Scott,
Thanks so much for your reply. I am going to pursue your suggestions, beginning with STARTING at full throttle to get the LSN out of the equation.
By that I mean I want to try to see if I can get it running a full throttle only, with the vent line connected.
A few more details: Dub Jett broke this engine in, and over the holidays I sent it back to him for examination because of its inability to run. He concluded that there is nothing wrong with it, and told me that it works perfectly at sea level, with the carb set up the way he set it up.
That's a nice data point, but it matters little. I still think the carb is set up wrong for my conditions, so I will get it to run at full throttle first, and then proceed to a "start over" on the LSN adjustment.
It is very hard (basically impossible) to get 20% where I live, so I have been living on the Jett-prescribed 15%, and will probably continue do have to do that. The fuel I am using is Cool Power 15%.
Right now it is going to be tough to make much progress since the weather in Colorado is pretty terrible. I'll post an update as soon as I can do some more running.
Thanks again for the help, I really appreciate it and will keep you posted on developments.