The thinner air at high altitude affects piston engine power a lot if it doesn't have a turbocharger or supercharger fitted. For example, a light aircraft piston engine full throttle power rating is:- Sea level -100 hp.
- 5,000 ft - 84.6 hp.
- 10,000 ft - 74.8 hp.
A motorcycle will have the same loss of power as altitude increases, so if you think that your bike feels like it only has 3/4 throttle at 9,000 ft, then your butt dyno must be well calibrated.
The 15M ECU has an air pressure sensor, (under that little black vent on top) but Meinolf has improved the barometric correction tables for the 15M ECU in his .bin maps he has shared around, so the automatic leaning needed as altitude increases is closer to ideal.
http://www.v11lemans.com/forums/index.php?showtopic=19815&do=findComment&comment=217735
If you try one of his maps, you may find the decel pops in the exhaust lessen, but still the laws of physics do not allow you to get sea level power at altitude unless you fit a blower.
I don't believe that valve clearances will have any effect on the loss of mass airflow that is caused by low air pressure at high altitude.
That example is for an old Lycoming O-235 variant.
I asked a pilot about this, and he took me seriously. He wrote:
"Density ratio at 9,000ft is 0.7621 so a bike down to about ¾ power is about right for normally aspirated engines.
The Cessna 172RG Cutlass (180 BHP @ 2,700 RPM MSL) can make 77% BHP at 8,000 ft at 22” MAP and 2,700 RPM, and 70% BHP at 10,000 ft at 20” MAP and 2,700 RPM. Manifold pressure lapse rate in lower layers is about 1” per 1,000 ft on a standard day.
Induction interference causes max MAP ISA MSL to be just over 28’’ Hg."
MartyNZ's post in Altitude Sickness - running weak in the mountains was marked as the answer