Using the Flow Area Graph Steps: First just enter the needle ID, main jet #,and C28 (slide open mm # taper start). Nothing more. Making it Richer Mid Throttle You can take note of the 1/2 slide number on the main graph (.41 in this example), then find a needle that bends the graph more with more taper angle to increase that number. Without changing the needle all you can do is screw up the low throttle jetting by changing C28 to be more mm or less mm. If you order another needle you can change the clip position because mid throttle jetting is more important than low throttle jetting. Different Needles On my spreadsheets Mikuni VM sheet it lists available needles. The #4 series are mostly of the correct length for my Mikuni VM18. The bigger the taper angle(s), the richer the jetting will be mid throttle. The 4F15 I am using with its 1.5 degree taper is just right but if I needed it richer then I would try the 4J11 needle with its 2.5 degree taper. Be aware that the steeper the angle the more easy the flow of gas is at WOT and so the smaller the main jet needs to be. If your current setup is good at WOT then write down the # at I14 and then with a differnt needle ID entered at D5 then keep changing the main jet # till I14 returns to the number it was before. Multi-tapered Needles I generally prefer single tapered needles but sometimes a dual tapered needle allows you to raise the needle more for more richness down low. Clip Position For most bikes the correct clip position causes the taper to "kick in" at any where from 1/4 slide open to 3/8 slide open. By that I mean that the beginning of the taper just starts to come out of the skinny part of the needle jet at those slide openings. (Note I rarely say "throttle position" because closed throttle isn't closed slide, and can vary a lot.) The clip positions are numbered 1 to 5 with 1 at the top and #5 being the lowest. Length of Straight Section That distance works along with clip position. For example the 4F15 in my VM18 is set to the 5th clip position which pulls the needle up the maximum. I could pull it up more with a home-made washer under the needle clip if I wanted to, but I don't need to. So this needle has a 26mm length but since it likes the 5th position that means I can use any other needle that is any where from 26mm to 21mm (because 26 - 5 is 21). All of the #4 needles are within that range. If I used the 4J11 at 21.5mm length then I'd need a 0.5mm washer under the clip set at the top #1 position. (26 - 21.5 = 4.5) Needle Diameter With Mikuni VM's the diameters are uniform but you can change the needle jet for different inner diameters which changes the flow area around the needle at closed throttle. This area affects the low throttle jetting. Old bikes have their needle jets wallowed out which allows too much flow and so causes low throttle jetting to be too rich. The larger the engine the less years that jet lasts due to more vibration. Keihin carbs have set diameter needle jets and so you can only try different diameter needles. Dellorto offers different diameter needles and different diameter needle jets. Flow Area Graph The 2nd graph in the jetting calculator shows the needle/main jet combo flow areas. In this example I've recorded the flow area for needle clip positions #2 to #5 and changed the main jet size to keep all four right around the same flow area at WOT. The details for each graph are listed under the "Store #" macro buttons. It shows the needle ID, needle jet ID, clip position, and main jet. Graphing Error The graph above shows a lack of graphing precision that you should be aware of on the Keihin and Mikuni TMX sheets. In the presentation to the left I've redrawn the graph line of #3 between 3/8 slide open and 1/2 slide open so you could see how it would look if there was a calculation for that point. But since the program doesn't calculate that point on those two sheets then it just draws a straight line between its 3/8 point and its 1/2 point which causes a distortion from reality. I've redrawn it here to correct that distortion. Now the VM/TM and Dellorto sheets are correct. This graph is further explained in THIS VIDEO. Using it is an alternative method to seeing jetting changes due to changes in the needle, needle jet, needle clip position, and main jet. Needle Clip Positions With Small Carbs The chart below shows everything the same except for the needle clip positions. You can see the big effect each clip position has on this small 14mm carb as to when the taper kicks in as you're opening the throttle. This is why 0.5mm thick needle washers are important for tuning small carbs so you can set them in half positions. Each needle clip position is 1mm apart, even for needles of large carbs.     Needle Clip Positions With Large Carbs This chart shows how small a change happens for the same 4 needle clip positions with a large 34mm carb. This gives you an idea of how important the distance is of the needle from its top to the beginning of the taper. In this example if you wanted jetting to be leaner below 1/2 slide open then you'd have to buy a needle with the same taper angle and same width but with more mm above the tapered section. Keihin PWK Needles This chart shows differences between Keihin needles with their mid throttle richness, from the moderately rich BEK (green graph) to the fairly lean N3WE (yellow). It also shows how different main jets were needed to keep all four graphs near the same WOT richness (#170-180).     Mikuni TMX Needles Here we see TMX needles which are more for trail riding due to being less rich mid throttle than the Keihin needles. The EN and EJ are about the same, and to have a needle that's richer mid throttle I had to find a VM needle for the job, in this case the 5F21. Due to this analysis I would consider a PWK for racing, and a TMX for trail riding or for use on the street since street/trail riding needs a leaner mid throttle. NEEDLE SHROUD These programs never cease to teach me something. I previously thought that by raising the needle shroud that the mid throttle jetting could be enriched. When I changed this graph to incorporate the shroud effects it showed me that a higher shroud has to be compensated for with a smaller main jet which leaves the greatest effect at low throttle positions. So a higher shroud which causes more vacuum at the needle will cause a leaner jetting at low throttle due to the compensating smaller main jet. Look at how the green graph with a 0mm shroud causes more richness at low throttle due to the larger main jet compared to the 2mm and 4mm high shrouds. GRAPH START The graph always starts at 0 just to make them more visible. The data for these 4 graphs come from cells C14 to I14 and the true flow areas of C14 are listed for #1 to #4 graphs just above the chart. So in this example #1 has .529 which is 7% more than the .494 of #4 so it causes 7% more richness at 1/4 slide open. HOME