Hello! I'm new to this forum. I ride a Yamaha Raptor 660, but I'm helping a buddy get his 2005 Arctic Cat 250 4x4 ready for a ride to New Mexico in a few months. We are in Texas at sea level, but the place he's riding in is between 8,000 and 10,000 ft in elevation. He currently has a Keihin carb with a 138 main jet and I'm wondering how many sizes I need to go down to get him close so he can have a nice ride in that high altitude?

 

Rule of thumb is down 10 so 128