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Jetting questions, HELP
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[QUOTE="skhasky, post: 311917, member: 29836"] Nozzles and needles and jets - oh my! The main jet screws into the bottom of the nozzle. The nozzle screws into the bottom of the carb. The needle rides in the hole in the center of the nozzle. At 0 to 1/16 throttle. a little air seeps past the slide, and the vacuum draws fuel from the pilot circuit. As you twist the throttle, the slide/needle raise allowing more air, and allowing fuel to flow from the nozzle (which has the main at the bottom). If you have a leaner nozzle, then it has a smaller hole in the middle. Think of it this way. the needle has a diameter - let's say tapered 1mm to 2.5 (not really, but for example). The nozzle has a 2mm hole. As the needle withdraws from the hole in the nozzle, there is space between the needle and the nozzle walls. This is where fuel gets by. If the needle were leaner (thicker) then there would be a smaller gap ther and the bike would run leaner at part throttle. If the hole in the nozzle were leaner (smaller) same story.... smaller space between the needle and the wall of the nozzle. So... you can get basically the same results by using a fatter needle or a smaller nozzle. See the relationship? It works the other way around too. You can change one or both the other way to get a richer part-throttle mixture. Now... your twisting the throttle to 3/4 and the needle/slide are coming up farther (more out of the hole in the nozzle) this allows more air, since the slide is lifting - and more fuel since there is less obstruction by the needle to the nozzle. Finally, you get to 4/4 throttle. The needle no longer effictively obstruts the flow of the nozzle which is being fed by the main jet attached at the bottom. Now here's the $40 million question. If needle taper and nozzle diameter do the same, which is better? If you run lean at 3/4 to full then the nozzle may be obstructing the flow from the main... use a larger nozzle. But that may end up in a rich condition at 1/4 throttle since the space between the needle and nozzle tube is too wide (too much flow). So, use a leaner needle. If it is rich on top and lean on the bottom, switch them! Use a richer needle with a smaller (leaner) nozzle. Don't get wound up to much as to what jet controls what. It is a system. To some extent they all effect each other! [/QUOTE]
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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Jetting questions, HELP
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