Low Engine Temperatures on Rotax 503 and 582
The fuel mixture on the small Rotax engines used on Challengers has a profound effect on the engine temperature.
A number of factors affect the fuel air ratio.
Even changes in ambient temperature from season to season affect the fuel air mixture and thus the engine temperatures.
In areas such as the northern United States and Canada where the outside temperatures change vastly from season to season the engine user must make adjustments to the carburetors to accommodate these changes. If not, temperature problems will occur.
A new Challenger owner may not be aware of these issues.
One new Challenger owner purchased a used aircraft from the northern US and moved it down to the southern states where the airplane would now be flown.
The pilot soon noticed that the engine temperature never climbed into the normal operating zone indicated in the engine manual and mentioned by other Rotax users. The other symptom that this pilot noticed was very high fuel consumption, much high than the 4 gallons per hour that was expected.
The reason for this was the change in climate. In the northern US the ambient temperature was much lower and the aircraft was purchased during the colder months. Now operating in the hot summer months of the southern climate, an adjustment to the mixture needed to be made.
The carburetor needle has 4 grooves that enable the fuel flow to be adjusted to match the changes in the ambient temperature. A circ-clip at top of the needle must be moved down to raise the needle for use in lower outside air temperatures. The circ-clip must be raised up so as to lower the needle and reduce fuel flow for higher ambient temperatures.
The rule of thumb is simple:
Temperature Down = Circ-clip Moves Down
Temperature Up = Circ-clip Moves Up
Further information on carburetor adjustments for outside temperature changes is found in the article Seasonal Carburetor Needle Adjustments.
The other situation that will affect the fuel air mixture and therefore also the engine operating temperatures is altitude.
One Challenger owner that lived in the Rocky mountains began operating his aircraft at an airport with an altitude over 6000 feet ASL. He noticed that his engine operating temperatures were very low. Why?
The Bing carburetors used on Rotax engines are factory set for use near sea level. For the vast majority of engine users this is acceptable.
As we climb in altitude air density decreases and this changes the fuel air mixture ratio.
To compensate for this change the user needed to replace the carburetor jets with ones optimized for his high altitude.
The jets are located at the bottom of the carburetors and feed the fuel to the engine. Whereas the needle varies the flow, the jet sets the baseline fuel flow volume. The size of the jet is a reference to the diameter of the orifice drilled in the jet.
Consult your nearest Rotax engine dealer for the jet that is most suitable for your ambient operating temperature and altitude.