Just How Fast is the RS125?

The RS125 delivers unrivalled performance in the 125cc class. Check out these specs.

Engine

• Single cylinder
• 2-Stroke
• Liquid cooled
• Aluminium cylinder with Gilnisil coating on barrel
• Reed intake and exhaust port control valve
• Separate lubrication
• 54 mm bore
• 54.5 mm stroke
• 124.8cc displacement
• 12.5 ± 0.5:1 compression ratio
• Dell’Orto PHBH 28 BD carb
• CDI capacitor discharge electronic ignition
• Electric start
• 12V – 180 W generator
• Separate mixing lubrication system with variable displacement volumetric pump
• Six speed gearbox
• Chain driven
• Multiple disk wet clutch
• Cast aluminium sloping twin-spar frame with thin walled structural shell and cross rib reinforcement
• Upside-down hydraulic action front telescopic fork (40 mm diameter inners) and wheel travel 120 mm)
• Cast aluminium swing-arm rear suspension with asymmetrical members, thin walled structural shell, single hydraulic monoshock, adjustable spring preload, wheel travel 120 mm
• 320 mm front disk brake with floating caliper and four parallel pistons
• 220 mm rear disk brake and twin piston caliper.
• Aluminium alloy wheels 3.00 x 17″ front and 4.00 x 17 ” rear
• Tubeless radial tyres
• Front tyre 110/70 ZR 17″
• Rear tyre 150/60 ZR 17″
• Maximum length 1,950 mm
• Maximum width at handlebars 720 mm
• Maximum height at windshield 1,135 mm
• Seat height 805 mm
• Wheelbase 1,345 mm
• Tank capacity 14 litres (3.5 litre reserve)
• Price £3,724 OTR

The Aprilia RS 125 is a GP derived replica that looks the part but offers performance that is practically unrivaled in the 125cc class. In full-power trim, the bike develops 33bhp which is the maximum you will be able to ride on a restricted license. This translates to 0-60 in about 6 seconds and a top speed of around 100mph.

All that power from a 125cc engine means that it won’t run forever and it will need occasional rebuilds. However, if you respect the bike, allowing it to warm up properly and avoiding wide-open throttle for too long then the engine will offer surprising longevity.

Click here to read about buying a second hand Aprilia RS 125.

The bike offers two power outputs; a restricted 14bhp and a derestricted 33bhp. If you’ve just done your CBT then you can ride the 14bhp format with L-plates. Once you’ve got your fill license you can pay somewhere between £50-£400 (depending on what’s already on your bike and who does the work) to derestrict the bike and more than double its power output.

Click here to read about derestricting the Aprilia RS 125

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Diagnosing the problem when your bike won’t start can be difficult. Check out this guide for common reasons for the RS125 not starting. Work your way through the checklist from top to bottom.

RS125 Non-Starter Checklist

• If there is a nick in a tube or you have a leaky carburettor, your fuel could have escaped and evaporated over the course of a few hours (Lift your filler cap and check that there is fuel visible in your tank).
• Someone could have played with it since you last rode the bike or you might have forgotten.
• Put the bike in neutral and push along with your feet (or have someone push you).
• When you have reached a good speed, put the bike into SECOND gear and hit the starter button.
• If she starts up, pull the clutch in. The problem would appear to be in the starter motor.
• The starter relay can be found behind the radiator and the connection means that it is possible for wires to fall out of the unit.
• If you can bump start the bike and the wires are all connected, the problem could lie within the relay.
• Turn the engine over and make sure that a good volume of air is coming out of the exhaust.
• If possible, take it off the bike and look for blockages.
• Make sure the choke is not flooding the engine.
• Remove the seat and lift the tank up.
• Either remove the spark plug or take a good plug that you are certain work, and connect it to the HT cap.
• Touch the end of the plug to the frame via the insulated cap as you are in for a shock (haha) if you hold the plug itself.
• Hit the starter button and look for a spark.
• Check the original plug in the same way to make sure that it isn’t the source of the problem.
• Make sure that the HT lead sits snuggle onto the plug.
• Make sure that everything is plugged into the CDI as it should be.
• This is located underneath the pillion seat. Look for bare or cut wires.

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.