Vapor-lock and the Rotax engine

[ibgarrett]

I was giving rides for some friends of mine this weekend and ran into a new problem that I haven't encountered. After the 2nd ride of the day I had landed and picked up my third passenger. For obvious safety reasons I had shut the engine down between each ride.

The day was hot - between 95 and 98 degrees with the density altitude of around 8,000 feet (here in the Denver metro area). The first two flights went off without a hitch. The third flight, I taxied out, did my run-up without a problem and then taxied out onto the runway. When I throttled up to take off, the engine suddenly started to vibrate and loose power, going from around 5500 RPM, dropping down to 4300 RPM. I immediately throttled back and called and abort take-off and taxied off the runway.

The first time I thought maybe the carbs got fouled as I had added fuel right before the 2nd flight and the fuel tanks were just recently filled up at the FBO, so some debris might have gotten mixed up. The engine idled normally, so I taxied back to the run-up area, did another run up, paying close attention to the drop with each mag check - no problems there.

The engine was within the normal operating limits of heat for the CHT and the EGT, so I taxied back and with the exact same problem. I called it a day at that point in time because I wasn't sure of what was going on.

My partner in the plane is also a licensed Rotax mechanic for our plane and operates a repair business for dozens of LSA planes in the area, so my first call was to him. After he thought about it for a bit, he said he experienced the exact same issue a couple of years ago under very similar circumstances. He spent weeks at the time trying to determine exactly what the cause was, working with Rotax and various other sources. The only thing they could come up with, after not being able to reproduce the problem, was that it was possible that it was vapor lock.

Never-the-less, I canceled my flight for the next day until after I could get the plane in for a good once-over from the mechanic. The plugs have all been checked and the plane has been determined to be safe again. Given that he's had this problem before I'm comfortable that I experienced the same thing he had run into with the combination of the hot day and multiple starts and stops in a short period of time.

My former instructor who has over 4,000 hrs of time instructing happened to be nearby as I taxied back off the runway. He and I spent a few minutes double-checking to make sure I didn't miss anything. From his observations outside the plane he said the plane looked to be running rich and during a couple of times of trying to restart the plane, some black smoke came out of the exhaust. At that point in time I just pulled the plane back to the hanger as I didn't want to even try turning the engine over again.

I'm reasonably new to flying with around 150hrs of time and this was my first true experience of an engine problem. I'm very comfortable in how I handled the aborted take-off (first time I've really HAD to do that) and I'm very happy that I was at a non-towered airport to do it at. I was more bummed out that I wasn't able to take a few other folks flying that day.

I'm writing this up not so much as to solicit advice (although any and all is appreciated), but to pass along my experience in case others may encounter the same or similar issue of what our findings were.

[3Dreaming]

A customer had a problem like this a month ago. After checking things like plugs ect. we did the 200hr carb inspection early. the airplane is 3 years old with a 170 hours. There was a bunch of black junk on one of the slides, and I think this was causing the carb to stick. After the inspection everything seems fine. If you can get to the back of the carb you can stick your finger in and move the slide to see if it is sticky. Tom

[ibgarrett]

Good to know - I'll definitely pass that along. Thanks.

Brian

[drseti]

Thanks for sharing this experience. Which Rotax engine are you running, Brian - 912 ULS, or something else? What kind of fuel are your burning - avgas or mogas? Ethanol or not? Any fuel additive used? Did you try carb heat? Did it make any difference? This may give some clues as to whether you were experiencing vapor lock, or something else.

[FLA-CFI]

Was the electric fuel pump on or off?
What was the fuel pressure indicating?

[ibgarrett]

Paul,

The engine is the Rotax, but I know we have the 100hp Rotax not the 80hp one, so I'm checking on the exact model. We were burning to the best of my knowledge mogas - but it's probably a mixture of mo/avgas as we do fly it around to other airports that don't offer mogas. I had just put 6 gal in of mogas, which doubled the tank volume. There's been no fuel additives that I've ever put in and I really doubt that our other partners would be putting any in either.

The plane we're in (a StingSport) there's no option for carb heat, so even if I wanted to try it, there's no lever to pull for it.

The electric fuel pump was off. I can't say that I looked at the fuel pressure...

All great questions quys - I've got a lot more checking to do on this.

Thanks,

Brian

[drseti]

Paul,

The engine is the Rotax, but I know we have the 100hp Rotax not the 80hp one, so I'm checking on the exact model.

That would be the 912ULS. Useful information, as that's the engine most of us on this forum are using.

We were burning to the best of my knowledge mogas -

OK, then, critical question: are you 100% sure that the mogas you're using is ethanol-free? I know that ethanol can increase your susceptibility to vapor lock.

there's no option for carb heat, so even if I wanted to try it, there's no lever to pull for it.

Oh well, one less thing to worry about.

The electric fuel pump was off.

Sometimes (not always) turning on the boost pump can help to clear vapor lock. If this happens again, it's something to try.

Very glad you made the prudent decision to abort the takeoff and terminate the flight.

[dstclair]

FYI.

The Sting has fulltime carb heat using a product from SkyDrive.

[drseti]

FYI.

The Sting has fulltime carb heat using a product from SkyDrive.

thanks for that piece of info, Dave. I guess we can therefore rule out carb ice as a contributing factor.

[garbageman]

Had the same symptoms on my Tecnam Eaglet with 912ULS. Cleaned the black gunk off the carb slides and engine ran fine for a few flights, then problem came back. Pulled the carbs and did a complete cleaning, installed rebuild kits, and reinstalled the carbs. No more problems. I run 100LL.