Hey. I've been on 3 and 4 wheelers all my life, but I've never modified one. So forgive my ignorance. I need to know about the needle valve. After I rejet it, I know I am supposed to use the needle valve for adjustments. But what am I adjusting? I'm guessing it's rich/lean mixture. If so, which direction do I turn for which effect? Also, do I need to order my jets through Yamaha, or are they universal? Thanks.

 

Usually you don't have to replace the needle, you simply change it's position. The needle is attached to the slide and plugs the needle jet when the throttle is closed. As you open the throttle the needle is pulled out of the needle jet allowing fuel to flow. The needle has a clip the sets how far into the needle jet it slides. There are 4 or 5 grooves in the needle where a C-clip fits limiting it's movment into the needle jet. Rasing the needle (lowering the clip) richens the mixture at 1/4 to 3/4 throttle. The opposite leans the mixture. If your engine needs either the bottom or top groove, another needle may be a better solution. Most carbs you can simply unscrew the cap and pull out the slide which will have the needle on it. It's easier to understand once you see it.