Rotax four stroke aircraft engines have proven themselves as reliable pieces of machinery. They can be bought as experimental (UL) or certified (A and F) engine and there are four main models (injected too) to date. The original engine is the 80 hp 912, from which the 100 hp 912S was developed and Rotax also has a 115 hp turbocharged 914 engine. In 2015 Rotax announced the 135 hp fuel injected turbocharged 915iS.
Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the french market), the 912iS is green but without the carburetors and the 914 has red covers and a turbo. The 915 is a turbocharged version of the 912iS with blue covers. These are the most obvious differences.
Maintaining these little engines is not much of a problem as things are just a little bit different compared to standard aero engines. Below some tips and tricks. Keep in mind that this list is not complete, visit flyrotax.com for the latest technical information.
As time goes by, field experience by several builders (myself too) have resulted in a number of items which require some more love and attention to keep the engine running happy.
The engine is equipped with a standardized electronic ignition unit, see Service Instruction SI-912-013-914-016. Personal experience has led to the following recommendation: when flying near high powered radio transmitter stations do keep a safe distance from those antenna towers. Normally you need to be 1000 ft higher to any obstacle within 600 meters, in this case stay at least 2 miles away.
Certain radio stations can emit high powered electromagnetic signals and flying close to them has the effect that these radio signals sometimes can be heard in your intercom. If this is the case, the electronic ignition can probably 'hear' them too.
The result of these unwanted signals in the electronics will be that the ignition might be firing irregular/erratic and could even fail at some point and the engine will start running rough, missing power strokes or even stop working altogether resulting in an off airport landing.
Also, make sure that the spark plug connectors (VD05FMH, resistive 4.4 to 6 kΩ) are properly installed and make good electrical contact with the high voltage cable, rough running and/ or un-even EGTs may result should this not be perfect. Cleaning the spark plugs can be done with an ultrasonic cleaner, which is the best method. If in doubt: use new ones!
You can verify the high voltage circuit by disconnection the spark plug connectors and measuring the resistance: from the spark plug connector (4,4 - 6 kΩ each) through the high voltage coil (6,1 - 6,7 kΩ) and the other spark plug connector for a total between 14,9 and 18,7 kΩ. You will find the schematic in the operators manual (electrical circuit: cylinder 1 to 3 top/bottom and cylinder 2 to 4 top/bottom).
With an intermittent rough ignition check and a normal drop, in some cases the core of the high tension coils have been the problem due to corrosion. These may oxidize and split.
Both ignition units and the coils need good ground contact with the engine and to the airframe/ battery (no ground loops). If this is not the case then expect either rough running, no drop at the ignition test or even a stoppage at runup. Intermittent RPM drops while flying are also possible due to vibrations.
The wires in the older looms were of such quality that they would break eventually, with the above mentioned results. Update: newer engines have a much higher quality teflon/ tefzel-like wiring which does not chafe and damage its insulation so easy anymore. Should you have an older Rotax, do check the wiring annually.
The yellow wires from the stator run to the regulator/ rectifier and under high load (>15 amps) these could get warm. If possible, replace them with a good quality tefzel wire capable enough to handle these loads.
The builtin alternator can only deliver about 15 amps at 5000 engine RPM. Some installations need more power so the manufacturer or homebuilder then chooses to install an auxiliary alternator driven by the gearbox or with a v-belt. The latter option is preferable as these can deliver enough power at idle engine RPMs where as the gearbox driven can not.
The heat stress at the turbo (914 and 915) may cause the stainless steel brackets to become brittle after some time. Make sure to check these for cracks at every oil change/ annual. As the turbo sits at the lowest point on the engine, oil wants to go there. When the ceramic seal is not so fresh any more, the oil may exit the turbo at the exhaust or in the compressor section. Oil then enters the air hose going to the carburetor 'lubricating' it and it will slip off and the engine loses its turbo pressure and power.
The waste gate needs attention too in the form of regular lubrication at the annual or 100 hour check. Make sure it and the bowden cable operates smoothly.
For a smooth running engine both carburetors need to be mechanically synchronized (step one, and make sure to tighten the locknut on the throttle cable adjustment when done). This step also involves making sure both throttle cables work in sync opening the carb simultaneously, if they do not, you will not get the carbs into pneumatic synchronization (see below).
For this you need to synchronize them using a pair of manifold pressure gauges to make sure there is no imbalance (step two). This problem usually shows itself as rough running between idle up to around 3500 RPM. The Rotax maintenance manuals are very clear on the procedure you will need to follow.
Also, have the carbs rebuild every 500 hours or so as the rubber parts tend to deteriorate due to alcohol in the fuel.