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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How to fit steel braided brake lines to your Aprilia RS125

Thanks toAndyS_RS on TJ Tuning who inspired this post.

Fitting braided brake hoses will not give you greater stopping power, but it will give improved feel which is crucial to effective braking. This guide describes how to bleed the front brake. The process is the same for the rear brake, but bleeding is easier and can generally be done without removing the calliper.

Install the steel braided hose and bleed your brakes

• Empty the line by bleeding the brake.
• Connect hose to the nipple and loosen
• Pump the brake and it should eject fluid, which you can collect in your container.
• Keep the brake attached to the bike while you do this.
• Once the reservoir is empty, stick a clean rag in, soak up remains and prevent intake of dust.
• Loosen the master cylinder banjo bolt (beneath the banjo bolt there is a small ball bearing, be careful not to lose it).
• Use a rag to mop up any drips.
• Take the loose end of the hose, place it into your container and then place the container as low as possible.
• Fluid should drain into the container.
• Disconnect the hose from the caliper once it has stopped.
• Remove the calliper from the fork but there will still be fluid in it so keep it upright.
• With the hole into which brake fluid flows carefully positioned over your jam jar, squeeze the pads apart and it will eject most of the remaining fluid into the container.
• Remove the pads from the calliper and clean it thouroughly.
• Check that your new hose is of equal length to the rubber hose it will replace.
• Ensure it is properly routed.
• Take the new bolts from the box and reinstall the calliper
• Connect both ends of the hose and tighten up the new banjo bolts.
• Ensure that the small ball bearings have been replaced and that copper washers have been installed on either side of the new hose.
• Fill the reservoir half full with your new brake fluid and start bleeding the brake(this will take a while as the system needs to be primed with new fluid).
• Top up the reservoir as required.
• When you reach a point that you feel is acceptable take the bike out a few times and the leaver will get firmer with each use.
• On your return, flush a few more measures of fluid through the line and top up the reservoir (the instant you open the bleed valve it will go spongy, but close it up and it will get firm very quickly).

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.

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.