Gentleman:

We fly at 13070 feet above sea level (In La Paz, Bolivia) and here are some tips:
. ANY gas engine will work no matter the size, but it will loose power power proportionally with altitude. In our case we loose around 20 to 25% of its power compared with sea level.
. In most cases you don't touch the LOW needle, if it is already been set up at factory, this applies to zenoah and dle engines. what you do regulate is the HIGH needle. but it has to be set up 200 to 400 rpm less at the rich side of peak power due at high altitude there is less air to cool the engine. In other words: set up the needles as if you are at sea level trying to find the highest peak and THEN set up a little rich on HIGH

Any further details I will be more than glad to post

Gary