The Avenger carbs are pretty lean in the transition / idle bleeds at this altitude and may need to be adjusted. My carb had an off idle stumble because of this.
If you have changed the jetting and other items to get around the off-idle stumble, then the tune may be too rich.
It also sounds like the choke is not opening correctly?
I guess it is electric choke and not the mechanical?
Are you using a thick carb gasket it spacer between the carb and intake.
On my RPM intake/Avenger combination, the fast idle screw on the carb passenger side would hit the intake unless I used a thick carb gasket.

Not sure what heat range plugs or ignition system you are using. You may need hotter plugs, and a MSD type ignition also helps keep plugs from fowling.