Rotax 912 ULS failure after takeoff

Hey Guys, 

I am just looking for any advice that anyone may have regarding a recent engine failure I had with my Rotax 912ULS.   The engine and my home-built aircraft (VM-1) both have 170 hours on them.   All flying has been done by me and the engine has never previously had an issue.   Last week the weather was relatively hot (32C/ 90F) and humid.   I was running 98 octane automotive gas, which I had purchased that day and strained prior to flying.   There was probably a small amount of leftover 100LL in the tanks from previous flights.   Run-up had no issues, and takeoff was uneventful.  I climbed out at 60 knots/1800FPM which gave me a nose-up attitude of about 20 degrees.   This is a bit more aggressive than I normally climb out, but nothing I haven't done before.   At about 1 minute after takeoff the engine sounded like it was sagging slightly.   Nothing major, but enough to start listening carefully.   Then about 2 seconds later it completely failed.   I immediately pitched the nose down, switched fuel tanks and applied carb heat.   My auxiliary fuel pump was on at the time as I was still in the takeoff climb.   The engine never fully stopped and caught back in after windmilling for about a second.   There was also a pretty strong burning smell came into the cockpit for only a couple seconds then went away completely.  It didn't smell like a fire, more like a very hot engine.  It remained on idle power and I turned back to the field.   After getting back over the field I attempted to apply power but it ran very rough and almost died again, so I landed the plane without attempting to advance the throttle again.   Once on the ground, I ran up the engine again without issue.   I then chocked the plane down and ran up to full power.   I noticed some minor RPM variation between 5300-5500 RPM at full power, but it otherwise ran normally. 

During this episode my oil pressure, oil temperature and CHT were all normal and remained normal throughout the event.   I do not have a fuel pressure gauge.   

I have since pulled everything apart to inspect.   Fuel lines all look fine but I have replaced them anyway.   Filters are all clean, there's no water in the gascolator or the carb bowls.   Fuel flow looks fine and the tank vents are clear.   Both the mechanical and electric pumps seem to be working fine.   Spark plugs are all intact with normal gaps and magnetos are both working.   Air intake is fine.  I did note that both in the exhaust headers and valves, and on the plugs there is a whitish coating on them.   This is the same on all 4 cylinders and I am guessing because I just did a cross country flight for about 10 hours on AVGAS but this is just a guess.   

In summary, so far I have not found a smoking gun.   I have replaced all the lines and filters, added a fuel pressure gauge, and flushed out all the fuel.   My best guess is either some fuel contamination or fuel starvation, or possibly vapour lock given the conditions.   If my repeat ground runs all go fine I am planning on filling one tank with AVGAS and the other with mogas, and doing an easy enroute climb to a decent altitude on 100LL before switching over and putting the engine through its paces at an altitude that gives me some safety margin.  This is mainly just because mogas is more prone to contamination and vapour lock.

If anyone on here has had similar issues or any suggestions, it would be much appreciated before I do another takeoff.   I am flying out of an area that is not overly forgiving for a forced landing, and if my engine failed less than a minute after takeoff climbing out at Cessna speeds I doubt I would have come out of it as well.