The exhaust power valve is definitely one of the spare parts that need to be purchased more often on the RS 125. Often the valve is not installed at all on restricted bikes and has to be retrofitted or it is damaged due to lack of maintenance and improper repair attempts.
The original part now costs about 160 €, while replicas are available for less than 30 €. Of course, one has to ask oneself if the difference in price is a risk.
This page shows an overview of different exhaust power valves available on the market and tries to show the differences between the different models.
- Motorize (Power Valve Kit)
- Moto RRT
- Noname aluminium replica
- BC Engineering
- Michelotto Racing
- Italkit (Pneumatic)
Original Power Valve
The original power valve is made of coated cast aluminum.
It is not a decorative coating, but a technical coating with a high surface hardness. The original part works, is easy to clean (combustion residues can be brushed off the coating without damaging it) and can last for the life of the engine if properly maintained.
Old power valves (extended blank valves)
Older Rotax 123 engines, before the RS 125, had a steel power valve. The flat part of the slide is identical to the blank valves which were also used later on and were installed on factory throttled engines.
From time to time these components are still offered and sold in used condition. Since they are made of steel, they are much heavier and possibly also more stable – but since the most critical load is caused by the impact when closing the valve, the high weight is not an advantage.
Replica parts and tuning parts
Basically: Both about the original part and the replica part you can find in various online forums or online stores reviews both reports of massive problems and statements that the parts can be driven without problems over a long time. There is no miracle product with an unlimited lifetime.
Replica power valves have been around since the RS 125 became popular, around 2000, but since the parts are not always branded and have been built and sold by different companies over these decades, the quality cannot be generalized. They are or were made of chipper-friendly aluminum, high-strength aluminum, stainless steel or promotional material fantasy names.
The valves all differ in small geometric details. This ultimately leads to the fact that all of them in the closed state come close to the pistons differently, function differently effectively and are differently safe.
Motorize Power Valve Kit
The Motorize shop offers a complete kit consisting of valve, seals, spring and housing. Vavle and housing are each compatible with the original components. Especially the housing is interesting because it costs only about one third of the original part. Also the slider is extremely cheap with a price of only 24,95 €.
Before the transition from flat to round, the slider is wider on the flat side than the original part, resulting in a material cross-section of approximately 140 instead of 125 mm². At the transition from flat to round, a chamfer is still visible, which further reduces the risk of breaking at this point.
There are small deviations in the geometry compared to the original part.
When closed, this valve is at about the same distance from the piston as the original valve. When open, a deviation of about half a millimeter from the original geometry becomes apparent, resulting in a small step in the exhaust port. Should the power valve break, it cannot slide into the piston if the guide notch in the cylinder is intact.
A problem is the spring included in the kit, which with a spring constant of approx. 2.4 N/mm and an unstressed length of 35 mm produces a much higher restoring force than the original spring with a spring constant of 0.94 N/mm and an unstressed length of 30 mm. This can lead to the fact that the magnetic coil can no longer open the slider and leads to a more violent shock at the end of the closing process.
Power Valve by Moto RRT
An interesting replica exists or was available from Moto RRT. Material, processing and coating appear of very high quality at first glance. The price of £ 50,00 seems to be cheap without taxes and shipping, but with shipping costs and taxes from Great Britain I had to pay a price of 96,99 €.
Here, the angle of the front surface is slightly different from the original, so that no exactly flat surface is created. Also the bevel at the front edge is missing, here the surface is not tilted backwards but at right angles to the plane. It is noticeable that the valve comes extremely close to the piston when closed.
This small distance to the piston is problematic when the first signs of wear and tear become apparent. If the power valve regularly strikes the cylinder, it hammers a small imprint on the surface of the cylinder over time. In this case the slider would sit too close to the piston and touch it. It would also be problematic if the slider would break off, because it would not be held back in time by the guide notches on the sides of the cylinder.
Noname power valve replica
A further valve replica without further information about the manufacturer for 24.99 € is sold for example on eBay. At first sight chatter marks are visible on large parts of the surface. This is especially annoying on the upper stem, because here the valve stem seal is supposed to slide. Also the pocket and the groove for hanging the cable are bigger here than on other models.
The replica is similar in fit to the Motorize replica, but again the original geometry is not exactly reproduced. At the transition from the flat part to the round stop, the material cross-section is also larger than the original part. The front edge is cut off at right angles instead of a bevel like the original part and is much wider.
The cheapest replica valve that can be found looks exactly like this.
BC Engineering
You can get a high-quality replica for 54,90 € from BC Engineering. The slider was coated with a “hard nickel coating” (according to the sales description).
Also regarding the accuracy of fit in the open as well as in the closed condition one cannot complain about this replica: This valve reproduces the original geometry most accurately of all replicas.
Michelotto Racing
A replica, whose geometry is well adapted to the exhaust port itself, is available on eBay from Italy for 59,00 €. The slider from Michelotto Racing is coated with a hard anodized layer.
Also the bevel at the front edge is correctly directed backwards here. Unfortunately this slider was the thickest of all replicas, small dirt in the slot must be removed before installation. The distance to the piston is also small compared to other replicas, in total and because of the correct backward pointing bevel this distance is not critical.
This model is my personal favorite in terms of engine power because of the successful, optimized form of the reproduction of the surface in the exhaust port and the small but even distance to the piston when the slide is closed.
Italkit Pneumatic Power Valves
Italkit offers two pneumatic power valves which replace the conventional RAVE consisting of control electronics, magnet, cable pull and valve mechanism as an independent assembly.
This pneumatic power valve assembly is available in two versions:
- A simple version that opens the valve slowly between 5,000 and 6,000 rpm
- With an attached setting mechanism with which the opening speed can be set in 9 steps in a range from 6,000 to 8,500 rpm.
Basically, this system works in such a way that the pressure in the exhaust acts on a surface that converts this gas pressure directly into a force that is proportional to the pressure. This is opposed (in both parts) by a spring force, which in turn depends only on the spring compression. If the pressures in the outlet channel are high enough, the diaphragm will open the vavle. Although the pressure in the outlet channel is pulsating, it increases on average with speed. It is strongly dependent on the installed exhaust system.
The vavles themselves differ only in the shaft, which is screwed in the simple version, the component with adjustment possibility has an outlet slide, which was manufactured in one piece.
Both exhaust valve assemblies have the disadvantage that the sliders do not lie flat in the exhaust channel when open, this is worse in the basic version. In contrast to the electronic RAVE, the opening speed can be adjusted for one of these products, but not the overall stroke (for both).
In the simple version, it is noticeable that it is not flush with the outlet channel when fully open, and the distance to the piston is also large.
The version with adjustment option opens a little wider and is therefore much better suited for high performance. However, since the sliders are otherwise geometrically identical, the slider does not reach far to the piston.
In terms of service life, the pneumatic power valves are clearly superior to the electronic version, since the slide is not accelerated to the hard stop by the spring. The pneumatics also make closing significantly slower due to the counterpressure that is still present.
Comparison of all power valves
The images of the individual sliders can be browsed in the following gallery views to compare them directly.
