The Jetting FAQ All carbs are basically the same components. Jetting should be accomplished by modifying the lower end first and changing towards the wide open throttle after the lower end is right (idle, bottom rpms, mid range, and then wide open). Start with number one and change through number six. NAME IT AFFECTS 1) Air Screw- idle to just off of idle (not the idle screw) 2) Pilot Jet- just off of idle to 1/4 throttle 3) Jet Needle- 0- 1 /2 throttle, work with the needle shape first, then the clip position 4) Needle Jet- 1/8-3/4 throttle, also called nozzle, most Keihn's don't interchange 5) Slide Cutaway - 1/4-3/4 throttle, the angle of the cut affects air intake, this is a high dollar item. 6) Main Jet- 1/2-full throttle Notice that every component overlaps the throttle range of one or more of the other components. Needles run about $5, jets (pilots & mains) from $2 to $4, slides from $30, nozzles from $6 (all ballpark numbers). Most people only change needle clip position, and pilot/main jets. Some older Mikuni's also have a power jet (1 /8-1/2 throttle). Most people check jetting by looking at the plug or the exhaust spooge. I jet by how the bike sounds and feels in response to throttle input at different rpm starting points. I always jet by the numbers listed above, one first, then two, etcetera. I don't mess with the cutaway because of price, nor the nozzle because my Keihn doesn't change. I've never known anyone who can jet a bike right the first time. All jetting is trial and error, because every bike is different. And different from the one designed by the manufacturer on the flow bench. Castings are different, gaskets, assembly, etcetera. As said before, higher -> leaner, warmer -> leaner, wetter -> leaner, and vice versa. The trouble comes when it gets cold (richer) and wet (leaner). The most sound advice is this: If it is too rich, you foul the plug, but if it is too lean, you'll eat the engine. Step 1: Basic Assumptions OK. We will assume that your air cleaner is clean and that you are using the fuel / oil mixture you plan to stay with. (I use my favorite brand at 50:1, have used that with great success for years. I believe that most oils are really great and are probably not very different from each other. Timing needs to be set to factory specs. Muffler and spark arrestor should be relatively clean. Use the recommended spark plug. Step 2: Main Jet First. The main jet controls the mixture at full throttle. Other jets have negligible effect at full throttle. Your objective is to get an understanding of the mixture at full throttle (wide open) operation. You need a long up-hill straigntaway for this test so you can be in the top gear with the engine under load and running up in the RPM range. If you hear pinging, it is running lean; so go larger on the main. If full throttle causes gasping and poor pulling at mid RPMS, it is again an indication of running lean; so go larger on the main. If the bike runs clean select a larger main jet until you find the jet that causes a blurbbing (four-cycling) sound. When you experience that sound, you have found the jet that causes you to run too rich. So back off one size to a smaller jet. This is the safe main jet to use. You could go another size leaner; but you need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side. Step 3: Needle Jet clip position and needle selection The needle jet controls the mixture from 1/8 to 3/4 throttle. At the upper end of this range the main plays a part too; but you have already selected the main jet. Again you need an up-hill straight away. This time use 1/2 throttle and allow the RPMs to reach the upper RPM range and leave the throttle there. Now do your listening. Any pinging means too lean. Blurbbing means too rich. Excessive smoking is also an indication of running too rich To adjust to a richer mixture, take out the needle and place the clip one notch lower (nearer to the sharp end of the needle). To adjust to a leaner mixture, take out the needle and place the clip one notch higher (further away from to the sharp end of the needle). Be careful when you remove the clip. They can flick away and disappear. I use a small flat screw driver and pry the clip away from the needle and I hold the needle and clip between my thumb and finger while I do this. If you find that you are up at the top notch of the needle (there are usually 5 notches) you should get a needle with the next lower number (leaner) and place the clip in the center notch to give the equivalent mixture as the richer needle with the clip at the top. Similarly if you find that you are up at the bottom notch of the needle you should get a needle with the next higher number (richer) and place the clip in the center notch to give the equivalent mixture as the leaner needle with the clip at the bottom. You should not have to go to any more than one size leaner needle than stock. And I would not expect you to need to go richer than the stock needle. Next go along slowly in one of the lower gears at less than 1/8 throttle and move the throttle quickly to 1/2 throttle. If the engine does not pull strongly. Sort of gasps for breath and only runs well after the RPMs build up, it is an indication that the needle is too lean. (I had a 175 Penton that came jetted that way. I thought it was characteristic of the porting and the pipe. The dealer helped by telling me that the needle was too lean and sure enough I found nice mid RPM / MID throttle power after dropping the clip a few notches.) Step 4: Pilot Jet selection and Air Correction Screw Adjustment Regarding the pilot jet. When you are at 1/4 throttle and more, the pilot jet has virtually NO effect. After you get the needle right, get the bike idling by adjusting the idle stop up where it will idle slowly on its own. The air correction screw should be at 1.5 turns out to begin with. OK now slowly screw in the air correction screw 1/4 turn. Try to find the air screw adjustment where you get maximum idle RPM. You may have to open the screw to 1.75 or 2.0 turns to get the right setting. As the idle RPM increases turn down the idle stop to return the idle RPM to a slow correct idle. You objective is to find the pilot jet that will give you maximum idle with the air screw set at 1.5 turns out. Remember that opening the air correction screw admits more air during almost-closed throttle making a leaner idle mixture. Now when you find that jet size you will want to do some final air screw adjusting to improve throttle response. Assuming that the needle jet is properly adjusted. Let the engine idle for 5 seconds then open the throttle abruptly and be aware of how the engine responds. If it almost dies, then you need a slightly richer idle mixture so that as that mixture is gulped at abrupt throttle openings it will be just a little rich and therefore give good response. You can test this as you ride say slowly in 2nd gear with the throttle closed so that the engine is drawing only from the pilot. Wick it open quickly. If you find yourself doing a wheelie, your air correction screw is set right. If instead the sound is uhhhhhhhhhwwaaaaaaa and you find your nose a few inches from you triple clamp, then your idle mixture is too lean and you need to close the screw a little. ------------------------------------------------------------------------ [Image] Back to Index. 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