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Rotax and Hirth engines use the same carburetor. On early model engines, carbs used a
system which connected their two floats together, and had a small weeper hole in the
bottom of the float bowl. This lead to the carburetor always dripping fuel out of the
weeper hole at idle. It is recommended the this older style of carburetor be updated to
the new model which uses two separate floats and has eliminated the weeper hole.
Many two place Challengers use a dual carburetor
setup. When supplied from Rotax the
engine is set up for use in an upright engine application (spark plugs facing up.) When
used in an inverted installation, like on the Challenger, it is recommended that the two
carburetor intake manifolds be turned over. Proper installation should have the
carburetors at an upward angle.
All dual carburetor engines should use a DUAL FUEL PUMP, not a single fuel pump with a
"T". On the 503 Rotax SINGLE carburetor engine Rotax is now supplying the
engines with a DUAL PUMP. The two output lines run into a "T" with the one line
then running to the single carburetor.
The fuel pump should be mounted ABOVE the impulse hole in the engine with the impulse
fitting on the pump facing DOWN. The pump should be rubber mounted, away from heat and
located not more than 12 inches from the impulse outlet on the engine. The line used to
supply vacuum to the pump should be of a very thick wall so as not to collapse, and all
lines should be clamped.
On early model Challengers the carburetor was reported to turn in flight. Later model
engines are supplied with knurled intake manifolds and carburetors which prevent this from
happening.
Another update has the two separate vent holes found on the sides of the carbs being
replaced by one line with vent holes in the bottom. This prevents improper fuel to air
mixture in the carburetor.
In Canada many Challengers are using engines meant for use in snowmobiles. WARNING: Rotax snowmobile engines are different from those built
for aircraft use, and should not be used on aircraft!
Fuel pumps supplied for snowmobile application SHOULD NOT BE USED ON AIRCRAFT. The fuel
pump supplied by Rotax, FOR AIRCRAFT USE, has a small hole in the impulse elbow of the
pump.
This prevents fuel being caught between the pump diaphragm and fuel pump body, which
could cause pump failure. Also note that this elbow on the pump should be facing down,
thus allowing fuel entering between the diaphragm and body to drain out of the cavity.
Also check out Troubleshooting the Rotax engine series vol
1
Also check out Troubleshooting the Rotax engine series vol 2
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