America Running Poorly At High Altitude.

More air would have been my recommendation. Many of the race cars double stack the air cleaners for hill climbs to increase volume of thinner air

 

If you think about it, you need smaller main jets and a tweak of the pilot air screw as there is less oxygen at altitude hence the rich condition.....simple matter of maintaining the correct fuel/air ratio.

This should get you in the ball park:
http://www.bonnevilleamerica.com/dinqua/Triumph/survey.html

 

At higher altitude the air is thinner (less dense) so the engine runs richer. You have the same jet=same amount of fuel, but less air so the air/fuel ratio goes lean. Could turn out your screw some for a temporary help, long term would require re-jetting with smaller jets.

 

This is the dilemma with carbureted bikes. I have the same problem at times. My home and shop are at about 3200 ft. elevation, and my vacation home is over 6400'. You have to compromise. Main jets and air filter opening size are the main criteria. Pilot jets are no big deal. If they are a bit rich, just screw the needle in a bit.. Mine seem to do OK up to about 5000-5500'. Above that, I just live with it. ...J.D.