Aprilia RS 125 Workshop Page

Here you will find instructions, collected tips and tricks about the old Aprilia RS 125 with 2-Stroke Engine. The content of this page is mostly the result of many hours of repair and work and is aimed towards all motivated mechanics who have similar questions about this bike.

This is a translated version of the original site and always work in progress. Everything that is linked is translated and everything that is not isn’t. Most of the text is translated with help of the DeepL translator, so if there is anything named totally wrong please contact me.

This page is not connected to the manufacturer in any way and is operated and maintained by a private individual. Please read the disclaimer and notes.

I am always open for suggestions, questions and qualified feedback (which I really appreciate), please use the comment function on the corresponding page.

• Cutaway model Rotax 123
• Stock Shock Absorber 3D CAD model
• SP Shock Absorber 3D CAD model
• 3D printing parts (STL) on Thingiverse

General

• What to know about the RS 125
• A guide for buying a used bike
• Differences between Rotax 122 and Rotax 123
• Maximum Speed Table (vmax)
• Engine warming-up
• Dethrotteling and unrestricted bike
• Differ Rotax 122 and Rotax 123 by the frame from outside

Repair and Workshop Tips

• Enginge not starting – what to do now?
• Common problems with low engine power
• Carburetor setup guide
• Using the right gearbox oil (Avoiding clutch problems)
• Power Valve (RAVE)
  • Operating Principle
  • Removal and Disassembly
  • Maintenance and Service
  • Trouble-Shooting and common problems
  • Adjustment and Setup
  • Correct Direction of Installation
  • Original and aftermarket power valves
  • Pneumatic RAVE 2
  • Control Unit for 8400 1/min
• Balance shaft gears installation in Rotax 123
• Bearings and gaskets for Rotax 122 and 123, proper removal of crankshaft bearing
• Function and diagnostics on the SEM ignition on the Rotax 123
• Rotax 123 Gearbox Assembly
• Test Thermostat and Thermostor
• Oil or coolant spill from a hole in the engine
• Sealing the exhaust flange
• Stock rear shock absorber unrepairability
• Headlight disassembly (bikes before 2006)

Good to know for self-repair

• Reset Service display (RS 125 from ’06)
• Compression Ratio and Compression Pressure
• Overview: Electronic Control Units
• Remove Power Gap from 071000-0910 QCA91
• Carburetor Jetting Chart for Dell’Orto PHBH 28 BD
• Carburetor Jetting Chart for Dell’Orto VHSB 34 LD
• Carburetor Jetting Chart for Dell’Orto VHST 28 CD
• Exhaust Front Pipe Labels
• Cylinder Labels Rotax 122 & 123
• Portmaps
• Light Bulbs

Accessories

• Exhaust Gas Temperature Meter
• LED Turn Indicator Lights (bikes before 2006)
• Installation Manual for Domino short stroke thottle handle
• Tuning Manual
  • Exhausts
  • Carburetor
  • Reed Valves
  • Airbox and Air Filter
  • Power Valve
  • Cylinder
• Polini 154 cm³ big bore test
• Pneumatic RAVE2
• Steel balance shaft gears
• Comparison of high and low beam light bulbs (bikes before 2006)
• Change speedometer backlight (bikes before 2006)

For more tips, help and for further questions I have put together a link list with some websites where you can get further help.

• Recommended Links

Please note that I do not carry out any contract work and I will not repair other people’s motorcycles. Inquiries about this via the contact form are of no use and will always be answered with a reference to the nearest specialized workshop.

Disclaimer and notes

Any liability for any damage that may result from the instructions and data published here is generally excluded.

This site is not intended as a substitute for specialized workshops and will never make this claim. A competent consultation by experienced motorcycle mechanics can never be replaceabled by any internet webpage!

Anyone who carries out work on vehicles themselves needs more than instructions from the Internet as you can find them here: Experience, specialist knowledge, high-quality tools and suitable premises. Such things are a prerequisite which is not dealt with in detail here every time, but which are usually available in specialist workshops.

Saving the money for the professional workshop for purely financial reasons and instead working in a dusty shed with worn out cheap tools is not only a bungle but a life-threatening bungle, also for other road users.

Acknowledgements

I owe a considerable part of the knowledge and my experiences that I share here to people who have shared this with me and wide parts of the internet.

In particular, in Apriliaforum.de these were, besides the great team, Persilia, DellOrto, Kojo125, Mooserider and many others. Thanks to everyone in the German Two-Stroke-forum, especially moe589 and Manuel, for what is readable and freely available for everyone there.

A special thanks goes to my father for not just putting me in front of the TV as a child (as it is common today) but taking me with him to his workshop.

Why the RS 125?

When I got my motorcycle driving licence at the age of 18, it was limited to 34 hp for the first few years, so an unthrottled RS 125 fitted well for the beginning. That was in 2007, after repairing a crankshaft bearing damage and some other issues, I had to deal with the technology behind this bike, first by need, then by interest. In the german Apriliaforum, which is meanwhile closed and sold, I was able to contribute under the nickname “sonic.noize” with a little over 4.500 posts over the years and had already written some manuals there. To continue this is a motivation for this workshop page.

After a few years it happened that I became less and less enthusiastic about the topic, especially because of my studies and job. Reparing and technology have always fascinated me, working clean is fun but loud exhaust systems and a life for the sloping position did not suit me very well, I could enjoy nature better on my bicycle. As a motorcyclist you have a hard time if you can’t get anything out of empty mufflers (I think it’s just selfish and inconsiderate) or if you just don’t want to race around badly visible curves like a maniac. I always found restored oldtimers more fascinating than worn out tires. Also the atmosphere in some internet forums was just too impudent for me at some point, which is why I had almost completely withdrawn from this topic. Everyone knows everything best, does everything right, everyone else is a bumbler and has no idea of anything and is insulted in a general way.

However, I still like to work on machines and engines, nobody can take this fun away from me, and I am a convinced two-wheeled cyclist – especially in times when cars are getting bigger and bigger. Effective technology fascinates me. While the large SUVs, which are unfortunately becoming more and more widespread today, consume over 10 liters/100 km and much is being discussed about traffic reversals and CO2 savings, two-wheeled vehicles offer the possibility of reaching your destination with significantly less fuel consumption. Even the old two-stroke RS-125 can be driven under 5 litres/100 km and the new RS4 125 even manages to get there with less than 3 litres/100 km. But two-wheeled driving in the everyday life is to be seen today felt only in the former East Germany here and is otherwise at extinction.