Also note that the Rotax dipstick’s “fill” and “top” lines are not
a quart apart. (Just as with any other engine, the Rotax doesn’t
like to be low or high on oil.) Determine the true meaning of the
marks on your own airplane. Since the engine has a dry sump, the
“proper” level is a function of where the oil tank is mounted and
which end of the airplane has the third wheel.
Coolant is special and must be checked with the engine cold at
the pressure cap. Be sure there is some amount of coolant in the
overflow tank, too. Leaks—oil or coolant—should be investigated
and addressed before engine start.
Once you’ve checked the levels, you’ll climb inside and note
that there is a choke but no mixture control. The Rotax’s Bing
carburetors are of the constant velocity (CV) type, and they do a
decent job of metering air while taking density altitude into
account, so Rotax doesn’t let you mess with mixture directly; the
choke makes starting possible.
Startup (turning the key) is familiar, but with a twist: it
requires a closed throttle. Warm-up may feel unusual; if the Rotax
idles too slowly (below about 1100 rpm), it has a tendency to
cause gearbox chatter. This noisy low-speed running warns pilots
that they’re using too few rpm. The choke is then gradually
closed; when the engine runs smoothly and responds instantly to
throttle at 2000-2100 rpm (without choke), it’s warmed up.
Since the 912 is primarily water-cooled and has a remote oil
sump, it may take a little longer to reach operating temperature
than an air-cooled, wet sump engine. There is an aftermarket oil
thermostat that lets a small amount of oil quickly reach operating
temps. Either way, it takes a while for all the oil to get happy; give
it ample warm-up time.
Speaking of oils, do not operate your Rotax with typical aero
oils. It’s a high-rpm engine with a gearbox and a wet clutch, so
the closest analogy would be seen in a wet-clutch motorcycle
rather than a standard aircraft engine or a car. (Auto oil generally
contains “friction reducers” that are quite effective at making a
wet clutch slip.) Unlike classic engines, the 912 runs its camshaft
100LL leaves deposits in the oil, requiring frequent tank cleaning and
twice-as-frequent oil changes. Additionally, lead deposits build up
internally, as seen here.