CV Carb operation and tuning
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Thread: CV Carb operation and tuning

  1. #1
    Junior Member Fzyace's Avatar
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    May 2018
    Martville, NY

    CV Carb operation and tuning

    I am reposting this from Basic CV Carburetor Tuning Guide | High Lifter Forums, I also added the float adj section from the same site.

    Due to a multitude of carb related issues on the HL Forum, I figured I would start a thread to explain the basics of carburetor tuning. This may help many of you to gain confidence to adjust your carburetor yourself. Many people believe that carbs are very complex, but with a basic understanding of how carbs work, you will have no problem with performing your own carb adjustments.

    First off, there's 2 basic fuel related problems. You either have a rich mixture, or a lean mixture.

    A rich mixture is caused by too much fuel compared to the amount of air being used during combustion. Rich conditions can be detected by the engine spitting and sputtering, blurbling, or acting like a rev limiter, rapidly losing and regaining power. In severely rich conditions, you may be seeing black smoke coming from the exhaust. The black smoke you see is actually raw fuel that is not being burnt and is being wasted. By looking at the spark plug, a rich condition can be detected by a black, sooty plug.

    A lean mixture is caused by too little fuel compared to the amount of air being used during combustion. Lean conditions can be detected by the engine losing power, yet retaining it's engine speed. For instance, the engine sounds to be accelerating to higher RPMs, yet feels as if it has no power. By looking at the spark plug, a lean condition can be detected by a white, blistered plug.

    Secondly, there are 3 basic carburetor circuits: Pilot Circuit, Mid-range Circuit, and Main Circuit. These 3 carburetor circuits can be troubleshooted by knowing the throttle opening they control.

    The Pilot circuit is responsible for throttle openings from Idle (0 throttle) - around 1/4 throttle. This circuit consists of pilot air jet(s), the pilot fuel jet(s), a pilot screw (either fuel or air screw), and pilot ports inside the carburetor throat (a.k.a. Venturi).

    There are 2 types of pilot screws: a fuel screw and an air screw.

    The fuel screw is located on the engine side of the throttle slide in the carb, and controls the amount of fuel that is drawn into the Venturi by the pilot ports. By turning the fuel screw out, you are allowing more fuel to pass the screw, effectively richening the mixture. By turning the screw in, you are restricing fuel, effectively leaning the mixture. Another way to determine whether it is an air or fuel screw is that a fuel screw has a rubber o-ring to keep air from entering the pilot circuit around the screw.

    The air screw is located on the airbox side of the throttle slide in the carb, and controls the amount of air that is drawn into the Venturi by the pilot ports. By turning the air screw out, you are allowing more air to pass the screw, effectively leaning the mixture. By turning the air screw in, you are restricing air, effectively enrichening the mixture.

    The air jets are hardly ever changed, so we won't go over that. The pilot fuel jet(s) can be changed to bigger (richer) or smaller (leaner), depending upon your problem. A good rule of thumb to use is that if you have to adjust the pilot screw more than two turns either way if it's stock setting, then you need to accomodate by changing the pilot air or pilot fuel jets accordingly.

    Remember, the Pilot Circuit is only effective from 0 throttle to around 1/4 throttle. It still functions during the rest of the throttle positions, but it's effect is minimal, and goes un-noticed.

    The Mid-range circuit is responsible for throttle openings from 1/4 throttle - 3/4 throttle.

    This circuit is controlled by 2 things: the Jet Needle, and Needle Jet (a.k.a. the Main Jet Holder).

    The Jet Needle, or needle as many call it, is attatched to the throttle slide, and drops into the Needle Jet. All needles are tapered. Either the Jet Needle is adjustable or it is not. If there are more than 1 grooves for the needle clip to sit in, then it is adjustable. By raising the clip on the needle, you are allowing the needle to sit deeper into the needle jet, which restricts fuel, effectively leaning the mixture. By lowering the clip on the needle, you are raising the needle out of the needle jet, which allows more fuel to pass, effectively enrichening the mixture.

    When the slide raises, it raises the needle out of the needle jet, allowing fuel to pass by the needle and into the Venturi. This is where needle taper comes into play. Unless you are extremely fine tuning the carb, you don't need to worry about taper. You change which part of the taper is in the needle jet by the position of the clip.

    Remember, the Mid-range circuit is only effective from 1/4 throttle - 3/4 throttle. None of the other circuits have a drastic effect on this circuit, so if your problem is in the mid-range circuit, then it can't be the main jet or the pilot jet.

    The Main circuit is responsible for throttle openings from 3/4 throttle - Wide Open Throttle (you'll see me refer to this at WOT later on).

    This circuit is controlled by 2 things: the Main Jet, and the main air jet. The Main Jet is the #1 thing that people change in a carburetor when it comes to tuning them. This is often a big mistake, as it only controls 3/4 - WOT, and NOTHING ELSE. Remember that. A larger main jet will allow more fuel to pass through it, effectively enrichening the mixture. A smaller main jet will restrict fuel, effective leaning the mixture. With the main air jet, it allows air to premix with fuel as it goes up into the Venturi.

    The Main Jet only functions at 100% when the slide is open and the jet needle is pulled completely out of the needle jet. At this time, the only thing restricting fuel flow into the Venturi is the size of the Main Jet.

    Now for tuning.

    If you read above, you should know the difference in feel of rich and lean mixtures. By knowing at what throttle opening the problem is occuring at, you can figure out what circuit the problem is occuring at.

    If it's the pilot circuit, there are 3 basic way to tune the circuit. You can adjust the pilot screw, change the pilot air jet, or change the pilot jet.

    Adjusting the pilot screw is simple. With the engine running at idle, warmed up to normal operating temps, turn the screw in until it starts to idle rough, then turn the screw out until it starts to idle rough, then turn the screw so it's between those two extremes. To check the position of the screw, you can count the number of turns as you turn the screw in until it seats SOFTLY with the carb body. Reason I capitalized SOFTLY is that the screws (especially the fuel screws) are easily damaged if over tightened. So screw them in until they SOFTLY seat the carb body. Compare your counted number of turns to soft seat and compare it to stock settings (stock settings are determined by counting turns until soft seat before you do any adjustments whatsoever). Again, if you had to turn the screw more than 2 turns either way, you need to change pilot jets (air or fuel) accordingly.

    In the mid-range circuit, there are 2 basic ways to tune the circuit. You can adjust the jet needle, or change the needle jet. Raising the clip will lower the needle, leaning the mid-range. Lowering the clip will raise the needle, enrichening the mid-range. You can also change the needle jet, but only if your jet needle adjustments make no difference in the way the mid-range circuit operated. If you are running lean on the mid-range, and you've raised the needle as far as it will go and it doesn't get any better, then you should go up in the needle jet size. Many carb manufactures don't have different sized needle jets, so the aftermarket may offer them, or they may not.

    In the main circuit, there are 2 basic ways to tune the circuit. You can change the main jet, or change the main air jet. Changing to a larger main jet will effectively enrichen the circuit. Changing to a smaller main jet will effectively lean the circuit. You can determine which you need to do by first determining whether you are rich or lean. Changing main air jets, again, is for very fine tuning. Once you have the main circuit functioning properly, you shouldn't have to worry about the main air jet, because the air for the circuit is mostly provided by the air passing through the Venturi. On many carbs, the main air jet is not changeable. They may be pressed in.

    So there you have it. I basically touched base with carburetor internals and how to adjust them to tune the carb. Every brand carburetor has different ways of accomplishing the same main goal of every carburetor. That goal is to precisely and efficiently mix air and fuel in the right ratios for efficient engine operation. This efficient operation comes from complete combustion, which cannot occur if you are too rich. Whether Mikuni, Keihin, or whatever, they all do the same thing, just in different ways. Hopefully this will help some of you to understand the functions of the carburetors internals.

    RE: Basic CV Carburetor Tuning Guide Wednesday, April 04, 2007 6:43 PM (permalink)
    The importance of proper float height.

    As far as I know, all float heights are set with the float just resting upon the needle, or slightly compressing the needle to it's seat. So consult a service manual for specific adjustment instructions. MOST of the time on ATV carbs, the carb is in an upside down postion during this adjustment. So if float height increases, fuel level decreases in the float bowl. And if float heigh decreases, fuel level increases in the float bowl. Got it? Good. []

    The height of the float in your float bowl is very important when it comes to driveability concers. For example, if your float height is out of adjustment then you may experience rich or lean conditions. This is because as the level of fuel in the float bowl changes, so does the air/fuel mixture that is being drawn into the engine.


    Well, first off, a carburetor functions on pressure difference principles. We won't go into specific principles, but if yall want to study up, do a Wikipedia search of Bernoulis Principle. Just know that in theory a higher pressure ALWAYS goes to an area of lower pressure.

    As yall know, yalls carburetors have vent lines. These vent lines are manditory so that areas such as the float bowl and the underside of the slide diaphragm always have an Atmospheric Pressure (referred to as AP for the rest of this topic) of 14.7PSI (at sea level, it varies with altitude but it's not important). If these vent lines do not work, then air cannot replace the evacuation (fuel being drawn from float bowl) of fuel. Why is fuel drawn from the float bowl? Keen on keepin on...

    As air velocity (speed) increases in the throat of the carb (Venturi), it's relative pressure is reduced. Velocity increases as engine speed increases, mainly with throttle positions. Any pressure under 14.7 PSI )at sea level) is known as vacuum. As vacuum is created inside the Venturi, the AP that is applies on the fuel in the float bowl "pushes" the fuel from the float bowl into the Venturi. It does this because the higher pressure (AP) moves to the area of low pressure (vacuum) that is in the Venturi. But it's not actual pressure that moves, it's actually the fuel that the pressure acts upon.

    So, how does this all tie in with float levels?

    If you have a high float level (low fuel level), then in theory you will have a leaner air/fuel mixture. This is because the amount of vacuum in the Venturi cannot draw as much fuel into the Venturi as it would with a correct fuel level. The weight of the fuel also helps. With less fuel in the float bowl, you have less fuel pressing down on itself which also makes it more difficult for fuel to be drawn into the Venturi because the AP has to work harder to "push" the fuel up into the Venturi.

    If you have a low float level (high fuel level), then in theory you will have a richer air/fuel mixture. This is because the amount of vacuum in the Venturi can draw more fuel into the Venturi than it would with a correct fuel level. The extra weight of the fuel in the float bowl helps the AP "push" the fuel up into the Venturi.

    So where does this all tie in with tuning?

    Well, say you have let your bike sit for awhile and you tear it apart to clean it. When you are turning the carb over to clean it and such you drop it on the float and you accidentially bend the float tang but you don't know it. Or, when you remove the float bowl to clean the needle/seat assembly, you accidentally bend the float tang that varies the height of the float. You don't notice this either, so when you go to start the bike up, it runs badly either rich or lean.

    So how do you adjust it? The float tang. Some carburetors do not have metal float tangs and therefore are not adjustable. So if yours is adjustable, then follow the instructions in your service manual. They are basically the same, but the specs are what vary.

    There is one more way, however, that even without ever going into a carburetor, the float height may become out of spec. This is through wear of the rubber needle tip against its brass seat. After repeatedly coming in contact with the brass seat, the rubber needle tip forms a ring around it. Over time, this ring becomes deeper and deeper. As it becomes deeper, the needle has to seat itself deeper and deeper into the seat in order to stop fuel from flowing into the float bowl. So as the needle seats deeper into the seat, the float itself is also rising more, effectively lowering the float level which in turn enrichens the fuel mixture.

    Most of the time, it takes awhile for the needle to get this ring in it deep enough to cause a major fuel issue, but it happens. Now you know how to fix it. []

    And whenever the needle needs replacing, always replace the seat also. This is because the soft brass material can actually allow dirt and other particles to imbed themselves into it, which doesn't seal very well, and fuel can leak. Anyways, most of the time you can only buy them as an assembly, but if you can buy them seperately, don't. Buy them both for the added insurance. If you happen to have a non-removable seat that's pressed in from the factory, take a Q-tip and some aliminum polish and polish the brass needle seat to buff out any scratches from debris, and to clear the sealing surface of any debris/gunk/etc...
    Last edited by Fzyace; 06-20-2019 at 01:23 PM. Reason: Added info
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    2007 TW200
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  2. #2
    Senior Member Fred's Avatar
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    Sometimes a simple animation can help envision the elements at play.
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    2003 TW200 "Betty Boop"
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  3. #3
    Junior Member Picacho Peak's Avatar
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    Oct 2018
    Las Cruces, New Mexico
    After reading your tuning information I have a question. My stock 2016 TW runs hot at slow speed in the desert. A Yamaha 30X-14342-21 pilot jet was purchased (next step up) to improve the lean condition. The stock jet has a very sharp needle cone that engages the needle seat and the replacement jet has no needle and is more blunt and actually has a small passage (hole) at the end. Based on the carb diagram information the correct part number was ordered for the replacement jet.

    Have TW folks had this same issue and is the blunt jet, 30X-14342-21 the correct jet? Any information would be greatly appreciated.

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  5. #4
    Super Moderator Purple's Avatar
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    The better the shape/taper of the needle matches the jet seat, the better the pair will perform together – are you sure that new one is OEM for this bike ?

    The other thing I will mention, is that it’s normal for an air cooled engine to start smelling hot at low speed high load – to keep the temperature down, there are a few things you could try – none of which will work well independent of air flow …..
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  6. #5
    Junior Member Picacho Peak's Avatar
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    Las Cruces, New Mexico
    Thanks for the reply. Yes actually two needles have been ordered from two different vendors both are Yamaha. The two new pilot jets are the same from the two vendors. The TW is like brand new for a 2016 and the carb has never been off the bike. A few days ago a 130 main jet was added and the pilot jet was going to be enlarged, at the same time, when the difference in jet configuration showed up. It maybe the blunt tip jet with the passage in the end is correct when going to a larger jet. The larger size jet is the same all except for the long needle section at the tip of the jet replaced with a blunt tip.. I probably just need to try it and see how it performs. Thanks for your help.
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  7. #6
    Junior Member glowspud's Avatar
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    Sep 2019
    Eagle Mountain, UT
    Thanks for reposting this guide. I hadn't seen it before and I really appreciate the well written description of how the carb works.
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  8. #7
    Senior Member methamphetasaur's Avatar
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    Jun 2015
    I'd like to add something if I may:

    And yes, before I start, I know it says in the title "CV" carb tuning, and I know this is only minimally relevant to tuning CV carbs- but I know some of you have put mechanical carbys on, or may have Dirtbikes or something else with mechanical cable operated carbs you might want to tune too so here goes anyway.

    Not really mentioned in your post is the slide cutaway.[This is much more important in tuning two stroke carbs or high performance carbs like for race dirt bikes or whatever] It's simple, but somewhat counterintuitive, so worth mentioning I think. Some people may think that since a larger cutaway increases airflow, that it there for increases fuel delivery. But actually higher airflow makes lower vacuum pressure, which results in less fuel being 'sucked' up through the jets. A smaller cutaway increases vacuum and therefore provides more fuel delivery. Basically smaller cutaway=richer, larger cutaway= leaner. The slide cutaway only affects the engine from 1/8 throttle to 1/4 throttle [the jet needle/ needle jet combo take over from there at the 1/4 to 3/4 range], so basically used to tune for a better 'hole shot', but is still applicable for all engines for all applications.

    ^this is all just information that exists, that may or may not be relevant. Feel free to disregard it if you like.
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  9. #8
    Senior Member methamphetasaur's Avatar
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    Hey LT or Purple- I think this should be sticky-ed to the technical write-ups section. It's a very simply explained and well written explanation of a task some might find too intimidating to tackle on their own otherwise.

    Done deal bro, thanks for bringing it to our attention.

    Edit: Oops, I didn't realize moving it meant no more posts. I hope no one else had anything important to say.
    Last edited by methamphetasaur; 10-01-2019 at 12:59 PM. Reason: request to move thread
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