Constant velocity (CV) carbs are a little different to carbs with manually operated slides. However, one of the elements that they do have in common is the needle and emulsion tube (needle jet). It is often referred to as the needle jet because it is a jet, a variable jet. One of the most common pieces of advice that you will find on the web is the "change your needle clip position". That inspired me to write this post. The needle and the emulsion tube are a matched pair. The needle taper determine the change in fuel flow rate for a given slide position. Those tapers are matched to the power characteristics, or state of tune, of the engine. Changing the needle clip position shifts the fuel flow rate to occur sooner or later relative to the position of the slide. This graph shows the effect of changing the clip position Let's start with the emulsion tube. This can get a bit difficult to follow so as an example; lets imagine that you have a Yamaha FZR250 and you are forced to run E10 (10% ethanol fuel). According to RFA: "Ethanol chemically enleans the air/fuel (A/F) mixture. As an example, in engines set at an A/F ratio of 14.7:1 on all hydrocarbon fuel, the introduction of 3.5% oxygen in the fuel would enlean the A/F ratio to about 15.2:1." There also seems to be quite a few references that suggest increasing fuel flow by 10% to compensate for E10. Notice that it says 'fuel flow' not jet size/number/diameter. The amount of fuel that can flow through a jet is determined by the area of the orifice not its diameter. Just as you need to change the main jet you will also need to change the emulsion tube. NOT the needle clip position. Your engine has the same power curve that it always had. It just needs more fuel. The 3LN1 has an N-8 emulsion tube so a diameter of 2.59mm and thus an area of 5.268 sq mm. We will need an area of 5.7948 so a Q-3 will be required. Similar calculations will be needed with the other jets. Now lets consider the needle. The first number is the length. The second and third letters represent taper and the last number determines where the first taper begins. If the needle only has one letter it only has one taper. So the difference between a 5CT7 and a 5CT8 is that the tapers on the 5CT8 needle begin 3mm closer to the top. Therefore a 5CT7 with the clip in the centre position is like a 5CT8 that has its needle clip lowered 3mm below centre. Imagine that you are tired of the peaky power delivery of your fizzer and you have decided to drastically alter your engine by fitting Acme Grunt Cams. This will change the power curve of your engine; it will move the maximum torque to a lower rpm. The engine will able to burn more air fuel mixture at a lower rpm because of less pumping losses. To put it simply you need more fuel down low and less up top. The standard needle is a 5CT7. This is a double taper needle represented by the letters C and T. Maybe with these new grunt cams your engine has a perfectly linear power delivery and you can switch to a single taper needle like a 5C3. In any case moving the needle clip is not going to do it for you. I hope this gives some idea of the complexity involved in altering carburettor settings. I am not saying that moving your needle clip is the wrong thing to do, but too often it is suggested as a panacea for everything from worn rings to a dirty air cleaner. As an oversimplified example, if I were to fit a 'race can' to my exhaust system then theoretically this will shift my power to the right, higher up in the rpm range. I would consider dropping the needle (commonly referred to as leaner) and using a larger main jet. Whereas the common suggestion would be to raise the needle (richer) and increase the main jet. I could be completely wrong and testing would reveal that. The point is that 'more fuel' does not equal 'more power'. The data for needles and emulsion tubes can be found as a spreadsheet on my links page. It is in German but you will get the gist. I have seen the same sort of information based on the taper angles somewhere but I don't recall where that is. cheers Blair
Two stroke carbs and jetting are little different. Most modern two bangers have power jets and no accelerator pumps.
Hey Blair Hear what you are saying but do not understand why all manufacturers had these 5 settings on the carbs if in fact, according to you they are not required?????? Can you explain why they are there, Why would additional expenditure be included if not required??? I don't get it.
Going off what maelstrom has noetically highlighted it also (for me at least) brings into question the life expectancy of the mod, for both road and track maintenance/costs. And I'm thinking sooner or later it'll still come back to the rider in the end -- and MotoGP
Hi Phil, I can't answer that question. I think most of it is tradition as most carb needles have had grooves and a clip since forever. Having said that, there are plenty of Keihin CV carbs that don't have any grooves, like Kiffsta's NC30. If you want to mess with them you have to use a shim. I used to think that as the internet grew in popularity that knowledge would spread but what has happened is that 'popular' knowledge has spread more. As an example, how to read correct mixture from a spark plug, the 'popular' version of this is the generic model that you see in the front pages of the aftermarket motorcycle workshop manuals (with any luck they don't do that anymore). Such information may be suitable for an Austin A40 but has little to do with high performance motorcycles. However, the publishers don't want to alienate their readers with 'esoteric information' about fitting new plugs and examining mixture rings with a magnifying glass. Getting back to the needle issue, I just wanted to make it clear for any interested readers what moving the needle position actually does and how the needle jet and needle work together as a pair. I also wanted to point out that richer does not mean more power. cheers Blair
Not sure about your 4 strokes but at least with the 2 bangers, they are pretty susceptible to temperature and altitude. If you get the jetting spot on in the height of summer, when the air is hotter and less dense, may mean you end up with a bike running lean in the middle of a cold winter when the air is much denser without any change to the settings. Same goes if you move from high altitude down to sea level. On the 2 stroke dirt bikes I have owned over the years they almost always come from the factory set up a bit rich to account for this I think. I almost invariably end up going down maybe one main jet size or leaning off the needle a little bit. The same bike would probably have spot on jetting in the South Island of New Zealand or something.
Agree fox, I think the factory jet fat to gain engine longevity as well as cover what you mentioned. Ie. altitude and average temp.
Excellent information Fox. Just to add to the conversation, for four strokes only, to set the main jet size this is my preferred method and it is the technique I used for racing motorcycles (they did win races). Keep going smaller on the main jet until the power suddenly falters. I always did this at the race track and I was not riding the bike. I would stand towards the end of the straight and listen for the burble noise that would come from the exhaust, on a wide open throttle. By wide open throttle I mean at least red line and preferably more. It will make that noise when it is too lean. If you have a restrictive exhaust then you may never hear it, so you will have to rely on the more seat of the pants loss of power effect. Now, go back up one size and you will have a screaming banshee. This works perfectly for that bike on that day at that location, so record the atmospheric pressure etc and make note of your elevation. I know that in this day and age it is all dyno this and dyno that, and they are very good for direct comparisons of different parts etc, but rolling road dynos are just just approximations not the real thing. To explain a bit more; on a rolling road dyno the motorcycle has to rotate a drum which is used to emulate the resistance that the bike would be faced with on the average flat road. However, there are so many variables that an accurate result is not possible. As an example the resistance that John Britten's brilliant motorcycle would have to overcome at 200mph (described as driving through a fruitcake by James May when testing the Veyron) is completely different to that of what a Kawasaki ZX14R would have to deal with. That is why professional dyno work is done with a brake dyno and the jetting is mapped for different loads by using throttle position and other sensors. To relate this to my previous post, I used the leanest possible setting not the richest. Rich is slow and vague. cheers Blair
