What to look for
The list of faults and potential issues on a used RS125 is fairly substantial, which isn’t surprising from a 125cc two-stroke learner bike that lives its entire life on the red line. For a start you have to consider its legality. As a 125cc learner legal bike, it should only produce 15bhp, but de-restricted RS125s can produce in the 34bhp area. The restrictions are in four stages on post-2003 bikes – an electronic one in the ECU, a lack of power-valve system, catalytic converter in the pipe and a smaller carb. The ECU can be overcome by breaking into the ECU (the internet is full of 'how to' guides) while the power-valve needs to be physically installed. If there is a blanking plate located above the exhaust manifold then you are lacking the power-valve (complete kits can be located used for around £100 on eBay while a pneumatic valve is £120 but requires drilling the cylinder) and the bike should be road legal. Derestricted bikes are far more fragile than road legal ones and are best avoided, while several bikes that have spent their life in the aforementioned Superteen championship also crop up in the market - occasionally returned to road use. These will have had a hard life and are also best avoided.
The RS suffers quite a lot from electrical faults down to really poor wiring, so always check that all the electrical items such as the dash, horn, lights and so on work. If the bike has been left standing then the carb will probably need a rebuild as they get gummed up or the spark plug will need changing, although don’t just assume this is the issue if the bike fails to start. Never buy an RS that you haven’t heard run as you are asking for trouble. A bike that stutters or runs rough will generally just need a bit of setting up of the float height or its carb rebuilding, a non-runner may have far more serious issues. When the bike is running, listen for any knocking sounds from the engine that hint at a big or small end going, and look for any leaks from the radiator. Crash damage is very common on an RS, so expect a few scratches. Check the frame is straight, the lock-stops aren’t smashed off and that the wheels aren’t buckled and the radiator is intact. Complete new fairing sets cost less than £350 from China and will make a bike look like new, which is great news but means you need to be very wary of a really tidy looking bike in a modern race rep paint scheme! Check engine and frame numbers well as theft is always an issue on popular 125s and give the bike an HPI check to see if it has been involved in any major accidents and previously written off.
Any updates?
The RS125 was first introduced in 1992 and early bikes are easily identified by their rectangular front headlight. This model was updated in 1996 when a revised Rotax motor was introduced alongside a new look, before the bike received yet another facelift and more technical updates in 1999 (pictured top). The ‘bulbous’ look of this generation apes the bigger RS250 and is the one that most riders remember. This bike continued fairly unchanged until 2007, when it gained an RSV-style angular fairing and radial brake caliper (bottom picture). This final two-stroke RS125 received a few very small technical changes but was essentially unaltered until emissions laws killed it off in 2012. Over the years there have been various race rep paint schemes such as the Chesterfield, Banacia and 'Spain’s No1' as well as the traditionally lurid stock Aprilia colours! The later bikes are generally considered more reliable and it is only tuned ones that tend to grenade themselves. That said, don’t skimp on quality two-stroke oil or it will eat pistons.
