912 ULS Engine Misfire

It was detonating from bad gas and/or being set too lean. I had put in 91 octane mogas (ethanol free), and when I took it back to the mechanic this morning I was able to get it to fail in front of him and it was determined to be detonation. It only happens after it warms up and when it would start to detonate under load he would pull the choke and it would subside. After draining the mogas out (and putting it to good use in my truck) and refilling with 100LL and decalin it is working perfectly and not "misfiring" (which apparently was detonation and not a misfire). Either way I'll report back if it comes back but I think that's what it was... The gas was only in the tanks for 2 months (it's been in and out of the shop) but apparently that's enough for it to go bad. I will probably run 100LL from now on with Decalin as this experience has left a bad taste in my mouth with mogas. I operate from a high elevation field in Colorado so I'm not sure if that makes things worse in terms of octane dropping or running too lean. I know I've heard people go back and forth and have heard these "prefer" autogas, but after this it appears mine prefers 100LL. I think 25 hour oil changes and the cost of decalin is worth it compared to the alternative of losing power and detonation on takeoff / under load.

If you usually operate from high level terrain you may consider re-jetting the carbs to avoid detonation. Just a thought.