My Engine Took a Piss!

[MileHighCTLS]

This is absolutely the strangest thing I have ever had happen with the Rotax 912. Here is the series of events:

1. I noticed fuel dripping from white tube:    
2. Looks like it is coming from here: 
3. Since I could find nothing visibly wrong anywhere, I decided to do a preflight of the engine, beginning with "burping the oil"
4. I slowly hand cranked the prop and the plane sounded like a horse taking a piss!
5. Gasoline was infiltrating the oil tank and coming out of the oil tank overflow rubber hose and onto the hangar floor!

I could find NOTHING on any ROTAX website or chat room or forum ... so, I did some a.i. research and my ChatGPT told me, "It sounds like your carburetor may have a stuck float or needle valve allowing fuel to leak into the engine. When you hand cranked the prop, it pumped the fuel in through the engine"

ChatGPT then asked me, "Have you done any recent maintenance or made any changes to the fuel system?"

As it turns out, the last time I flew (3-weeks ago), I had filled up with Avgas – for the first time ever – because I had run out of auto fuel. I thought one time wouldn't do any harm.

ChatGPT suggested: "A sudden change from auto fuel to Avgas could've dislodged debris, causing a float or needle to get stuck."

(BTW, one other possibility suggested was possibly a bad diaphragm in the fuel pump but I doubt this because it it relatively new and the dripping seemed to be confined to the left side of the intake manifold.) 

I have since drained all of the oil, which was mixed with fuel.

Now I am at a loss as to where to go from here?

I have never heard of anything like this before.

Any suggestions?

[GrassStripFlyBoy]

ChatGPT has you pointing in the correct direction, it is fuel shut off issue but you'll need to diagnose the specific carb (or is this an injected engine?).  Also, do you not close the fuel shut off valve?  That would not have prevented the issue but would have kept it from being so severe.  If the fuel was shut off, then there's a second aspect to this.

I've had a little fuel spill on the drip tray that was related to the height of the float arm that drives the needle being a bit out of tolerance, which allowed higher level of fuel in bowl.

So it may be the needle / seat being damaged, it may be a piece of debris, it may be the arm is out of tolerance, and if the fuel shut off was closed that needs replacement.

[Roger Lee]

You should always turn off the fuel when parking the plane. Gravity can cause fuel leakage and hard starts due to flooding.

I see this now and then. It's usually caused by a high fuel level in a carb. It's easy to fix. It can be caused by three things. First you need to weigh the floats. If they are heavy the fuel level will be high in the bowl. replace the floats if they are over 7 grams when weighed as a pair. Second it could be the brass float armature. It may need adjustment to the 10.5mm height. Third it could be the needle valve that controls the fuel flow. It may need replacement or the brass seat it's in needs to be polished.

Your welcome to give me a call if you need more info or help.

[FredG]

As noted above by Roger and Darrell, if the engine is carbureted, then a float may be heavy (saturated with fuel) or you may have debris (it doesn't take much) preventing the needle valve (operated by float level) from fully closing.  Damage to the needle and seat will also cause this, but are less likely to occur spontaneously than a heavy float or a bit of debris preventing the needle from sealing against its seat (together these are the two parts of the needle valve that controls the fuel level in the float bowl).  

I am not an aircraft mechanic but I have rebuilt these carbs on both of my 912 ULS engines and on my 1982 BMW R100RS motorcycle (that uses a virtually identical Bing carburetor).

[Tom Baker]

Shutting the fuel valve would have stopped this from happening, but that just prevents the issue from showing up. Oddly enough shutting off the fuel valve at the end of the flight is not on the factory checklist for the CTLS, but they do tell you to turn in on at the beginning. As others have said, there is a carburetor issue. The float valve is not shutting off the fuel and letting it overflow out both sides of the carburetor, to the intake manifold and airbox. The possible causes have already been mentioned.

[MileHighCTLS]

My AP recommends rebuilding both carbs (to be safe). Then, of course, changing the oil filter before refilling with oil. Then he recommends checking oil after running to be sure all of the fuel has been purged from lower end.

He doesn't necessarily think the avgas, after just one time, would cause this, but the sitting in hangar for 3-weeks, with avgas, could've contributed because he said avgas is not great for Rotax engines.

[Roger Lee]

Avgas wouldn't cause this and the carbs don't need an overhaul. You just need to address the three things I listed above. This isn't that uncommon. Plus you don't need to purge out your oil. Go ahead and change the oil, but any residual  fuel will vaporize and go out the vent tube when you get the oil temp up over 200F. Like I said what you're going through isn't that uncommon. I talk to a number of people every year about this very thing and it's always easy to correct.

Here's why an overhaul won't help. First you need to weigh the floats, then pop the carb off and measure the brass float armature height to 10.5mm. If this doesn't stop it then you'll need to remove the float needle and polish down inside. If you overhaul the carbs and it's the polishing you need the overhaul won't help. If the floats are heavy and you need to only adjust the brass float armature then an overhaul won't help. Might as well just do the three things. Just throwing hundreds of dollars ($500 - $750) at it for an overhaul may not fix it so do the most common cheapest fix first.

This may also go away if you make sure you shut the fuel valve off when you're done flying for the day.

[Tom Baker]

I agree with Roger on this. One thing that I might add depending on the history of the carbs is to replace the float needles, part number 261-708 and 261-710 clip. The tips of the needles do get hard with age, and they don't seal as well.

Honestly if it were my airplane I would change the oil and weigh the floats. If they are within limits I would run the airplane. If it runs okay I would fly it, and make sure to shut off the fuel after each flight and move on.

Also just like Roger I have seen this before. Once I had a seat that needed polished, and that was right after a carb overhaul. That one cause a rough running engine. The other time it was heavy floats.