. On tuned PX200s the settings will be very different and could probably be in the range of 2 to 3 turns from out, depending on the tune and conditions!


The Si 24mm carb. has two types of throttle slide. The Si E found on the PX200 has smaller cutouts compared to the slide on the Si G carb. found on the T5. The throttle slide cutouts allow air to pass through and mix with the fuel at idle. The T5 G carb. with it's larger cutaways allows more air in at idle compared to the Si E cutaway. This will also have some effect on takeoff as it will enable more air in and hence with the jetting, more fuel. This is the reason why the Si G carb has a richer idle jet than the Si E carb.


Setting the Mid Range and Main Jet

Firstly, as was stated earlier in the setting the fuel screw and idle section, there is a huge overlap between the components on the Si carb.


Main Jet Stack:


Air Corrector


This is the hole that a set amount of air comes in to mix with the fuel coming in. The larger the number, the more air is delivered. A 160 air corrector is therefore 'leaner' than a 140 because it has a larger air hole and lets more air in. Changing the air corrector will affect the mixture throughout the rev range from 1/3 throttle to full throttle!


Atomiser/Mixer Tube


This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. Therefore, the BE4 is 'leaner' than a BE3.


Main Jet


This allows a set amount of fuel into the atomiser/mixer tube to exit the carb and out the spray bar into the venturi then into the case inlet. On the Si Carb. the fuel exit on the main jet is a metric size. Eg a 116 main jet has an exit hole of 1.16mm. The larger the number, the 'richer' the main jet. A main jet of 118 is therefore richer than a 116. The main jet stack has its effect from about 1/3 throttle to full open. This is a much wider range than most other carbs that have three jet circuits and not two, like the Si.




As you can see, there is a huge overlap between the two circuits that supply fuel/air to the venturi!!! Many people merely concentrate on the main jet when tuning but fail to look at the idle circuit, which overlaps. An example would be someone putting an expansion chamber on a PX200. They usually up the main jet by 2 or 3 points but fail to increase the richness of the idle circuit. Many engine seizures happen in the low to mid throttle range, I'll leave you to draw your own conclusions..........