Posted on 10 December 2009 by John
In this series, I’ll be taking you through the workings of your Aprilia RS 125’s carburettor and talking you through the process of jetting your carburettor.
Introduction
Jetting is the process of adjusting the air and fuel jet sizes in order to fine tune your bike’s carburetion to suit the load demands on the engine and attempt to optimise power delivery. If you make any modifications to your engine (air box and exhaust included!) you need to know about jetting your carburettor. In this series you will learn the correct way to setup your carburettor after modifying your bike, how to tell if you are running lean or rich and what you can do about it.
In order to understand the best setup for your carburettor, you first need to understand how it works.
What parts make up the carburettor?
The purpose of the carburettor is to provide your engine with the correct ratio of fuel and air. Although the carburettor can’t provide the ideal mixture at all loads, the different parts of the carburettor work to try and create this ideal mixture for proper combustion.
The RS125 carburettor consists of the following parts:
- Fuel bowl
- Shut-off needle
- Floats
- Main jet (provides mixing above 1/2 throttle)
- Pilot jet (provides mixing from idle to 1/4 throttle)
- Jet needle (regulates flow of the main jet as the throttle is opened; 1/4 to 3/4 throttle)
- Needle jet (passage to which the jet needle resides)
- Air screw (adjusts the amount of air from idle to 1/4 throttle)
- Idle screw (adjust the idle speed)
How does the carburettor mix air and fuel?
Fuel from the tank enters the bowl through a small passage until the floats rise up enough to force the shut-off needle into the passage (prohibiting any more fuel from entering the fuel bowl). The main jet is usually found near the bottom of the fuel bowl. As the piston moves down, it draws air into the cylinder. This air flows through the throat of the carburettor and creates a vacuum which sucks fuel into the cylinder via the main jet.
The mixture screw is able to adjust the air in small amounts in order to fine tune the mixture (usually at idle). Most carburettors require adjustment of this screw and a jet change. When you change a jet, you are simply giving the fuel a smaller or bigger hole to flow through, thus changing the amount of fuel that reaches the cylinder.
What is meant by running rich or running lean?
These terms refer to the amount of petrol being delivered to the engine. If you are running lean, then the ratio of fuel to air is too low, that is, there isn’t enough fuel reaching your engine for the amount of air that is reaching the engine. This can cause the engine to run hotter and leave you with a seized piston. Running rich means that there is too much fuel reaching the engine for the amount of air that is reaching the engine. Running slightly rich is generally acceptable and protects the engine as the excess fuel helps to carry heat out of the engine. If you are running very rich, you will have a very smokey exhaust and performance will suffer.
The ideal ratio of air to fuel is called the stoichiometric ratio.
The next part of the tutorial will look at preparing your carburettor for jetting by servicing it. Remember, you can’t rush perfection!
Posted on 22 July 2009 by John
Another question from reader Paul:
Hi John
So sorry to bother you again, i went to clean my air filter took off filter lid and there isn’t one just an old pair of tights wrapped around the top plastic tray,would you have any ideas why air filter would be removed would it be for more speed.do you know what benefits or downsides putting a K&N filter on would have.
Again sorry to bother you.
Paul
Don’t apologise for getting in touch Paul! I relish the opportunity to share my knowledge with the world! I started this site to help people like you and the questions help me to create content that you want.
I wrote back to Paul:
Paul,
Can’t think why they would have done this it won’t give you any more speed but it will ruin your engine. If you fit the conical filter it will give you a raspier sound but you will need to rejet your carb and you shouldn’t expect any performance gains. To be honest, I’d just look for an air filter from TJ Tuning or CMP Racing.
Let me know if that helps.
John
Posted on 29 March 2009 by John
Adjusting the carburettor needle can make a huge difference to the way your bike behaves. Begin by removing the tank (pop the seat off, remove the bolt through near the head stock and pop the tank up). Turn the fuel tap off and remove the fuel line.
Undo the screws that hold the top of the top of the carburettor in place and lift out the slide. If you are using a 34mm carburettor, you can unscrew the top and lift out the slide. Disconnect the slide from the cable by compressing the spring with your fingers and pushing the cable down into the slide. Do not let the spring fly off!
Once the cable has been disconnected, turn the slide upside down and allow the shim to drop out. This is a small plate onto which the spring pushes in order to keep the needle down. Once you’ve done this, you can remove the needle. There will be a circlip on the needle.
If the clip is on the lowest groove then you will need to upjet your carb. Otherwise, try moving the clip down to the next notch (towards the tapered end) and take your bike for a spin. This process pushes the needle up into the carburettor slide more, allowing more fuel to squeeze past the tapered end.
If moving the clip has improved performance, but it is still not perfect, try moving the clip to the bottom groove. If that still isn’t right, upjet your carburettor and reset the clip to the middle of the needle.
If these adjustments make the performance of your engine worse, try moving the clip up a notch and test the engine.
Posted on 15 February 2009 by John
Derestrict your bike to unleash its full 33bhp!
The Aprilia RS125 restricted to 14bhp for learners or derestricted to 33bhp for those who hold a full license. This guide will tell you where to find restrictions and how to remove them.
How can you tell if your RS125 is restricted?
- Ride it and look for a significant increase in power at 800rpm
- Lift up the tank and check the right hand side of the battery compartment to see if a powervalve solenoid has been installed (the solenoid is tubular and mounts via two bolts).
- If you don’t get an increase in power but the powervalve has been installed then the system is not working correctly.
- If it has been fitted ensure that the cable is correctly connected to the blade and the system moves freely (check out this guide for loosening a seized powervalve).
- Test the solenoid by disconnecting the 2 pin multiplug that connects the solenoid to the wiring harness.
- Earth one of these pins and connect the other to the positive terminal on your battery (it will move if it is working)
- If it has seized then it will need to be replaced.
Removing the exhaust restriction
- Bikes produced since 2003 have a catalytic convertor in the exhaust expansion chamber. You can either
- Purchase a new expansion chamber and end can (you MUST rejet your carb)
- Take the expansion chamber from an older bike
- Cut open your pipe to remove it and weld it back together.
Installing a powervalve and solenoid
- Installed the blade with the flat side facing up and the tapered side facing down (when retracted, the slope on the end of the blade lines up with the exhaust wall).
- Install the solenoid on the right hand side of the battery box.
- Connect the cable to the blade
- Adjust the solenoid once it is in place.
- The more the powervalve protrudes into the exhaust port, the more low end power you will have and the less top end power you will have.
- The less it protrudes, the less low end power you have but the more top end power you will have.
- You must now install the RAVE unit.
- Remove the tail fairing to get to this area.
- The RAVE unit is a similar size to the CDI unit (also underneath the pillion seat) and you will find a multiplug in this vicinity that connects to it.
- Change your spark plug or you will destroy your piston.
- A restricted bike should run a BR8EG
- A derestricted bike should run a BR10EG.
Removing the CDI restriction
- The stock setup retards the ignition at around 6000rpm so the bike can pass emissions tests.
- This causes very poor running at this engine speed and you will notice a drop in acceleration.
- Derestricting the CDI will invalidate your warranty, so buy another from ebay if you bike is still in warranty and derestrict that.
- The CDI is located under the pillion seat. Remove the tail fairing to get to it.
- The back of the CDI is filled with rubber and the bridge is underneath it.
- Remove an area of about 10mm from the bottom of the CDI and about 15mm either side of the centre line of the unit.
- You will eventually reach a bridge between two terminals.
- Snap this and then refill the hole with silicon.
Carburettor
- Newer bikes have a 28mm carburettor whilst the original bikes had a 34mm carburettor.
- Fitting the 34mm carb gives better mid-range performance and better top end power without affecting your pertrol consumption significantly.
- Get hold of a Dellorto VHSB 34MM and a new inlet manifold (the original will be too small).
- The original airbox should be fine with the bigger carb.
