Originally Posted by Pedro Navaja
The Sherpa is jetted lean - very lean - from the factory. Dr J's kit makes it less lean at all engine RPMs at very little loss (maybe no loss) of fuel mileage.
As altitude increases, engines run more rich as they gulp that thinner air while using more fuel. Therefore, without any other changes, the engine is now running more rich - which means that the Sherpa - again, with or without rejetting - is running more like a well tuned one would be running at sea level.
Actually most newer (carburetor) bikes will behave this way. It's not something unique to the Sherpa, or say, my west coast KLR, which spends a lot of time at 7,000 to 9,000 feet. It runs fine up there. I really don't feel any difference.
Back in the "old days", 2-stroke engines would blubber and foul plugs more quickly up in the mountains.
People who are up there full time sometimes alter their carb settings, which involves more than just turning a screw, but others don't bother and have no problems.
The Sherpa is well known for being "cold blooded". Start the engine, adjust choke, and it will often stall immediately if you attempt to ride off right away. That's at sea level. My wife is usually on the Sherpa and likes to warm it up for a couple minutes before riding it, but when I ride it, I can immediately ride away with the choke opened about 1/2 way, and push the knob down gradually as I ride.