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RS125R-92 Tune-Up

My RS125R-92 in its new colors blue-green metallized with the numbers and designs in a metallized grey ...

Official Honda Documentation

Engine Tune-Up

The first operation you should do when you receive your brand new machine is to dismount and measure everything ...

These bikes are sent in a configuration that lets them use standard petrols (commercial super gazoline).
Though, the power is low and you cannot fight even in a national championship. You need to improve the power.
Honda publishes a pamphlet giving some indications (courses are also given in some important countries) that lets you improve both the engine and the frame.

For the 92' model, Honda showed two main tune-ups: standard Avgaz and racing Avgaz.

A word regarding the various fuels

The standard Avgaz can be found on smal airports for the airplanes that use piston engines. It's a petrol sligthly more expensive than classical super gazoline but it's not very good for racing purposes because it does contain a lot of anti-freeze additives and is intended for engines that run regularly at low rpms, very far from a pushed 2-stroke engine ! I use it (mixed with 40% of super) for trainings after having slowed down the engine, of course.

The racing Avgaz is developped specifically for the motorcycle racing. Several makers do it and Elf, for example, furnishes also the special oil for Honda that should be used with (HTX976).
It contains additives that prevent the mixture to explode and lets you use high compression ratios while maintaining the octane ratio allowed by the FIM rules. This petrol burns very quick which lets you have high rpms and an ignition timing not to much advanced.

In the world championship, they can use better petrols because they don't have the same rules as the other FIM championships on this point ...

Before 93, several other petrols were available with different octane ratios and the richer (it's the proper word !) were up to 124 octanes !

From 98', unleaded petrol will be obligatory: the engines will suffer ...

Standard Version

Here are some indications for the RS125R-92 as it comes out of the box.

Influence of the ignition timing over the engine power curve. Warning, don't play with this to much, it's the better way to destroy an engine ...

Here is an operation you must do when you re-plate your cylinder. The small bar in the middle of the exhaust port is heated a lot and it expands in the cylinder with a seizure risk, you should grind it according:

Two important points on the exhaust you should NOT modify:

There is a step between the cylinder and the exhaust, it's absolutely normal !
There is a restricting convex at the end of the exhaust, don't grind it (loss of 0.5-1 hp !).

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Honda Standard Avgaz Tune-Up

This basic tune-up needs a petrol with at least 102 octanes.

Tune-up details:

Grind off the head of the cylinder for 0.15mm
Modify each port according to the design. Slightly grind the corner radius of each port to avoid the piston ring to be damaged. Clean the transfers to the ports.
Base gasket: 0.4mm
Combustion chamber volume: 10.7-10.9 cm3
Ignition timing: 1° (1 mark on the ignition stator) more than the normal timing up to 9.5°/12'000 rpm or 23.5°/7'500 rpm
Reeds: Boyesen 606R

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Honda Racing Avgaz Tune-Up

This basic tune-up needs a petrol with at least 110 octanes.

Tune-up details:

Grind off the head of the cylinder for 0.2 mm
identical to the previous tune-up
Base gasket: 0.4mm
Combustion chamber volume: 9.8-10.0 cm3
Ignition timing: 2° (2 marks on the ignition stator) more than the normal timing up to 10.5°/12'000 rpm or 24.5°/7'500 rpm
Reeds: Boyesen 606R

Head, 92' version

Head, 93' version

Exhaust, 93' version

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My tune-up

I'm used to take the "footprint" of the ports on my cylinders, here is the standard one:

And those of my tuned-up one:

Some other mods:

Head modification like in the racing Avgaz Honda tune-up
Crankcase cleaning
Carburator: Keihin PWK 39 overbored to 40.5mm (standard: PJ 38 mm)
Reeds and reeds-box: Aktive special (see schematic)
- The main reed is soft and plain
- A small reed avoid the contact of the two reeds at low flows
- Another reed gives rigidity at high flow
- The stopper is short
Exhaust: Jolly Moto, carbon silencer

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Frame Tune-Up

An extremely important point on the RS125 is the attachement of the engine on the frame. The engine has no secondary shaft to equilibrate the mainshaft, this gives heavy vibrations the Honda engineers could master only by mounting the engine on silent-blocs (it's better for your balls !).
If the front silent-blocs are worn, the engine will move a lot in the frame and force the exhaust to break ...
These silent-blocs should be replaced frequently (at least twice a season), the rear nylon tubes could be changed less often. Before mounting them, I inject silicone in the new silent-blocs: they last more.

Here are a few mods I did on my bike:

The weakest point of the RS (until the 94' version) are the suspensions.
I replaced the fork, rear damper and steering damper by racing models from GCB-Ceriani (Dirk Raudies won the 93' world championship with this material). I can adjust the spring constraint and the hydraulic damping in compression and extension.
The original wheels are a bit heavy and I replaced them with magnesium Marvic Penta.
The brake discs and pads are from Braking. I usually use the original caliper absolutely correct for the stopping power but a bit heavy. I've tried some times a six-pistons ISR caliper (not very good, not rigid enough !) and a six-pistons Alcon caliper extremely powerful.
This last caliper distorded a disc (like a dome !) after a bad mounting, the pads were acting only on the outside of the disc which overheated it while remaining cold on the inside, a funny shape ...
I did a lot of testing for Braking and sometimes they gave me some special stuff like plastic brake tubes (3mm external diameter !) or aluminium discs. Keep cool, they're friends, they've never tried to kill me ...
The footpegs are replaced by Nylon bars, they cost less ...
The outer parts of the gearbox and brake pedals are also replaced by small Nylon bars for the same reason, they break without damaging the pedal itself