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  • Carb help

    Ok, I am trying to re-jet my carb for a 2000 1600 sliverado. I ordered the Dynojet kit from JP and it doesn't seem to be the correct jets... The Main looks too small and the pilot is just a stub jet, not the long one that is in the carb already. I am officially lost? Did i get the wrong kit? Am I supposed to use these parts that don't even look similar? Any help will be appreciated. Thanks in advance.

  • #2
    Pics of what you got, including the label?

    Wherever you go in life, ride there if at all possible.


    • #3
      If the parts look different then they are and you have the wrong kit. Send it back and order a Barons kit and also order a 37.5 mikuni pilot jet to complete the set for the proper jetting for your bike. A dyno jet kit is cheaper but don't really have anything in it that is usable except the adjustable needle. And that is even a little different than the Barons that has proven itself as best the best kit.


      • #4
        Here are some photos.


        • #5
          So what parts are different? None of the jets that come with that kit are the correct size for your bike. But that is what Dynojet corp thinks is right for the bike and they are wrong. That is one of the problems with the dyno jet kit. Their jetting recommendations are wrong for your bike. You can use the needle from that kit but order some mikuni jets. You should order a 167.5 Mikuni main jet and a mikuni 37.5 pilot jet. The dynojet 37.5 that comes in that kit is not the same as a Mikuni 37.5 the Dynojet 37.5 is the same fuel flow as the OEM 35 Mikuni pilot jet.


          • #6
            OK so i called JP cycles and they are going to have a Barons kit sent over-night. I will just send the dynokit back. They said the Barons did include the 167.5 and 37.5. Are those going to be the correct ones? I have stock pipes with baffles removed and k&n air filter. I am also porting a intake manifold for it. Also, Also, I am trying to go pump-less and installed the new bowl needle. I'm sorry if this is all in a thread somewhere already, I'm still getting used to navigating the new forum. Thanks!


            • #7
              Originally posted by V-Max View Post
              OK so i called JP cycles and they are going to have a Barons kit sent over-night. I will just send the dynokit back. They said the Barons did include the 167.5 and 37.5. Are those going to be the correct ones? I have stock pipes with baffles removed and k&n air filter. I am also porting a intake manifold for it. Also, Also, I am trying to go pump-less and installed the new bowl needle. I'm sorry if this is all in a thread somewhere already, I'm still getting used to navigating the new forum. Thanks!
              The Barons kit does not include a 37.5 pilot jet you have to buy that separately the Barons kit only includes a 35 pilot jet.


              • #8
                , I should have done all this during the winter....I am guessing the 35 will not work correctly?


                • #9
                  The 35 is a bit anemic. Fine for stock, but not for modded exhaust and intake. Pick up a n224.103 in the correct 37.5 from Jetsrus, or one of the others like PJ’s Motorsports.


                  • #10
                    I don't know. I used all Mikuni parts in my 1600 with, my true duals, Curt's manifold & K&N 1980. Runs great


                    • #11
                      This is a good understanding of how the carb works, and that jetting is relative to the actual bike in question, what works for one may not work for another, so, find a baseline and work from there .
                      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, you’ll 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 it’s 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 it’s 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 it’s opening when at it’s lowest position and allowing full flow at it’s 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 it’s greatest volume. Non-CV carbs don’t 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 Circuit’s 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 carb’s “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 it’s air (as real chokes do, hence the name), it has a circuit that infuses extra fuel into the intake tract. The enrichener (we’ll 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 it’s effectiveness. If the throttle is opened significantly, the “choke” may completely stop delivering it’s 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. It’s 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 carb’s 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 it’s strongest when slowing from road speeds, with the throttle closed. That’s 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 it’s 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 RPM’s) 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. That’s 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 it’s called 15-PSIA in the lab. The “A” stands for absolute, because it’s 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 it’s diaphragm (which gets it’s “pressure-signal” from a connecting channel above and upstream of the throttle plate), rises and carries the needle with it. The rising needle, with it’s 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 engine’s 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 it’s lowest (so the pilot system is virtually shut off), but air velosity above the needle jet is at it’s highest and the needle jet orifice is uncovered as much as allowed by the needle’s 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. That’s 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.
                      You’ll 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, Baron’s 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, it’ll come with instructions for removing the plug covering the Pilot Mixture Screw (PMS). On the Road Star’s 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”.

                      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 you’ll drill a small hole in its center and then screw in a sheetmetal screw, which will be used to pry out the plug. Be careful that when you drill through the plug you don’t 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 you’ll 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 ½ turns off the seat, so while counting the revolutions of the screwdriver handle, back it out that much. You’ll 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 you’re 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, it’s probably an air adjustment type. On carbs with the PMS below the float bowl gasket, it’s 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). It’s shape is altered by exchanging it for another of a different profile (rate of taper) and that’s typically done by purchasing a jet kit. The OEM needle isn’t easily adjusted, other than by raising it with the addition of washers placed under it’s 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 you’re 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. It’s 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.

                      You’ll be rejetting the carb to compensate for alterations to the engine’s 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 engine’s 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 engine’s 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 doesn’t burn as efficiently as the proper mixture will when the engine has warmed up, but will insure that the burn rate doesn’t 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 you’ve 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, it’s probably too rich on the pilot circuit, or the float level is too high and it’s flooding OR the new, “high performance” parts you’ve installed actually reduce the engine’s 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 hasn’t 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 doesn’t actually richen the mixture at most times. The PMS is still the primary fuel controller at idle, because it’s 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 hose’s inside diameter. Adding a nozzle on the end will allow throttling of the flow to anything under the bare-hose’s 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 don’t wiggle it side-to-side. That can break the tabs in the bottom of the slide and then you’ll 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 aren’t 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. You’re 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 you’re running at WFO a lot! In which case, you’re 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 don’t 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 you’re 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 shouldn’t 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. That’s because the dyno can’t 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, you’re 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 you’ve got no reason to waste your time with such folly.
                      Sorry could't post link as from old forum,
                      Sometimes it takes a whole tank of gas before i can think straight


                      • SKWEARpeg
                        SKWEARpeg commented
                        Editing a comment
                        I always liked that write up. If not done already, maybe the Mods could find a way to add it in the Tech Articles section.
                        A lot of the recommendations by Davej and a few others, are based on actual real time use with an AFR gauge mounted on their bikes.
                        When I first joined the Forum a half dozen or so years ago, the idea the stock 35 Pilot was not large enough, was roundly “poo-poo’ed” as un-necessary. Great discussions were undertaken about just how far out you needed to back out the PMS, to make things run right. A very small minority had already been stepping up on the Pilots to get rid of the off idle lean cough, with good results.

                    • #12
                      Originally posted by V-Max View Post
                      , I should have done all this during the winter....I am guessing the 35 will not work correctly?
                      The 35 will not work as nice as the 37.5. The 35 can and will in most cases cause a off idle stumble that is corrected with a 37.5. Think of it like this, When opening up the intake and exhaust with aftermarket assemblies it will "lean" the fuel mixture with the stock jets. When it leans the nixture it leans it ocross the board in all ranges and all carb circuits including the pilot circuit. Now with that in mind how do you think the oem pilot jet will handle the leaned out condition?The Main Jet can't that is the reason we rejet. With that said, to properly rejet you have to rejet all circuits. The kits try to compensate the lea spot in the pilot circuit by opening up the PMS adjustment to more turns out. In many cases that leads to loosing the PMS screw due to loosing the spring pressure under the screw that helps hold it in place. Turning it out also don't correct the transition stumble between circuits or off idle stumble. Installing the 37.5 pilot jet corrects both and the pms can be set to 2 - 2 1/2 turns out and that keeps spring pressure on the screw to keep it from vibrating out. The larger pilot also does not negatively effect MPG it actually helps in performance and maintains mpg. Keep in mind that most of the riding we do under about 45 - 50mph the pilot circuit is still in play. It isn't just at idle. I'm sure that these companies that sell the kits can justify the jetting with a dyno sheet but haven't done any real life long term testing like we have here on the RSC. Dyno testing for performance like they do to impress the buyer with a dyno sheet is nothing more than a 0 - wide open throttle test and not the on the road with a person setting on the bike from light to light like we ride in real life.

                      Edit: Here is another thought. When Yamaha had a performance product line called "Speedstar" There jet kits included a couple main jets, an adjustable needle and larger pilot jets. For the Speedstar BAK and Speedstar exhaust the jetting recommendation was to change the needle, leave the stock size main jet and only install the #50 pilot jet only.That was because the OEM main jet was already large enough to supply adequate fuel at WOT, fuel for the other ranges was supplied by the adjustable needle and the larger pilot jet.
                      I will say the # 50 pilot was a bit large and in my testing and dyno experimenting on my 1700 I found that a #40 pilot was a better choice with all of the jet kits for the 1700 R* and the 37.5 with the 1600 R*.


                      • #13
                        ......and, the reason the Speedstar needle worked fine with the big fat 1700 182.5 Mikuni Main, was because it is not as sleek as the Barons or DJ needles. Either of the latter two needles, will have you reducing the mains to a 172.5 or a 175.


                        • #14
                          Originally posted by SKWEARpeg View Post
                          ......and, the reason the Speedstar needle worked fine with the big fat 1700 182.5 Mikuni Main, was because it is not as sleek as the Barons or DJ needles. Either of the latter two needles, will have you reducing the mains to a 172.5 or a 175.
                          That is correct. the smaller main jets can be used with the other needles. The slimmer profile of the Dynojet and Barons needle allow an adequate amount fuel to pass with a smaller main jet due to the smaller diameter and taper profile. My point about the Speedstar was to point out that the Speedstar theory was to change the pilot circuit jetting to correct lean conditions from a BAK and exhaust change.

                          The single groove in the OEM needle is in the same location as the 4th groove on a Barons needle. MR Shamrock posted that he machined a OEM needle to match the profile of a Barons needle and it worked fine. I also tried it on a couple needle and installed them into carbs at the Celina M&G for some members that had carb issues but didn't have a jet kit available. The modified needle works fine and the members were happy with the results.
                          When modifying the needle you just have to have the proper measurements in I think it was 4 different spots to insure the taper is correct and the diameter falls right into place when the proper taper is achieved. I haven't done 1 in a couple years but I have a couple that need to be modified.


                          • #15
                            Yea this makes sense about the 37.5 pilot, as here in the uk the needle has 5 grooves and i do jet off idle stumble sometimes, the pms just seems to get better the more you turn it out, my main is big, dont know what size as it has no numbers , the jet on the left is 170 mikuni, the other is unknown, and the std needle with 5 grooves
                            Sometimes it takes a whole tank of gas before i can think straight


                            • brianmac
                              brianmac commented
                              Editing a comment
                              hotrod460 what kind of fuel do you get in the UK, may need slightly different jetting?

                          • #16
                            The PMS, is really just for the idle air/fuel mix. It regulates the amount of fuel available at idle. If you’re turning it out in hopes of fixing something else, you pretty much negate anything you tried to accomplish at idle.
                            If your bike was born a “Wildstar”, it came from the factory with the 5 groove needle.
                            If stepping up on the Pilot and richening the transition by raising the needle with a shim or a full groove still seems to leave a lean stumble, you may want to think making sure you have a good seal at the manifold to head flanges.

                            Without knowing who made the larger main, its difficult to determine the size. Mikuni sizes their jets according to flow rate. It’s why a DJ jet compared to a Mikuni of the same size marking results in a completely different size hole.


                            • hotrod460
                              hotrod460 commented
                              Editing a comment
                              Yea, true, im gonna ditch the unknown jet and stick with mikuni, gonna try the 37.5 as well, get the primary dialed in, then the rest, ive done the leak test so thats ok

                            • Roadie2007
                              Roadie2007 commented
                              Editing a comment
                              Pleassssse can you share a pic/diagram explain where the PMS is? Running rich (smells) 25mpg down from 35mpg I think a simple adjust will do the trick (more air in mix). 2007 Road Star.

                          • #17
                            I have RC-1980 air filter, ported manifold, AIS delete, bub exhaust w/o baffles. Replaced main with 167.5 and pilot with 37.5. I set pms to 2.5. This is the needle the po had in carb. Not sure who manufacturer is or if clip setting is right.
                            Bike seems to run fine except for having to give it more throttle from a stop bc of hesitation.
                            1999 Roadstar


                            • #18
                              It's hard to see but it looks like the clip is in the 3rd groove from the flat end. It should be in the 4th groove.


                              • #19
                                I just looked at your profile and it says the clip is in the 3rd groove. Move it to the 4th groove down from the flat end and your issue will be resolved.


                                • #20
                                  I've got a 2000 RSS with a stock carb. I have Freedom Performance 4" duals, a ported intake, a Barons BAK and no AIS. It runs great and gets 40-42 mpg at 75-80 mph. I'm running a stock pilot, 167.5 main with the needle on the fourth clip and a washer underneath. My RS pulls smoothly from idle all the way through to 4500rpm. I have great throttle response without having to grab big throttle to get out of a flat spot. Occasionally I get a cough through the carb when not fully warmed up, but other than that, no issues once it's up to operating temp. Going up a grade wide open in 3rd through 5th gear it pulls away slightly from a friends 2016 HD Dyna Street Bob with a 103 and a 2 into 1 exhaust!

                                  I don't like the Dynojet kits because the numbers on the jets they send are not similar/equal to Mikuni jets so you have to go off that chart in the tech section to find the equivalent jet size. I fiddle farted around with mine for 8 years...the previous owner used the Dynojet kit, it always ran rich, it popped and backfired! I could never get it to run right until I bought the jet kit from Shane at SS Custom cycles...he specializes in Road Stars and is very helpful. His kit only comes with Mikuni jets so the numbers make more sense.


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