Possible Vapor lock?

Ken Ryan wrote:

Why don't you run a tank of 100LL and see if it goes away?

It did go away. I just descended and 1k ft and was gentle on the throttle.

I'm more curious if this is a Rotax problem (I presume not) or an RV-12iS problem, (beginning to sound more likely).

Rotax also published a safety notice: "Incorporation of the mandatory fuel return line of ROTAX Engine Type 912 (Series):"

https://www.rotax-owner.com/en/flightsafety/476-ai476

It strongly urges that ALL rotax 912 engine fuel return lines should have a restricter jet (to hep avoid vapor lock.)

The fuel restrictor on the 12iS is part of the fuel bypass fitting.  See attached. 

There is no need to insulate the fuel line on the engine itself.
Any Vapors in the fuel lines will form in the suction area before the Fuel pumps.
Most likely just after the Coarse Fuel Filter.
The pumps may become starved for liquid fuel on the input side, but the high pressure on the output side will condense the vapors back to liquid.

The Bulk Fuel Distribution Terminals had to purge their winter fuel stock by May first.
The Retail Gas stations had until June 1st to switch over.
If any of the fuel in the tanks was purchased more than 4 weeks ago, some of it might still be low vapor pressure winter fuel.

If the liquid fuel is at 90°F ...
100LL has a service ceiling of 22,000ft.
Summer E10 has a service ceiling of 14,000 ft.
Winter Blend has a service ceiling of 5,000 ft.

At 70°F, the Winter Blend will be good to 14,000ft.

There is an assumption in the auto world that you are not going to find temps above 90°F in January in Colorado.

Fly the pattern down to minimum fuel and then top it off with a fresh fill.
Or ... drain the tanks and put the fuel in your car.

Bill has this nailed (as usual), but I have to say that this story is much to common with Rotax owners.  The reason is that auto fuel is recommended for these engines, but very little attention is given to educating folks “why” using winter fuel in warmer temperatures is dangerous. If the typical certified aircraft used auto fuel, there would be an entire chapter on this in ground school, complete with a charts and graphs showing the “fuel ceiling” of various fuel vapor pressures.  But unfortunately I have never seen any educational material on this for pilots, and instead we are left depending on tribal knowledge, or personal experience.     

I also had this happen to me (one time) when using an ethanol free 93 octane fuel.  At the time I did not know enough to enquirer about the RVP of this specialty fuel, which turned out to be very high. This event prompted me to educated myself on this subject, and I even made a simple apparatus to test the RVP of fuel, and a “fuel ceiling” calculator.  It has not happened or me since, nor do I expect it to.  

Knowing why a problem happens is often the best way to avoid it, so anyone flying with Auto Fuel should consider learning about Fuel RVP (reid vapor pressure), how this affects the fuel system, and how RVP changes with the winter and summer blend fuels. The Rotax manuals touches on this briefly, but this topic does not get nearly enough attention. This would be a great topic for a video!

Avgas has a year round vapor pressure of 7-8 PSI.  Summer auto fuel is about 8-9 PSI, and winter fuel is as high as 13 PSI, which is where you can get in trouble.  Living in California it can be warm just about anytime, so when winter auto fuel is being sold, I run a 50/50 blend of Auto Fuel and 100LL.  Blending equal parts of winter auto fuel and 100 LL gives you an RVP of around 10, which even at 90 def F. is good up to 10k feet.  We get summer fuel earlier than other parts of the country, typically by may.