When an ATV leaves the factory line, its carburetor is jetted for use at or near sea level. If you’re going riding at 6,000 feet or higher above sea level, you may need to rejet to meet the changing air requirements — especially if yours is a two-stroke engine. In these mountain environments, the air is thinner. If you leave the stock jets in place, the carburetor will be sending too much fuel for the air at altitude, and you’ll get a rich fuel-air mix.
This will cause a bogging, sluggish power delivery. Smaller jets will better balance the mix. Air temperature differences can also require jetting changes. Colder air is more dense; like there is more of it in the same amount of space. That means there’ll be more of it in your carburetor venturi, so you’ll need more fuel through the jets to readjust the fuel/air mix. That doesn’t mean you automatically rejet just when the temp drops, "you rejet when you need to rejet," to paraphrase Penland.[/b]
