Jul 29, 2000
South America
What I’ve Learned Different About Carb Jetting
1. the main jet affects the idle mixture because some fuel is pulled up past the space between the needle shaft and the needle jet, and the main jet although its flow area is bigger, still does affect the flow past the needle at that setting.
2. since the idle jet, air screw, throttle slide, needle, needle jet, and main jet are almost always overlapping in their effects there is a dire need to use a jetting calculator to figure out how to get perfect jetting. Also because anything within 5% of ideal jetting feels OK to us (although that is a big 10% range).
3. the most important factor in jet sizing is the carb bore area. Second is the design of the exhaust pipe. If all motorcycle pipes were the same I could tell you what jets are needed just by the carb size. On one story on a forum a guy jetted a carb right for his bike and then put it on two other engines, one being twice as big. He said the jetting felt right for each engine.
4. the hardest part about jetting is getting the right needle in it, although most people just leave in the original needle and just change the clip position. They usually settle on a jetting compromise, something that isn’t too bad.
5. triple taper needles are terrible. They always screw up the jetting curve. Keihin only uses single taper needles and Dellorto only uses single and dual tapered needles. Any time someone tells me his triple tapered needle works fine I know I am talking to someone full of shit who wouldn’t know fine if his life depended on it.
6. the needle height should only be adjusted to have a smooth jetting transition from closed throttle. It should never be used to adjust the mid range jetting at the expense of minimal throttle jetting. That’s what other needles are for, to get the taper right to match what the engine wants for mid range.
7. places that sell needles couldn’t give a flying fuck about you knowing all about their needles so you can make an informed decision. They just want you to buy as many needles as possible like shooting in the dark till you find something you like. I offered to the three main sources to measure their needles for them free of charge and they all turned me down.
8. One place I know supposedly sells “genuine” Mikuni needles but two out of three that I measured had their taper angles way off from what their letter codes said they should have. (Don’t ask me in public which place because I might need to buy something else from them in the future). RESULTS ON MEASURING TMX NEEDLES : 6GDY12 should have angles 1.75/1/6.5 degrees but I measured 1.15/1.83/3.7 degrees. 6DGY04 should have 1/1.75/6.5 degrees but I measured 1/1.83/3.45 degrees. Their 6CEM3 was pretty close to the .75/1.25/3.25 degrees it should have.
9. carb sizing isn’t as easy as using the common formula that we have seen on the internet. There is a range from high velocity racing carb sizing to medium velocity street/trail carb sizing and so there is no one formula for all since it’s all about max air velocity and what type of riding you do.
10. most people are full of crap and so if you want to learn about something do the testing and experimenting on your own to find out the true scoop. I don’t care how “expert” someone claims to be, I will never waste a dime on any special colored needle that is triple tapered. I guarantee you that perfect jetting can be had on any carb with just a single tapered needle. The exception to that rule is the PWK28 which has a selection of needles with tapers that don’t go lower than 2.5 degrees which is still too much. See my latest video on how I had to find a 1.5 degree Mikuni needle to substitute for it. That gave a near perfect jetting graph.


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