RPM Drop in 912ULS

SKGY airport is at 8389’ MSL.  All these events have happened at high altitude.  What fuel are you using?  

Sounds  like an intermittent fuel supply problem.  As you have blown out the fuel lines, what of the tank ? 😈

Francisco.  Given you are spending so much time trouble shooting this perhaps swap the carbs from the left and right engine.  If the problem moves with the carbs at least you know where the problem is. If the problem is still on that same engine now you know it is perhaps electrical or some other fuel supply issue to that engine.  

Intermittent suggests something other than carb in my mind.  ICE perhaps but then you would expect it on both engines.  

Cheers

Hola Francisco...

There are experts here with much more knowledge than me but, and open to discussion, I would like to share my 2 cents.

-. The engine running at 3,800 to 4,200 is too low. You are putting a lot of load on the engine. Why so low? What RPM can you see in static WOT? What about takeoff?
-. I would consider that your problem could be related to the engine temperature. Let me explain my idea. It is possible that when the engine is hot, the rubber on some wire may be expanding and, because the wire is broken, it may lose contact. Maybe?
-. I remember seeing a case describing a strange problem related to the ignition coils. The result was that when the engine was "cold" they worked fine but, when the engine was hot, it started to fail. There is a procedure in the MMH to test the coils. To simulate engine heat, you can use a hair dryer - hot air - and hopefully the reading will change. If that happens, maybe the problem is there. You didn't mention any engine roughness, but I would say it's worth checking.

By the way, I have flown out of that airport. The views are so cool! For sure you will recognize the location...

Good luck!

My only suggestion would be to use a tank full of 100LL fuel, it sounds like vapor lock to me. At that elevation, low octane fuel could easily vapor lock. Just a suggestion that may prove to be a very easy fix.