
Section 04
ENGINE
Sub-Section 02
(TOP END)
040-02-15
1. Measure here
CAUTION : Do not use acid core solder ;
the acid can damage the piston and cylin-
der.
– Take another measurement at the opposite
side of the piston.
– Repeat procedure for the other cylinder.
The lowest and the highest values of both cylin-
ders must be within the specified range.
If adjustment is required, replace cylinder base
gasket accordingly, using a thicker or thinner one.
RAVE SYSTEM
Theory
For a 2-stroke-cycle engine to have high power ca-
pacity at high crankshaft speeds, a high volumet-
ric or breathing efficiency is required and the fresh
charge losses must be minimized. The result is
achieved by opening the exhaust port early and
utilizing the resonant effects of the tuned exhaust
system to control fresh charge losses.
When an engine of this design is run at a medium
speed, efficiency falls off quickly. The relatively
high exhaust port effectively shortens the useful
power stroke and because the exhaust system is
tuned for maximum power, there is a large in-
crease of fresh charge losses. As a result, the
torque decreases along with a dramatic increase
of the specific fuel consumption. Higher torque
along with lower fuel consumption can be ob-
tained at lower engine speeds if the time the ex-
haust port is open is shortened.
Bombardier-Rotax has patented a remarkably sim-
ple system to automatically change the exhaust
port height based on pressure in the exhaust sys-
tem.
Located above the exhaust port is a guillotine-
type slide valve. This rectangular valve is connect-
ed by a shaft to a diaphragm which is working
against the return spring. One small passage in
the cylinder just outside the exhaust port allows
exhaust gas pressure to reach the diaphragm.
To the outside of the return spring is a red plastic
adjustment knob. Turning the adjustment in or out
changes the preload on the return spring which, in
turn, will change the RPM at which the RAVE
valve opens and closes.
TYPICAL
1. Slding valve
2. Diaphragm
3. Spring
4. Exhaust port
5. Adjustment knob
As the throttle is opened and the engine begins
producing more power, the pressure against the
diaphragm will overcome the pressure of the re-
turn spring and the RAVE valve will open.
Engine Squish
Gap
587
1.2 - 1.8 mm (.047 - .071 in)
717
1.3 - 1.7 mm (.051 - .067 in)
787
1.2 - 1.6 mm (.047 - .063 in)
A18C0OA
1
-
A18C01A
1
5
3
2
4
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