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Jetting What is jetting? How does it work?

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WHAT MEAN JET? : Carb jetting can be easily understood if we understand the basic principles of carb and engine operation. A carb mixes fuel with air before it goes into the engine. When the mixture is correct the engine runs well. The bottom line is a carb must be adjusted to deliver fuel and air to the engine at a precise ratio. This precise ratio can be affected by a number of outside and inside influences. If you are aware of these influences you can re-jet your carb to compensate for the changes. I'm going to show you some examples of how you can change your jetting for better performance and in some cases increased engine life. As with any engine work be sure you have good tools the correct parts and a good manual before you get your hands dirty! ALTITUDE COMPENSATION: For our first example let's say we find a new riding area WAAY up in the mountains. Our jetting is dialled in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 260 main jet at sea level you would drop down to a 240 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD!. Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 240 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder,,

Posted on Nov 10, 2008


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HI My KTM 200 2002 model uess way to mutch fuel bout 50km to a tank. Any ideas

Thanks for contacting FixYa with your KTM inquiry.
50 kilometers is equivalent to 31 miles. The tank capacity is 2.9 Gallons indicating you are obtaining a very sub-marginal 10mpg approximately.
A couple things to check would be:
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