Principles of modern motorcycle carburetor function specifically in application to the Yamaha XV1600 Road Star
by Ken “The Mucker” Sexton
Whatever the motorcycle or automobile, most carbs work on the same principles and internal systems to deliver fuel in the proper mixture ratio to the engine. The actual components within the carb(s) that use those principles vary quite a bit, but their ultimate execution remains the same. They can be broken down into separate circuits, called that because, like electrical circuits, they have defined paths of flow, cause and effect. The Road Star uses a Mikuni 40mm CV-type carburetor. The CV stands for constant velocity and refers to a theoretically constant speed of the air that passes under the throttle plate. But as you read further, youll see that the actual air speed is not so fixed. Still, at the outset, it must be mentioned that the OEM carb on the Road Star (and most emissions-legal street motorcycles, since 1978), being a CV-type carburetor, has a few significant design components that separate it from most pre-emissions and/or race carbs. Those carbs that are not CV-type are often called slide or throttle slide carbs. But more on that later.
The essential fuel delivery systems are:
#1- The Pilot Circuit (also called the primary, low speed or idle circuit) consists of a brass fuel jet- called the pilot jet (in the float bowl), the pilot mixture screw (outside of, but adjacent to the float chamber), and the pilot air-correction jet (in perimeter of the mouth of the carb). The Pilot circuit delivers its air/fuel mixture through a small hole in the floor of the carb outlet, downstream of the throttle plate. It regulates the fuel mixture at idle and small throttle openings, typically under one-quarter throttle. The pilot air correction jet admits air to the pilot system, through a channel above the pilot jet, as a fuel/air ratio modifier and emulsion improver.
#2- The Midrange Circuit, which is actually a component of the main system, is comprised of the needle, needle jet, slide assembly and throttle plate assembly. The slide has a diaphragm attached to its top, which serves to isolate the chamber above the slide from atmospheric conditions below it. The needle, which rides in the bottom of the slide and moves up & down within the orifice of the needle jet, acts as a throttle for the needle jet, by nearly closing its opening when at its lowest position and allowing full flow at its highest position. The midrange system takes care of the mixture between approximately one-quarter throttle and near-wide open throttle (WFO). The throttle plate, located between the slide and the carb outlet, acts to control ALL air that passes through the carburetor and, while doing so, controls the transition between the pilot and main circuits. At its (mostly) closed setting (idle), the throttle plate restricts the air passing through the carb throat to a minimal volume needed to maintain idle and, when fully open (parallel to the carb throat) the air flow into the engine is at its greatest volume. Non-CV carbs dont have a throttle plate (although they may have a choke plate in its place and therefore have no enrichener circuit), so they rely on manual (rider controlled) operation of the slide itself as the throttle.
#3- The Main Circuits ultimate components are directly below the needle jet and includes the midrange system (above) PLUS the main jet, emulsion tube (between the main jet and the needle jet) and the main-air correction jet (in the perimeter of the carbs mouth, opposite the pilot air correction jet). The function of the main jet is to limit the total amount of fuel available through the carb, at wide-open throttle. The main air correction jet admits air to the main system, through a channel that connects to the emulsion tube directly above the main jet, and that air also acts as a fuel/air ratio modifier and emulsion improver.
#4- The Starter or Enrichener Circuit: There is no true choke in the Road Star carb, or in most modern motorcycle carburetors. That is because, rather than strangling the intake tract of its air (as real chokes do, hence the name), it has a circuit that infuses extra fuel into the intake tract. The enrichener (well call it a choke for simplicity from now on) requires high intake vacuum downstream of the throttle plate to work, so opening the throttle during startup will actually reduce its effectiveness. If the throttle is opened significantly, the choke may completely stop delivering its expected fuel, until the throttle is closed enough to regain a high vacuum within the intake tract.
#5- The decel-enrichener system is a small device mounted to the side of the carb, containing a small diaphragm and spring. It adds an additional measure of fuel during the very high intake vacuum that exists during closed-throttle deceleration at road speeds. Its sole function is to help reduce exhaust backfiring during deceleration and it is not common to all modern motorcycles.
#6- The accelerator pump is just what it sounds like. A small diaphragm, acted upon by the throttle linkage and a plunger, gives a squirt of raw gas into the intake tract, whenever the throttle is applied from idle or near idle. The extra shot of gas is intended to compensate for a momentary lean condition, which occurs when the throttle plate opens and vacuum AND air velocity through the carb drops too low to draw fuel up through the normal fuel circuits.
Theory of Operation:
To understand how a carb works and how to make it work best for you, you should understand the simple laws of nature that allow the carbs systems to do their job. They are Vacuum and the Venturi Principle.
We all know vacuum is simply suction, or air pressure below that of ambient atmospheric conditions and it exists primarily between the throttle plate and the engine during idle, small throttle settings and deceleration. Intake vacuum will be at its strongest when slowing from road speeds, with the throttle closed. Thats because the engine is above idle speed and pumping strongly against the closed throttle plate. Think of a vacuum cleaner with your hand obscuring the hose opening. The vacuum cleaner may be pretty strong in its normal operation, but with your hand covering most of the hose end, the vacuum is so strong that the hose may collapse. By lifting your hand away from the hose-end vacuum strength drops and the hose can re-extend. In the Road Star carb and its intake tract, closing the throttle during deceleration creates a high intake vacuum and the higher the road speed (engine RPMs) the stronger the vacuum. When the throttle is opened again, the vacuum is progressively relieved as air is allowed to rush in.
Always remember that pressure and vacuum are relative conditions and directly related to atmospheric conditions. In fact, in scientific circles, they are considered the same things. Thats because, when compared to a total absence of air, any amount of air is a relative increase in pressure. If you were to remove all the air from the hull of an aircraft carrier and then add one cubic foot of air, you will have raised its pressure above what existed when it was totally evacuated. Even though it will still be at a pressure far below that which we live in and which would be commonly called a vacuum. We live in an atmosphere of nearly 15-PSI. That shows up as zero on the common vacuum gauges that most people see in tool and auto parts stores because atmospheric pressure is considered zero for most purposes, but its called 15-PSIA in the lab. The A stands for absolute, because its a measurement compared to a true lack of atmosphere. The common gauge that most stores sell actually shows PSIG, which stands for pounds per square inch gauge. Why do we care about all this It helps to illustrate how the various carb circuits work. Any differential pressures between two volumes will result in what we all consider low pressure, or vacuum, however strong or weak.
The Venturi Principle dictates that when a fast moving fluid (air in this case) is accelerated past the top of an otherwise enclosed volume (in this case the float bowl, by means of the needle jet, emulsion tube and main jet), the pressure above that chamber drops (the venturi effect). The area of the carb throat controlled by the rising and falling slide is called a venturi. The venturi effect creates a low pressure above the needle jet (essentially, a weak vacuum), which draws fuel up from the float bowl (which is kept at atmospheric pressure by means of a vent channel above the carb intake). At small throttle settings, airflow going under the slide and above the needle jet is minimal so the venturi effect is minimal and little fuel can be drawn up from the float bowl. But, as the throttle opens, the volume and velocity of the air passing over the needle jet rises, so the venturi effect creates a lower pressure above the needle jet (therefore a stronger vacuum pulling fuel up from the float bowl). The slide, which is controlled by mass-airflow and differential pressures between the volume under it and the volume above its diaphragm (which gets its pressure-signal from a connecting channel above and upstream of the throttle plate), rises and carries the needle with it. The rising needle, with its tapered shape, exposes an ever-greater amount of the needle jet orifice to the venturi, allowing an increasing amount of the fuel to rise from the main jet.
So, at idle, when the throttle is almost closed, there is a strong vacuum downstream of the throttle plate. With a strong vacuum between the throttle plate and the engines intake tract, the pilot system is the controlling system for fuel delivery. The needle jet, which is isolated from that vacuum, can deliver little fuel as a result. As the throttle is opened, allowing more airflow to the engine and causing the needle & slide to rise from its lowest position, intake vacuum weakens (causing a drop in pilot fuel delivery) and air flow above the needle jet rises in volume and speed. As the needle and slide continue to rise, they expose an ever-greater amount of the needle jet orifice to the airflow above it, causing a progressive rise in fuel flow with it. At WFO the vacuum within the intake entire tract is at its lowest (so the pilot system is virtually shut off), but air velocity above the needle jet is at its highest and the needle jet orifice is uncovered as much as allowed by the needles fine tip. The main jet is now in control. Then, when you close the throttle from road speed, intake vacuum rises to even greater levels than normally exist at idle. So during deceleration, the high intake vacuum can draw more fuel from the pilot circuit than it could at idle. Thats a good thing, to help the decel-enrichener system combat exhaust backfiring. But more on that later.
There are no absolutes in carburetors. The three circuits overlap, so at any given throttle setting, engine RPM, and intake pressure there will tend to be some gas delivered to the engine through more than one circuit. As an example, at a steady 60 MPH cruise speed in top gear, the pilot and midrange circuits will both be delivering some fuel, but because the throttle setting is so small, causing a high intake vacuum, the pilot system will be the dominant factor in determining fuel delivery. As the throttle is opened and intake vacuum drops, the pilot will be progressively retired and the needle, needle jet and main jet become more dominant. Conversely, even at idle when the pilot system is the primary fuel metering circuit, airflow above the needle jet may allow some fuel to be delivery from the main system. The Road Star in particular, has a slide which never drops to the carb floor (as most other slide-equipped carbs have throughout the history of the design), so it has a greater potential overlap between the pilot and main systems than many other motorcycles.
Youll rejet your carb to improve engine performance, fuel mileage, make it work with alterations to changes in intake and/or exhaust breathing (K&N filter, free-flowing aftermarket pipes, etc.) or correct for poor running characteristics related to fuel mixture. Generally this is best accomplished by purchasing a jet kit from manufacturers like Dynojet, FactoryPro, Barons Custom, K&N, etc. Such kits generally give you a needle with multiple adjustment grooves (compared to the OEM needle with only one setting, other than by shimming it up with small washer), several main jets to accommodate varying intake & exhaust modifications, a drill to facilitate removal of the PMS plug, varying hardware such as float bowl screws and needle adjustment washers and detailed instructions. Read all instructions that come with the jet kit and you should have no problems. The typical procedure for rejetting is as follows
If you buy a jet kit, itll come with instructions for removing the plug covering the Pilot Mixture Screw (PMS). On the Road Stars carb it is located under the carb outlet, adjacent to the float bowl. It is in a recess near the carb heater (which protrudes below the float bowl gasket) and centered under the carb throat.
Caution: There is also a flush-mounted brass plug between the PMS and the float bowl, do not attempt to remove or drill out that plug. If for some reason you do remove this plug, you will have to solder up the resulting hole in the brass plug from the drill bit. Then force the brass plug back in the hole from where it came.
In order to make adjustments to the pilot circuit, it is necessary to remove the brass plug, behind which the PMS is hidden. The plug is within an indentation of the float bowl edge (the brass circle above the jets, Typically youll drill a small hole in its center and then screw in a sheet metal screw, which will be used to pry out the plug. Be careful that when you drill through the plug you dont allow the drill to drop onto the brass PMS adjustment screw and ruin its screwdriver slot. Once the plug is removed, you can insert a small screwdriver and make adjustments to the idle/low speed mixture as needed. The PMS is an adjustable jet (car people tend to call them needle jets) which allows a fine adjustment of the air/fuel mixture delivered from the pilot air-correction jet AND pilot (fuel) jet. Start out by screwing in the PMS until it lightly closes onto its seat (excessive torque will damage the PMS screw and the aluminum seat within the carb casting). If you look into the carb outlet youll see the tiny tip of the pilot mixture screw protruding into the bottom of the carb throat. For most purposes, the baseline PMS setting is 3 turns off the seat, so while counting the revolutions of the screwdriver handle, back it out that much. Youll back it out more if the engine needs more fuel at idle and decel and turn it in for a leaner setting. Note: Backing out the PMS on the OEM CV-carb richens the mixture, while on many non-CV carbs the PMS meters air (not fuel) and so their function is the reverse; backing them out leans the mixture. If youre unsure of the type of pilot mixture screw on your non-OEM carburetor, the rule of thumb is the following: if the PMS is located above the gasket surface of the float bowl, its probably an air adjustment type. On carbs with the PMS below the float bowl gasket, its a fuel adjustment type. Most modern, non-CV, smoothbore or race carbs have a PMS that regulates air to the pilot circuit.
The pilot system also contains a pilot jet within the float bowl and it sets the maximum fuel possible through the pilot system. By exchanging it for one with a higher number, you set a higher potential amount of fuel that the engine can receive during high intake vacuum conditions.
The design (shape) of the needle and its height adjustment is the principle method of adjusting the mixture between idle and wide-open throttle (WFO). Its shape is altered by exchanging it for another of a different profile (rate of taper) and thats typically done by purchasing a jet kit. The OEM needle isnt easily adjusted, other than by raising it with the addition of washers placed under its head. Needles that come with jet kits (and race carbs) will have either 5 or 6 grooves cut around their top portion. The grooves allow adjustment by the placement of a small E-clip (often called a Jesus Clip because of the exclamation frequently made when the little bastards fly across the shop floor, when youre trying to slip them into the needle groove).
The main jet is screwed into the end of a tube (the emulsion tube), within the float bowl chamber. It is simply exchanged for one with a bigger or smaller orifice, depending on the need for a richer or leaner mixture at WFO. Its number, stamped on the side rises as the jet gets richer.
In the photo below, the three brass jets between the white fuel floats are the main jet (lowest), the starter jet (above the main jet and to the left of the pilot jet) and the pilot jet (on the right). You’ll have no need to alter the starter jet. Whenever the float bowl is removed, use extreme care that the float is not banged. Its adjusting tang (the T shaped silver part near the bottom of the picture), for float/fuel level, is easily bent out of adjustment.
Youll be rejetting the carb to compensate for alterations to the engines breathing efficiency, and/or correct for poor OEM setup (typically too lean as a result of EPA mandates). Most aftermarket exhaust systems will effect the engines breathing enough to call some recalibration. Changing the air filter and/or entire air-box/filter assembly to a less restrictive design will fairly scream for a jet kit. Failing to properly re-establish the engines correct air/fuel mixture after significantly improving the breathing characteristics can cause damage to the engine itself. Why is that The answer lies in some more theory
A theoretically perfect air/fuel mixture (called stoichiometric) burns in a controlled, rapid speed within the combustion chamber. The engine is designed to work with that rate of burn. If the mixture explodes (as opposed to igniting in a rapid growth from the spark plugs) or burns too slowly, the result can be anything from poor performance, poor mileage, reduced driveability, or damage to the engine itself. A lean mixture tends to burn too slowly and causes hesitation to throttle input, reduced mileage, intake backfiring (POP! up the intake tract), exhaust backfiring (BANG! from the tail pipe), surging at steady throttle cruise speed, and/or engine overheating. A rich mixture burns faster than a lean one, but may cause poor mileage, smell of unburned gasoline, and/or reduced power. Because thoroughly atomized gas ignites more reliably and burns more efficiently AND a warm engine helps to keep the fuel/air mixture atomized, it is necessary to compensate for a cold engine by adding extra fuel during cold starting. The extra gas added by the choke system doesnt burn as efficiently as the proper mixture will when the engine has warmed up, but will insure that the burn rate doesnt drop below a critical speed. If the mixture burns excessively slow at idle (as a result of poor atomization or a lean mixture), it may still be burning at the end of the exhaust stroke (called overlap) when the intake valves are opening. In that case, the still-expanding gases may rush up the intake tract and the result is a POP! which may even be severe enough to stall the engine.
So, perhaps youve installed a free-flowing K&N air filter and/or a set of pipes. Does it hesitate when you apply throttle from idle Or require an extended period on choke before it will run cleanly Has it suddenly begun to backfire up the intake or from the tail pipe If so, the pilot circuit is probably too lean. Conversely, if it suddenly has the miraculous ability to start on a cold engine, without benefit of the choke, its probably too rich on the pilot circuit, or the float level is too high and its flooding OR the new, high performance parts youve installed actually reduce the engines breathing ability (NOT as uncommon as you might imagine). A properly set-up carb will require choke on a cold startup, be able to pull away cleanly on half-choke within a couple of minutes and run well without the choke within about a half mile.
Usually a lean pilot circuit simply needs to have the pilot mixture screw (PMS) backed out more, to admit more fuel during idle, small throttle and deceleration. If backing the PMS out 4 or more turns still hasnt fixed the problem, swap the pilot jet to a larger one (#40 or #45 in the Road Star). In the photo below, the correct pilot jet design for the Road Star is on the left. The other three are examples of different models of pilot jets that will NOT work with the Road Star.
The larger pilot jet allows a greater potential fuel delivery from the pilot circuit, but doesnt actually richen the mixture at most times. The PMS is still the primary fuel controller at idle, because its orifice is normally restricted to something less than that of the pilot jet. But if the pilot jet orifice is smaller than that at the PMS, it limits the peak fuel delivery. Think of a garden hose with an open end. At any given pressure, the maximum flow is limited by the size of the hoses inside diameter. Adding a nozzle on the end will allow throttling of the flow to anything under the bare-hoses maximum. Even if the nozzle can be opened up to something larger than the hose ID, the flow is still limited to what the bare-hose can deliver. In this analogy the hose ID correlates to the pilot jet orifice and the nozzle is the pilot mixture screw. The difference between the hose analogy and your pilot system is that, at any given PMS setting, the larger pilot jet can deliver more fuel during very high vacuum (like decel).
A proper pilot setup should idle smoothly (consistent firing in the case of our Big Twin), accept throttle well, require choke during cold startup- but not for extended periods, and be free of any backfiring.
The needles setting is both straightforward and potentially time consuming. Most jet kits come with instructions giving an initial setting. Pay close attention to the assembly instructions. Be careful with the slide diaphragm. It is thin rubber and can be easily damaged. It may stick in the groove around the carb top, so cautiously coax it out. If you manage to put a hole or tear in it, replace it. The needle is held under a plastic retainer in the bottom of the slide. I suggest you use a set of duck bill pliers to pull out the plastic piece and be sure to pull straight out and dont wiggle it side-to-side. That can break the tabs in the bottom of the slide and then youll have to buy a new one. I like to lube the little o-ring on the plastic retainer with a thin coat of silicone grease, so it comes out easier thereafter. The kit manufacturer should tell you what grove (counted from the fine tip or the fat end of the needle) to put the E-clip into and on which side of the e-clip to put the original AND kit-supplied washers. You may vary from that recommended initial setup, after some experimentation on the road, but start with the recommended settings.
A lean condition, on the needle will give less than optimum acceleration, may result in surging during cruise and give poor gas mileage. Too rich will tend to give even worse mileage and, in extremely rich running, may foul spark plugs while yielding weak performance. Remember, the pilot circuit is a big factor at small throttle openings, so while cruising at 60 MPH in top gear (you arent opening the throttle much at such times), the pilot circuit is still a factor of the jetting. The best method to determine the optimum needle setup is through experimentation. After riding the bike with the kit suggested setup, try lowering the needle a notch (raise the clip to the next groove up) and ride it again. Is it stronger Weaker Does it surge at steady throttle Try raising the needle from the initial setup and note the changes. Youre looking for the setting that falls just a bit richer than the lean setting that runs poorly. With that, it should run smooth, respond well, accelerate strong and give good mileage.
The main jet is simply replaced with one supplied in the kit. Most kits give several main jets, so you can go leaner or richer than the recommended jet. Again, start with the suggested jet and swap it from there as subsequent testing suggests. Essentially, the main jet is to the main system as the pilot jet is to the pilot system. It sets the maximum fuel that the engine can get at WFO, while the needle sets the fuel flow at settings less than full tilt. For that reason, with big cruisers like the Road Star, the main jet is the least critical part of the jetting to get perfect (unless youre running at WFO a lot! In which case, youre probably worried less about the jetting than you are about being hauled away in a squad car or ambulance). Contrary to popular belief, many modern bikes actually come from the OEM setup fairly rich on the main jet, in an effort to compensate for lean needle parameters. So, depending on the intake and exhaust alterations, you may not need to go much larger than the stock main jet, if at all. Determining the best setup on the main jet is simply a matter of determining the best power at WFO. Only experimentation will do that.
Is jetting really that simple Well not always. There are other things that can give the same symptoms as a lean condition. Intake leaks between the carb and the engine can introduce enough air to cause a lean condition in a setup that would otherwise be fine. Exhaust leaks at the header-to-head mounts can cause backfiring from the tail pipe, by introducing enough air into the pipe to ignite unburned gas that would otherwise leave as unburned dinosaurs. Even the emission system, commonly called the Exhaust Air Induction System, can malfunction and introduce air into the exhaust that can cause backfiring. If the accelerator pump is delivering too much fuel or delivering it too late, it can cause intake backfiring as well. If the accelerator pump is delivering too little gas, the engine may simply hesitate when you apply throttle from idle. Float level can be too high or too low and that effects any, or all, jetting parameters. A high fuel level richens the mixture and a low level can lean it. Only experimentation will tell the tale, but first determine try altering the jetting. If conventional adjustments to the jetting dont result in improved performance and ride-ability, check for intake and/or exhaust leaks, adjustments to the accelerator pump, and/or float level depending on the prevailing symptoms. If youre careful not to knock the float out of adjustment when you are swapping jets (AND the factory set it right in the first place), you shouldnt have to alter the float adjustment.
A word on other tools and procedures to help in determining the optimum jetting.
Dynamometers can be an asset in finding a good setup quickly. They can save you the time it takes to suit up and go for a test ride. By doing some experimentation with the jetting, while monitoring power production throughout the RPM range (and often while also monitoring exhaust gas quality), a good setup can be found very quickly. But typically, the dyno will get you close quickly, with the lingering need to check final results on the road. Thats because the dyno cant mimic real road conditions. The way you ride, the flow of air past the intake and actual exterior atmospheric conditions may dictate some final jetting adjustments. A good tuner and his carefully calibrated bum can often do as well, or better, than a dyno and a team of technicians.
Reading spark plugs on a street bike, running modern, lead-free gas, to determine the jetting is of little use, if at all. Racers do it, while running consistently superior gasoline of known qualities, on race tracks and they can get very good results. Any time you examine your spark plug deposits, youre mostly seeing the results of combustion that existed at the moment the engine was last shut down. If you rode it, pulled into your garage in a normal manner, let it drop to idle and then turned off the ignition, the plugs will show the result of combustion at idle just before you turned the key off- NOT how lean or rich the main jet or needle setup are running. Add to that the dubious quality of modern, pump gas, which can give all sorts of doubtful deposits on the spark plugs and youve got no reason to waste your time with such folly.