Yeah, your mixture will change and you will lose power. I read here in a forum a few years ago that someone had calculated how much power loss per 1000 feet altitude MSL (Mean Sea Level) you would loose when flying at locations like Denver, and so on. If you spend a little time searching, you might find it.

I did the look-see for you. It works out to about 3% power loss per 1000 feet. Thebelow is for a 2 stroke engine that starts out at 1.6 bhp at sea level (OS .46 for instance).

Sea level 1.6
1000 1.552
2000 1.50544
3000 1.4602768
4000 1.416468496
5000 1.373974441
6000 1.332755208
7000 1.292772552
8000 1.253989375
9000 1.216369694
10000 1.179878603


CGr