How to Fix Rotax 912 Carburetor Leak

New 2022 carburetors for 1994 Rotax 912 UL

For more than 27 years, the Rotax 912 carburetors of our plane delivered trouble-free performance. 

That changed suddenly. It started innocuously enough: a small drop of fuel out of the cowl seen during a preflight check in Albuquerque, NM.

Guess 1: Expanding fuel in the center tank

Probably overfilled the center tank during the recent fueling. The fuel has expanded, hence the drop of gas on the floor. Let’s ignore this and fly back to Dallas.

Back home, next Sunday. Another drop of fuel on the floor after moving the plane to the front of the hangar. The expanding-fuel explanation is wrong.

We remove the top of the cowl. The underside of the air filter of the right carburetor is wet with gas.

Guess 2: The needle valve in the float bowl no longer closes properly.

No wonder. The needle is more than 25 years old. Easy fix: We order two needles for the two carburetors.

When the new needles arrive, we open up the carburetors, pull out the old needles.

Surprise: The needles show no sign of wear. No matter, there must be subtle wear. We replace both needles, look at the seats of the needles.

They are pressed into the carburetors, cannot be replaced. Guess that means they never wear out. We install the two needles.

Next day: Both carburetors are wet with fuel. Oh my, failure.

We go on the Internet and search for "leaking Rotax carburetors."

Several sites say that even low fuel pressure may cause the needles to leak tiny amounts.

Since the top of the center tank is above the carburetors, this case applies.

Guess 3: Installation of a shut-off valve for the main tank will solve the problem.

There is already a shut-off valve that friend Mel and I installed when we built the plane in 1994. Well, he was the expert builder, and I the apprentice.

That valve is almost inaccessible, right behind the firewall under the main tank. Very hard to turn, too. Used it in the past only during repairs.

What to do? Must install a new valve that is easily accessible and easy to turn. This is not a trivial problem.

Detour: Key rule for fuel shutoff valve

No matter where and how the valve is installed, the following rule must be maintained: The gascolator must remain the lowest point of the entire system except for the wing fuel tanks, since they are equipped with sumps.

Due to this rule, hoses weaving up and down to an accessible shut-off valve are not allowed.

Solution: Remotely operated fuel valve

We buy an easily turned fuel valve with small handle from Aircraft Spruce and construct a small torque tube with a handle so we can operate the valve from the panel.

Fuel Valve. Photo from Aircraft Spruce catalog

The entire system is held together by a single AN 3 bolt. When it is removed, one can pull out the short tube twisting the lever of the valve, the torque tube, and the handle sticking out at the panel.

Torque tube operating fuel valve below main tank

You can read "FUEL," so valve is turned on.

You can't read "FUEL," so valve is shut off.

We install everything and proudly go home. Problem solved.

The next day a first operational check. We open the valve, sump the gascolator, and turn on the electric fuel pump. After about five minutes both carburetors start leaking.

We aren’t out of ideas, yet.

Guess 4: The floats pushing up the needles, now more than 25 years old, likely don’t work any more.

We order two new floats from Lockwood Aviation, install them, turn on the fuel pump, and . . . both carburetors are still leaking.

Must be the seats of the needle valves. But they cannot be replaced.

First correct idea: Call Lockwood Aviation

"Oh yes, we know of this problem. We can try to rework the needle seats and test. If the needles then seal, you are in luck. Otherwise you need to buy two new carburetors since Rotax doesn’t sell the carburetor body as separate item."

We send the carburetors to Lockwood. They try to rework the needle seats, but cannot get the needles to close properly.

Conclusion: We must buy two new carburetors.

Astonishing, isn’t it? A trivial thing such as the needle seat cannot be replaced, and as a result we must replace both carburetors. The photo at the beginning of the post shows one of them, with proper modification for our rod actuator system for the throttle.

The only nice aspect of the entire repair process: We now can turn off the fuel supply to the engine with minimal fuss on the ground and in the air.

The "in the air" part would be important if there ever was an emergency situation such as a forced landing with a failed engine.

Installation

We install the carburetors and check for leaks: The carburetors do not leak anything during several minutes of pumping with the electric fuel pump.

Next we balance the carburetors as described in Synchronization of Rotax 912 Carburetors. A small adjustment achieves full synchronization.

A surprise: The factory setting for idle is exactly right: The engine runs smoothly at 1,800 rpm.

Test Flight

The first flight includes a pleasant surprise. Previously, when we reduced power to idle in the air, for example during landing, the engine would shake quite a bit. To avoid this, we always used some higher rpm, which complicated the landing approach. Our interpretation was that the engine didn't like being pushed by the propeller.

Not so with the new carburetors. When we now pull power in the air, the engine continues to run smoothly. There is just a bit of vibration, nothing like the previous shaking.

Fuel Consumption

During cross country flights, fuel consumption has been reduced. Flying at our typical cruise power of 4,400 rpm, fuel consumption has dropped from 3.5 to 3.2 gal/hr, a 9% reduction. 

The reduction has little impact on costs, but extends the range. We can fly 5 hours and still have more than an hour as reserve. This is particularly important when nasty headwinds slow us down.  

Conclusions

The leaking carburetors motivated us to install an easily operated shutoff valve. That's an important improvement.

The replacement of the carburetors was costly and annoying. On the other hand, Rotax apparently has changed the design of the carburetors to improve performance at low rpm. On the ground, engine idle is smoother. In the air, there is no more shaking when we pull power. On top, fuel consumption is reduced.

Have any questions or feedback? Please share your thoughts in the comments.