The Dennis Kirk Blog
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You’ve been planning this trip for a long time. It’s been in the makings for weeks, if not months, and you’ve thought of everything. But, while checking the boxes on your to do list for your trip, you realized that you might have to do some work on your carburetor before you head off into the mountains. Well, the good news is, you are in the right place. Rejetting snowmobile carbs due to altitude changes can be something of a mystery, but we are here to give our two cents on the matter.
First, we are going to dig into the science behind it all, just deep enough to understand why rejetting is even a point of consideration. As altitude changes, so does air pressure. As air pressure decreases, the air becomes less dense, meaning less oxygen per cubic foot. Your carburetor will continue to pull in the same amount of fuel, but, the amount of oxygen it receives will vary based on the amount of air pressure, and in turn, altitude.
So, look at it this way. You’ve planned a route that goes conveniently straight up the side of the mountain. You start at the bottom, with your snowmobile’s carburetor perfectly calibrated for your current altitude. You cruise straight up the side of the mountain, and as you go up, your snowmobile starts to bog down. It’ll feel sluggish, slow to respond to the throttle. This is a sign that you’re snowmobile is running too rich. It’s getting too much fuel and not enough oxygen, and it can’t burn it all efficiently. For the sake of learning, let’s ignore these signs and keep going up the mountain. The symptoms of a rich fuel/air ratio, which started as minimal, have become noticeably worse, until the snowmobile sputters to a halt thousands of feet up a mountain. This is the outcome you are trying to avoid, the whole purpose of rejetting.
Carburetor jets are what determine the amount of fuel that enters the carburetor at any given point in time. These jets are what keep it from getting too much fuel. By changing the size of your jets, you can bring that ratio back to an optimal level. Snowmobiles tend to be jetted according to the average altitude at which they are sold or manufactured. If yours needs to be changed, here is a general rule: For every 2000 feet that your altitude has increased, decrease your jet one size and go one clip leaner on your needle jet. This will leave your carb much better prepared for the new altitude.
Of course, it’s unlikely that you are going to ride at only one specific altitude. It’s not uncommon to find a single day’s riding with a 3000 foot difference in altitudes. The best thing you can do is plan for the average altitude that you expect to ride at, taking into account maximums, minimums, and where you’ll be spending the majority of your time. Also, keep in mind that, while altitude is the primary factor, it’s not the only one. Temperature and humidity also play a role, and in order to be optimal, your snowmobile might require a richer or leaner fuel mixture than this general rule allows for. Still, this is a good place to start.
If you have any additional tips for your fellow riders or any questions that we can answer, post them below! We’d be happy to help however we can. Thank you for reading, good luck on your ride.
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