How do I adjust my chainsaw carburetor for high altitude?

Turn the high-speed (H) screw clockwise in small increments—about 1/8 turn per 1000 feet of elevation gain—while the saw is running under load. This leans the mixture because air is thinner at altitude. Go too far and you risk seizing the engine; newer saws may have limiters or non-adjustable carbs, so start conservative and richen it back at sea level.