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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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.