Jetting
 

Hey guys. I built a 2003 KX125 this summer with a new crank and piston. I took it up riding at around 11k. When riding it around it would puff out smoke out the tailpipe like it should and didn't run amazing. But when I took it down to 7k the bike would have the normal 2 stroke puff. I know very little about jetting but wouldn't a bike run richer at high altitude. I don't know the history of the bike before I built it. I'm just confused with going up in altitude and the exhaust suggests it's running lean. Is what I'm thinking correct? Thanks