Turn the high-speed screw clockwise for altitude.

Yes. At higher altitudes, air is thinner, so your chainsaw runs richer (more fuel relative to air). You need to lean the mixture to keep it running right.

Find the carburetor adjustments—usually two screws: H (high speed) and L (low speed). Turn the H screw clockwise in small increments, maybe 1/8 turn per 1000 feet of elevation gain. Do it while the saw is running and cutting under load. The goal is a clean four-stroke sound that clears up when you rev.

A couple warnings: screw it in too far and you’ll run lean enough to seize the engine. And many newer saws have EPA-mandated limiters or non-adjustable carbs—you might need a different jet or a trip to a shop.

Start conservative. You can always richen it back down when you return to sea level.