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Can Rotax ULS operate on one Carb?

I had an engine quit on an RV-12 flight in a power off stall. It restarted and and ran a little rough back to the airport. At taxi RPM it barely ran. When the cowling was removed I found the left carburetor had pulled out of the socket at about a 20 degree angle due to not being fully engaged after removal and replacement. That’s the closest I’ve been to one carburetor operation.
 
It depends on what you call “running on one carb”. I suspect if you are flying at cruise altitude and somehow one carb acts-up and decides not to flow fuel/air mixture, you could power-back and land safely. Other than that, no, the engine would operate so roughly that it would possibly shake itself right out of the engine mount. Last year I experienced vapor-lock on takeoff. Engine was running 100% power and at about 500’ altitude the engine lost significant power and shook so violently I thought the engine came loose from its mount. I throttled back and engine ran slightly smoother with something way less than 50% power. I was able to execute 180 turn and land on runway - I know, you’re not supposed to do that…
 
The Rotax 912 iS is fuel injected, all 4 cylinders run off of the same throttle body. This engine either runs, or it doesn't run. No worries about balancing carbs or them falling off and running on 2 cylinders.

Maybe you'd like to consider that engine option instead of the 2 carburetor model.
 
Carbureted Rotax (ULS as example) are virtually 2 half engines joined at the crank, kind of. Left intake manifold & carb feed only that side of the engine. Same for the right side. So if one carb became in-op, only one half of the engine would run - extremely rough, you would think the engine was trying to tear itself off the mount.
 
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half engine works fine

Sorry to say so, but it's time to confess: I tried it.
Not on purpose but still:

apparently distracted after some technical work I performed a TestSpeedRun (some 2 - 3 flight hours...)
All looked well (for what I knew that time) at run-up, so I ran the speedrun.
This runway is fairly short, so at one time I was afraid not to be able to stop on time: Let's go!
I know y'all recognize, but I fell into the pit.
At 100ft or so I noticed there something definitely wasn't ok: fairly shaking engine. Looking for a place to sit down I noticed we still flew, so I took short circuit. Landed uneventfully, and after taxiing back opened up the cowling - only to discover no single top-plug was attached.

My '12 took off and landed safely on 4 bottom-plugs only.
Not to be repeated though, but from now my trust in this little engine has doubled.
And yes, I share this story and my shame where ever I can.
Thanks for reading, laughing and not-judging.
 
Thanks for sharing Hans!

Now imagine if the plugs on the Left side were connected, but the Right ones were left off. (similar result to in-op carb situation asked by the OP). I think you would have known there was a problem in less than a second.
 
When you ask what if only one carburetor is operating you need to think about what the failure mechanism is on the bad one. For example if the air filter was blocked on one side I would expect it to operate at reduced power, but if a carburetor completely fell out of its socket the resulting air flow into the induction manifold might make the engine not run at all. I don’t plan on testing the theory! I’m not sure it matters. Any time the engine is not performing as you think it should get on the ground and figure it out ASAP! I’ve had three partial engine failures: carburetor partially out of the socket on a 912ULS during cruise, stuck exhaust valve on a Continental O-200 during takeoff roll and broken fuel pressure sensor line in cruise on a Lycoming O-360 (pumped one tank dry). Answer was always the same: get on the ground and figure it out.
 
Carbureted Rotax (ULS as example) are virtually 2 half engines joined at the crank, kind of. Left intake manifold & carb feed only that side of the engine. Same for the right side. So if one carb became in-op, only one half of the engine would run - extremely rough, you would think the engine was trying to tear itself off the mount.

Exactly what Ralph says…

I flew my Sling 2, up just before Christmas, a beautiful, uneventful flight just over an hour. Landed and decided to top off with 100LL (I normally run 91 no ethanol) as the plane wouldn’t be flying thru the holidays and 100LL would keep the octane better than mogas.

Topped off 18 gallons, went to start up AND the engine wanted to jump out of the cowling. Scared the living **** out of me. Long story short, it wasn’t the fuel, but a heavy carb float. The floats were to be swapped out at the annual in July, but they sent only one pair of floats and no gaskets. We differed to my next oil change which would have been in 6 more hours.

Not sure what the outcome would have been had this happened in the air or on power reduction landing and hope I NEVER find out. I have faith in ROTAX, as a power plant. I am meticulous with servicing and have never had any of my previous 3 aircraft AOG for anything engine related, other than a flat tire.

So, my question, had this happened in the air, would switching right or left from Both, like a magneto, on a Lycoming or Continental have isolated the bad Carb?
 
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So, my question, had this happened in the air, would switching right or left from Both, like a magneto, on a Lycoming or Continental have isolated the bad Carb?

No, each Capacitive Discharge Ignition module fires two sets of coils – one coil for (2) left plugs and one coil for (2) right plugs. Wiring is done in this manner so that if a CDI quits all four cylinders will still have one plug firing. So, to answer your question, the ignitions can’t be used to isolate carb problem on Rotax 912ULS.

Lycoming and Continental can be similar where one magneto fires top plugs on left side and bottom plugs on right side and vice versa.
 
No, each Capacitive Discharge Ignition module fires two sets of coils – one coil for (2) left plugs and one coil for (2) right plugs. Wiring is done in this manner so that if a CDI quits all four cylinders will still have one plug firing. So, to answer your question, the ignitions can’t be used to isolate carb problem on Rotax 912ULS.

Lycoming and Continental can be similar where one magneto fires top plugs on left side and bottom plugs on right side and vice versa.

Thanks Piper, much appreciated.
 
John,

I’m curious as to why you think magnetos would identify a bad carb on Lycoming or Continental. We’re you thinking of using the ignition switch to identify bad plugs? Shut off one ignition at a time. The rough running mag or ignition module at least identifies which half of the plugs has the bad one.


Rich
 
Post 6 mentioned that his motor would not run without effort on only the lower plugs.

I would expect an aircraft motor to operate in a normal matter on either one of the ignition systems.
I sure hope there was a follow up on this situation.
 
John,

I’m curious as to why you think magnetos would identify a bad carb on Lycoming or Continental. We’re you thinking of using the ignition switch to identify bad plugs? Shut off one ignition at a time. The rough running mag or ignition module at least identifies which half of the plugs has the bad one.


Rich

Rich, yes, before figuring it was a carb issue. I had a Mag problem in my Tiger way back when on a trip to Florida. Bad magneto. This time, at first, I thought the same (my limited knowledge) then I observed 2 cold and 2 hot EGTs on the G3X. The lightbulb went off, heavy carb float, which thankfully all it turned out to be.
 
Thanks for the reply. I will add EGT and CHT to my grey matter as a possible indicator of heavy floats. If memory serves me the coil fires plugs on opposite sides of the engine while the carbs (if balanced) serve each side of the engine. Seems that would help resolve if the problem was a carb or ignition. Can anybody that had heavy floats confirm that?
 
Correct me if I am wrong but doesn't each carb provide fuel to only two cylinders, so if one carb failed you would be running on two cylinders? I am by no means and expert on this.

Rob

Does anyone know if one of our Rotax ULS carbs failed (ice, filter...) would the engine run on one?
 
Correct me if I am wrong but doesn't each carb provide fuel to only two cylinders, so if one carb failed you would be running on two cylinders? I am by no means and expert on this.

Rob

Not exactly true, the balance tube will transfer some air / fuel mixture to the other side.
 
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