I was having visions of that twin turning 14k rpms developing 150 hp! Yeah, elevation knocks the heck out of hp if you don't have a turbo.
Copied from the web.
Higher Altitude Means Less Power
Reduced oxygen in high altitudes can lead to inefficient, sluggish engine performance because there is less air to feed the internal combustion engine. Generally speaking, an engine loses three percent of its rated power for every 1,000 feet of altitude gained. This means you could lose as much as 20% of your vehicle’s horsepower if you drive it from sea level to Big Bear Lake (with its 6,750 elevation).