MIKUNI MAIN JETS EXPLAINED or HOW I SAVED BIG BUCKS JUST BY LEARNING THIS STUFF OK folks, here’s the full meal deal on Mikuni main jets. This is from various sources gathered over the years. Read this carefully 3 times. It helps to have an example of each style of jet at hand when reading this explanation.
I use a heat gun to warm up the post areas, then spray condensed air (used on computers) to spray the pin. Enough of the cold air spray will freeze the pin - you'll see frost. The pin comes out easy then.and use a proper sized center-punch (like our HCP17638) to gently punch out the pin. Some good info and images can be found here:andBut whatever you do, don't try to 'force' it out, or it might end up like this:HCP17638 Aftermarket miniature hand-held CENTER PUNCH, proper sized thin point tip is the perfect size for driving out the float pins on Mikuni carbs. Adjustable punch force control and a knurled, sure-grip handle makes this a 'must-have' tool when working on your carbs.$ 18.95HCP17689 Aftermarket replacement PUNCH TIP for the HCP17638 center punch above.$ 4.95HCP5188 OEM Mikuni FLOAT, float only (pin is not included and must be ordered separate), fits all XJ550, 1986-88 FZ600, 1986-1990 YX600 Radian, and XJ650 Turbo models. NOTE: the float pin tube on this float bracket is bushed as original.
Each:$ 42.95HCP5188SET4 OEM Mikuni FLOATS, set of 4:$ 164.95HCP9940 Aftermarket Mikuni FLOAT and FLOAT PIN SET, fits all XJ550, 1986-88 FZ600, 1986-1990 YX600 Radian, and XJ650 Turbo models. NOTE: the float pin tube on this float bracket is not bushed as are the originals. Each:$ 29.95HCP9940SET4 Aftermarket Mikuni FLOAT and FLOAT PIN SETS, set of 4:$ 109.95HCP9944 OEM Mikuni FLOAT, fits all 1984-85 FJ600, XJ700-X, XJ750-X, all XJ900RK, RL, N/FN, and F models, XJ1100, and all 1980-81 XS1100 models. Float pin is not included. NOTE: the float pin tube on this float bracket is bronze-bushed as original. Also fits the following models: 1980-82 SR250, 1982-83 XS400 Maxim and Seca, XS650 G/H/SH/SJ/SK, 1984-85 FJ1100, 1987 FJ1200 and 1989-93 FJ1200 carbs. Each:$ 59.95HCP9944SET4 OEM Mikuni FLOATS, set of 4:$ 229.95HCP1668 Aftermarket Mikuni FLOAT and PIN SET, fits all 1984-85 FJ600, XJ700-X, XJ750-X, all XJ900RK, RL, N/FN, and F models, XJ1100, and all 1980-811 XS1100 models.
Float pin is included. NOTE: the float pin tube on this float bracket is bronze-bushed as original. Also fits the following models: 1980-82 SR250, 1982-83 XS400 Maxim and Seca, XS650 G/H/SH/SJ/SK, 1984-85 FJ1100, 1987 FJ1200 and 1989-93 FJ1200 carbs. Each:$ 29.95HCP1668SET4 Aftermarket Mikuni FLOAT and PIN SETS, set of 4:$ 114.95HCP23266 OEM Mikuni brass FLOAT, fits all 1978-79 XS1100 models. Float pin is not included.
NOTE: the 1980-81 style plastic floats can be used in place of these original brass “barrel” floats. Each:$ 54.95HCP23266SET4 OEM Mikuni brass FLOATS, set of 4:$ 209.95HCP23267 Aftermarket Mikuni brass FLOAT, fits all 1978-79 XS1100 models. Float pin is not included. NOTE: the 1980-81 style plastic floats can be used in place of these original brass “barrel” floats. Each:$ 19.95HCP23267SET4 Aftermarket Mikuni brass FLOATS, set of 4:$ 69.95HCP22367 OEM Mikuni FLOAT, fits all 1992-98 XJ600 Seca II models. Float pin is not included. Each:$ 62.95HCP22367SET4 OEM Mikuni FLOATS, set of 4:$ 239.95HCP5190 OEM Mikuni FLOAT PIN, fits all XJ550, 1984-85 FJ600, 1986-88 FZ600, 1986-90 YX600 Radian, XJ650 Turbo, XJ700-X, XJ750-X, all XJ900RK, RL.
N/FN, and F models, XJ1100, and all XS1100 models, each:$ 4.95HCP5190SET4 OEM Mikuni FLOAT PINS, set of 4:$ 17.95HCP14532 Aftermarket Mikuni FLOAT PIN, fits all XJ550, 1984-85 FJ600, 1986-88 FZ600, 1986-90 YX600 Radian, XJ650 Turbo, XJ700-X, XJ750-X, all XJ900RK, RL. N/FN, and F models, XJ1100, and all XS1100 models, each:$ 3.95HCP14532SET4 Aftermarket Mikuni FLOAT PINS, set of 4:$ 13.95HCP22366 Aftermarket Mikuni FLOAT PIN, fits all 1992-98 XJ600 Seca II models, each:$ 5.95HCP22366SET4 Aftermarket Mikuni FLOAT PINS, set of 4:$ 23.50Unlike the Hitachi carbs, Mikuni carb floats used a precise-fitting hinge pin to secure the float to the formed 'tube' that the float pin slides thru on the float bracket arm. This design feature results in a float that typically has very little side-to-side 'slop' in its motion and movement. Any slop that does exist can, however, allow a bit of erratic movement in the float on the pin, and can contribute to difficulties with getting the fuel level set properly and consistently on these carbsWe offer some very precise SHIMMING WASHERS that allow you to take up this slack. Unfortunately, due to variations in the machining of the float post inside distance on various car bodies, there is no 'one-size-fits-all' solution that we can offer as to what thickness shims are going to be correct to use. The best guidance that we can offer is that you want to eliminate equal amounts of slop on both sides of the float pin tube, while still allowing for enough clearance on both sides to eliminate any chance of the float binding on the pin.Our research shows that a typical carb body on the BS28 series carbs (XJ550 models) will have float posts that have approximately an 18.20mm inside-to-inside distance, at the level where the pin goes thru the float posts. The pin tube on the float, however, is only about 17.60 - 17.80mm wide.giving a total amount of 'slop of 0.60mm between the pin tube and the float posts.note that this 0.50mm of slop is equally divided on both ends of the float pin tube, resulting in a clearance of approximately 0.25mm on each side of the pin tube.The BS30 series carbs (used on XJ650 Turbo models) will have float posts that have approximately an 18.10mm inside-to-inside distance, at the level where the pin goes thru the float posts.
The pin tube on the float, however, is only about 17.60 - 17.80mm wide.giving a total amount of 'slop of 0.50mm between the pin tube and the float posts.note that this 0.50mm of slop is equally divided on both ends of the float pin tube, resulting in a clearance of approximately 0.25mm on each side of the pin tube.The BS33 series carbs (used on XJ700-X and XJ750-X models) will have float posts that have approximately an 18.20mm inside-to-inside distance, at the level where the pin goes thru the float posts.
Mikuni Carburetor Tuning for VinMikuni Carburetor Tuning for Vintage 2-strokesOverview- There are many manuals and resources that offer tuning info for Mikuni“Round-slide” carbs, but most lack “real world” explanations of how to do theproper preparation along with that tuning. During the 1970s, we were the techsresponsible for the jetting of countless aftermarket carbs for much of theoff-road (and road) aftermarket. This tuning guide is written with thebackground and experience of that development and tuning work.This document contains alot of “before you start jetting” information that seemstedious.
However if you follow the outline, the actual tuning process will gorelatively quickly, and with a minimum amount of drama.Fine Tuning Vs Jetting from Scratch- Fine tuning of stock carbs is generally not too difficult because so much ofthe basic tuning choices (needle taper and needle-jet) are already done.Tuning “from scratch” is much more time consuming because of the infinite numberof needle-taper / needle-jet combinations that should be tested.About Jet Needle Tapers- Needle tapers deserve mentioning before any other jetting subject. Mikunineedles are made in several different lengths to suit the various differentsized carburetor bodies. In addition to that, the needles are made in countlesssingle and double taper angles, as well as differing starting diameters forthose tapers. OEM manufacturers conducted weeks of testing to isolate theideal needle taper for each particular model of bike.
Sadly, there is no way tocalculate or predict which particular taper will work best for any given engine,so the only way to know it to try them. An added complication is that everydifferent tapered needle will need to have a differentpilot-jet/slide/needle-jet combination to yield it’s best results. And thereis no way to calculate those variables either.
If you are fine tuning carbs ona “close to stock” engine setup, it is best to stick with the stock needletaper. However if you are starting from scratch, you could have a world of workin front of you.Jetting From Scratch- An Example of the way it was done in the day- Many 70’s off-road bikes were equipped with tiny carbs, and racing versionsof those machines needed much bigger carbs. In most cases, the needle from thestock carb was too short to be used in the new larger racing carb. This meantthat aftermarket race shops had to calibrate all the jetting specifications inthe larger carb to a new needle taper.In a typical development project, we would gather six or more carbs that wereall identical except for having a different needle-taper.
We would then spend afew hours tuning with each individual carb, to get the best possible overallresponse and acceleration that we could get with that particular needle taper.We would then set that finished carb aside, and repeat the tuning process witheach of the other 6+ different needle-taper carbs in the test. We learned thateach carb had a very different combination of pilot-jet, slide, and needle-jetthat worked perfectly with each particular needle. We called these varyingcombinations “jetting spectrums”. Once we had found the ideal jetting spectrumfor all of the different carbs, we then conducted a series of back-to-back tracktests with all these carbs to determine which needle taper/jetting-spectrum gavethe best acceleration and throttle control. The differences were sometimessubtle, but in every test there always was one clear winner. This entireprocess for one bike took about a week, but it was time well spent for anoff-road model that would sell 300-500 pre-jetted carbs in a year.Back to the real world of today, very few people have the time (and carburetors)for this kind of testing. Knowing this, if you are jetting from scratch, itwould be well worth the time to try to learn what the most popular needle taperhas been for the particular bike and carb size you will be working with.
Inmost cases, the data is out there.How“Signal” Affects Jetting- The subject of inlet signal can be very lengthy and complex, so we will staywith an abbreviated version as it relates to your jetting.Many folks believe that air entering the mouth of the carb is what picks up thefuel from the jetting circuits and that is false. In truth, the crankcase ofyour 2-stroke sends a negative pressure wave (aka “signal”) up the inlet portfrom the crankcase, and that negative pressure wave literally draws fuel fromthe jetting circuits, as well as air in through the carb mouth. While it soundslike two different ways of saying the same thing, the difference becomesimportant.
For instance, if you install an aftermarket inlet manifold that hasa much larger internal volume, the “signal” is very much weakened by that addedvolume. Weaker signal results in ideal carb settings that will change alot withchanges in altitude and temperature (as well as much harder starting). An inletwith “strong signal” offers much easier starting, and can easily accommodate bigchanges in altitude and temperature with no need for jetting changes. The tablebelow explains more.
Increases Inlet Signal (good)Decreases Inlet Signal (bad)Smaller Carb ThroatExcessively Large Carb ThroatMore Restrictive Air FilterLess Restrictive Air FilterPod Filter on Rubber Inlet TubePod Filter on CarburetorDecreased Inlet Tract VolumeIncreased Inlet Tract VolumeProper Fitting Carb SlideLoose Fitting Carb SlideTight Sealing ReedsFractured or Poor Sealing ReedsReed Cage StufferReed “Spacer”Thepoint of all this, is to show that many “non-carb” related items can make itmuch harder to tune your carb perfectly. A 125 that is fitted with a new largeinlet manifold and a 36mm carb will be very hard to jet properly because theinlet signal from the crankcase has been so badly weakened by the added volume.It is often wiser to increase to just a 32mm carb, and the smaller volumemanifold used for the 32. The 36mm carb can be made to work on a 125, but itwill need constant fine tuning for changing air conditions.
The 32mm will tunemuch easier, and require far less future tuning.The two manifolds on the left are both for a CZ400 motocrosser. Thecast manifold on the right was made very long so the carb would reachnicely to the airbox with a short rubber tube, but that addedinlet-tract length significantly increased the internal volume andthat added inlet-tract volume significantly weakens inlet signal. Thebillet manifold on the left has a much smaller internal volume thatstrengthens inlet signal. The two manifolds are marked with theirrespective internal volumes (in cubic-centimeters). Reducing thatvolume by 53cc will make for a huge improvement in overall throttleresponse as well as much easier starting.Increasing inlet volume to make for a closer fit to the airbox is muchlike putting shorter shocks on an MX bike so your feet more easily reachthe ground Both “sound” like a good idea, but functionally-speaking,both are a very bad idea.
The designer of this large cast manifoldcertainly had the best of intentions. But sadly, not much of anunderstanding of the importance of inlet signal.The photo on the left shows the carb mounted on the short billetmanifold. The reduced inlet volume resulted in easier starting, quickerthrottle response, and much better overall carburetion. A slightlylonger air boot was needed to reach the airbox, but that length does notharm performance in any way.Important Jetting Protocols– These tools will help to decrease tuning time, and offer clear indicators totell if you are going in the right or wrong direction.Find a Hill- By far the best place to do 0-60%-throttle jetting is on a long straight roadwith an uphill grade. The added load of heading up a grade will more vividlyshow the acceleration differences in low speed carb adjustments.Notch the Throttle Grip- While it sounds like a back-woods thing to do, it can be very helpful tonotch the large flange of the throttle grip to allow you to clearly see the 0%,25%, 50%, 100% throttle settings. A corresponding mark can be made on thethrottle body, so at a glance you can see exactly where in the throttle rangeyou are.
This is particularly helpful for tuning in the 0-50% ranges.Digital Tachometer- Evaluating the best main jet size by seat-of-the-pants testing is bothinaccurate and darn risky. Whether you are testing on the road, or on a dyno,the “only” way to perfectly evaluate the ideal main jet size is by viewing thepeak digital tach numbers in high gear.
For off road bikes, taller gearing maybe needed to properly load the engine for this testing. We consider analog(needle) tachs to be very poor tools because they do not display rpm increaseand decrease “trends” with near the accuracy that a good digital tach does. Themost effective and affordable digital tach is the “Tiny Tach” commercial model.This tach installs quickly and offers very accurate data (updates every halfsecond). Avoid digital tachs with longer update times (there are plenty).Warm Engine– Whenever you begin any meaningful carb tuning, the engine must be fully warmedup (and ideally “cleaned out”). Attempting to do carb fine tuning on an enginethat has not been fully warmed up and cleaned out, is waste of time.About “Plug Reading”- In the 1970s spark plug “reading” was a very commonly used means of finetuning race engines. Sadly, this method was only valid for main-jet setting,and it required that the rider get a clean “plug-chop” (simultaneously shuttingoff from full-throttle, and hitting the kill button while at peak rpms in highgear).
If you do not have a clean plug chop, the plug reading is useless. Inlater years, the real time deto-sensors introduced in the 1990s made plugreading a Neanderthal means of fine tuning, and no professionals do it anymore.It also bears noting that today’s pump gasolines (laden with oxygenates andvarying ethanol percentages) do not “color” spark plugs nearly as quickly asrace gas (or 1970’s pump gas). The long time it takes today’s pump gas toproperly color a plug is even more reason to not bother with reading plugs.There is certainly validity to looking at the spark plugs on a regular basisjust to confirm the there are no internal catastrophes in progress. But forfine tuning, plug reading is a process that takes up so much time and offerssuch questionable precision, no professional tuners rely on it anymore.Another tuning fable that still surfaces occasionally is that different areasof the spark plug tip/electrode are indicators of various low speed jettingranges.
This is absolutely untrue, and it always has been untrue.UseRace Gas for Tuning- We strongly recommend to use race gas for initial tuning, even on enginesthat will be run pump gas here is why. The great fear of testing high speedjetting is the risk of seizing a piston with a jet that is too lean. The truthis that 99% of these seizures are not caused by the lean mixture, but rather bya high-percentile of detonation strikes that overheat the piston. If you tunewith 110+ octane race gas, the risk of experiencing any detonation at all isvery remote. If you install a main jet that is too lean (while running racegas) the digital tachometer will show a significant loss in peak rpms longbefore the engine will experience any dangerous detonation. It is a false fablethat engines will perform their best at the brink of being too lean. Allprofessional tuners know that the ideal main jet always offers the besthorsepower (and best peak tach numbers).
Any jet that is leaner than ideal willalways show a loss of peak tach numbers.Carb Inspections Before Tuning- Needless to say, the carb(s) float bowls should be clean of any debris, and afresh in-line fuel filter should be installed to avoid future debris. It isalso a good idea to confirm that the pilot and main jet both have clear orificeopenings. Lastly, the float level should be checked, and the sealing of needle& seat should be confirmed. A leaking needle & seat, or a ”too-high” floatlevel will cause an erratic low-speed rich condition that you will not be ableto tune away.Engine Inspection Before Tuning– By far the most important engine inspection is to confirm that the engine hasno air-leaks. We have a document on our site that outlines how to pressure testyour engine.it is absolutely mandatory. Doing carburetor fine-tuning on anengine that has any air-leak at all, is a complete and total waste of time.
Anengine with an air-leak will be hard starting, idle erratically, and be at highrisk to score a piston skirt.About Tuning 2 and 3 Cylinder Engines- By far the most important preparation for tuning 2stroke twins and triples isto first confirm perfect carburetor synchronization. By synchronization, wemean that the slides are being lifted away from the full-closed (not idle speed)position at the exact same moment. Later twins (like the RD400) have removable“sight plugs” that allow for quick and easy visual synchronization. Howeverearlier twins and triples have to be done “by hand”. That is, holding twofingers up against the slides as they sit in the full closed position, thenslowly turn the throttle to confirm that both slides are being lifted at theexact same moment.
If not, the cable adjusters on the carb tops should beadjusted to achieve perfect synchronization.Tuning Vocabulary– If you are planning (at some point) to get advice from an experienced tuner,it’s important that you use the right terms to describe the symptom you aretrying to resolve. Terms like hiccup, stall, die out, etc. Don’t offer anyuseful diagnostic information.
Below is a list of useful and commonly usedterms.Bogor Hesitation– These two terms are common, and they usually describe a slightpartial-throttle lean condition in a particular throttle opening range. It iswise to see if the bog (or hesitation) disappears with a smaller or largerthrottle opening, however it is unwise to steadily hold the throttle at asetting that is causing a bog or hesitation.Loading Up- This term describes a rich condition, and describes how the spark plug isbeing overloaded with fuel. If the throttle is steadily held at a throttleposition that “loads up”, the spark plug will eventually be unable to sparkthrough the excess fuel/oil mixture referred to as “fouling” the spark plug.Once a spark plug is fouled, it should be replaced.Stutter- This describes a very slight rich condition in a partial throttle range, thatcan cause the engine to run roughly, but not quite load-up. Some tuners alsouse the term “four-stroking” to describe this condition.Cleaning out– Describes when the throttle is opened to a clean running rpm range, afterbeing held at a lower setting that causes a stutter (or loading up). Veryliterally, the excess fuel on the spark plug is being “cleaned out” of thecombustion chamber, and the engine suddenly runs very smooth.Hitting the Power-band- On many high-performance 2stroke engines, there is one particular rpm whenthe engine very suddenly enters the strongest part of the power-band, referredto “hitting” the power-band. Carb fine-tuning can sometimes reduce theintensity of this “hit”, however there is no jetting that can eliminate the hitaltogether.“Laying Down” or “Nosing In”- Describes when the engine is already in it’s power-band, and a suddenapplication of full throttle causes the engine to stop accelerating, or evenslow down. This symptom usually indicates a full throttle lean condition causedby either a very lean main jet, or a blockage of the fuel supply.
If thishappens, shut the throttle off quickly to avoid possible piston scoring, andincrease the main jet size or fuel flow abilities.TheFuel Circuit Ranges– Each of the changeable jets in Mikuni VM round-slide carburetors has oneparticular range where it has the strongest effect on fuel mixture. Each ofthese jets also has a “range of effect”, but the measure of that effect tapersoff as the throttle opening moves away from the point of strongest effect. Thetable below outlines these ranges. Pilot Air ScrewPilot JetSlideNeedle JetNeedleAir JetMain JetStrongest EffectIdleIdle20%40%50%100%100%Range of Effect0-20%0-100%15-50%20-60%25-75%80-100%70-100%Themost important point of this table is to show that many of these jets have“over-lapping” ranges that can have an effect on one another. Dealing with thesmall nuances of these overlapping ranges is what “fine tuning” is all about.An ideally jetted carb offers easy starting, smooth idling, no hesitations, noloading up, and clean carburetion at every rpm in the entire range. That is thegoal.
The text below describes the step by step process to get there. Oneimportant rule is to keep the same needle taper for the entire process, becauseyou will be tuning to find the best possible “jetting spectrum” for thatparticular needle taper. It bears noting that on twins and triples, alladjustments and jetting changes should be applied to each carb equally.About the Chokeand Starting - Virtually all 2strokes require an exceptionallyrich fuel mixture to allow an easy 'cold-engine start' Applying the choke(aka 'enrichener') for the first 10-30 seconds of engine operation is typicallyenough enrichment for an easy and steady start-up.
After this initialstart-up (and warm up) the choke should not be needed at all for future'warm-engine' starting. If the choke is needed for warm engine re-starts,you have an engine or jetting problem. It bears noting that on all roundslide Mikuni carbs, the choke works better as the slide get closer to the fullclosed throttle position. Opening the slide to 15-25+% throttle opening(during a cold start) makes the enrichening ability of the choke nearlynon-functional. On a carb with a perfectly setup idle speed circuit,steady idling is always achieved with a slide opening that is just barely off offull-closed. With an idle slide position like this, you should alwaysexperience 'one kick' starting with a fully closed throttle (choke-on cold.choke-off warm).It oftenhappens that a carb with an incorrect size pilot jet (whether too rich or toolean) will require the idle speed screw (that adjusts slide height) to be turnedin excessively. Thus raising the slide much more than is ideal.
Becausethe slide is open excessively at the idle position, both cold starting and warmstarting of this engine will be very difficult. The correct setup would beto reduce the idle slide height, and find the pilot-jet / air-screw combinationthat offers steady idling with a lower slide opening. The text below aboutidle circuit setting is not just important to idling and low speed acceleration.It is also fundamental to easy 'one-kick' starting.Idle speed- The fuel supplied by the pilot jet, and the air allowed in by the pilot airscrew, work together to deliver the ideal mixture for idling and warm enginere-starts.
The full adjustment range of the pilot-air screw is 3 turns out frombottomed out. At settings beyond 3 turns, it is not allowing more air.After warming up the engine, set the engine idle speed with the idle adjuster(that makes physical contact with the slide). With the engine idling, adjustthe pilot-air mixture screw in and out in 1/4 turn increments. As you getcloser to the ideal setting, the idle speed of the engine will increasenoticeably. When this happens, adjust the engine idle speed back down, andrepeat the ¼ turn fine tuning process until you achieve the smoothest and moststeady idling. Ideally, the air screw should end up at a setting somewherebetween ¾ - 2.0 turns out. If you find that the best idling comes with asetting under 3/4 turns out, it is wise to install a slightly larger pilot jet.If you find that your best setting is beyond 2.0 turns out, it is wise toinstall a slightly leaner pilot jet.
Keep in mind that Mikuni makes severaldifferent designs of pilot jets for their various carbs, so you want to be sureyou are purchasing the correct pilot jets for your particular model. Afterchanging the pilot jet, repeat the process of ¼ turn adjustments to get to yourideal idle speed setting.Initial Throttle Opening– The slide cutaway has a strong effect on the fuel mixture when the throttle isjust above idle. The cutaway number is stamped on the bottom of the slide. Thenumbers from richest to leanest are.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5. Since thepilot jet and needle-jet ranges overlap so strongly on the slide range, it’swise to test with different slide cutaways after you are already comfortablewith the pilot jet and needle-jet setting. The only exception to this, is ifyou find the slide to be so lean that the engine nearly dies (from a leancondition) when the throttle is just cracked open.Mid-Range Jetting- The mid-range metering is most strongly affected by the combination of thebrass needle-jet and tapered needle. Most Mikuni needles have five differentclip grooves that allow the needle to be raised for a richer setting, andlowered for a leaner setting.
To avoid confusion, many tuners refer to the 5grooves by number. #1 (closest to the top of the needle) is the leanestsetting, and #5 (closest to the tapered part of the needle) is the richestsetting.Thebest approach is to first try a richer needle setting and do test runs of steady30%-60% throttle riding, looking for any signs of stuttering or loading up.This is the time when you will find your long straight uphill, and digitaltachometer to be most helpful. Another effective road test is get under way ata low rpm (about 2000rpm), and then roll the throttle on to the 50% position onyour grip. Different needle settings will offer very different rates ofacceleration under this test. If you find that the needle can be raised to therichest position without inducing any loading-up or stuttering, you will want toconsider installing a richer needle jet.
If you find that you have mid-rangeloading-up or stuttering with the needle in the leanest position, then you willwant to consider installing a leaner needle jet.The 3 kinds of needle retaining clipsused in early Mikuni carbs.WRONG!! This clipposition can let the cable end jump out.RIGHT- This clip positionholds the cable end in.WRONG!! This clipposition can let the cable end jump out.RIGHT- This clip position holds the cableend in.Thebrass needle-jets are manufactured in several different size series to fit thedifferent size Mikuni carb bodies. The 3-digit series number is stamped on eachbrass needle-jet. Along with the series number, there is a letter/number codethat designates orifice size. From leanest to richest, the sizes are N-0, N-2,N-4, N-8, O-0, O-2, O-4, O-6, etc. Continuing up to the richest R-8.
Needlejets are not cheap, and they are not easy to change. That said, it’s best to dothe full gambit of needle adjustment testing to see if you can get by with thebrass needle-jet you have. It bears noting that the actual diameter incrementsare very tiny and precise.
It is very unwise to try to drill your own needlejets just buy em. If you do change the brass needle-jet, you will need torevisit the different settings of the tapered needle.The three needle jets to the left are all the same orifice size, butthey are all different “series” made to fit in different size carburetorbodies. It also bears noting that different needle jets can havevarying height 'atomizer stems'. These atomizer stems protrude into thethroat of the carb, but have different heights that accommodate slideswith different base reliefs. Also note that the main-jet threads in thebottom of the needle-jet will be different between hex-type main-jetsand round-type main-jets.Sometuners refer to the brass needle-jet as the 'spray-bar'.High Speed Jetting– High speed jetting is best done on a flat road instead of a hill or grade.High-speed road testing MUST be done in high gear at full throttle, andpreferably done with race gas. For larger street machines that run at very highspeeds, it is usually best to do high speed jet testing on a dynamometer.
Highquality dyno facilities are very common in today’s motorcycle shops, and buyingdyno time is money well spent.Main-Jet- It is always best to start out with a slightly rich main-jet, and slowly movetoward leaner main-jet sizes. Under this test, the reading on the digitaltachometer is the final word as far as best setting. In dyno testing, the dynowill also show an increase in HP numbers as you get closer to the ideal main-jetsize.
As mentioned above, if you go one size leaner than optimum, thetachometer will quickly show the peak rpm loss.Air-Jet– The air jet (at the 6-oclck position in the carb bell-mouth) is oftenoverlooked because it has a specialized function. The sizes (rich to lean) are.5, 1.0, 1.5, 2.0,. Since it covers the same range as the main jet, it isoften ignored, and in very many cases it is removed all together. The air-jethas it’s greatest effect in carb setups that are running (long hex) main jets inthe 400+ range.
Carbs that run these huge main-jets are very often setups thathave very poor signal, and throat sizes of 38mm or more. On such setups,installing a richer air jet can reduce the need for such large main-jets. Thereare also some specialized racing applications where the engine is held at peakrpm, and the throttle is then released just slightly (to 80% opening) to set upfor a turn. In some cases there can be a dangerous lean spot at this 80%setting that can cause a sudden scored piston. Installing a richer air jetresolves this lean spot, and reduces the scoring risk.Final Review- Once you have the carb as good as you feel it can be, we recommendexperimenting with a “one-size” richer brass needle-jet.
Since so many otherjetting parts have ranges that overlap into the needle-jets range, a slightlylean needle-jet can easily be “cloaked” by the other jets. It is an importanttest because a lean condition in the needle-jet range can so seriously harm lowrange acceleration. The same rule applies to slide cutaways. The othercircuits can easily “cloak” a slide that is one size off. In the case of boththe needle-jet and slide, the best evaluation test is a set of “roll-on”acceleration tests on your road-test hill.Jetting for 21 st Century Fuels- Across the USA, current pump gasolines are being blended with varying amountsof ethanol.
Besides the carburetor maintenance issues that the ethanol cancause, there are fuel metering issues as well. Ethanol has a significantlylower specific gravity than gasoline. When blended in the gasoline, the ethanolwill change perfect jetting into jetting that is slightly lean across the entirerange. Fortunately, Mikuni carbs offer an easy way to contend with thisproblem. As mentioned above, increasing the size of the pilot jet will increasethe fuel delivery by that increased margin from 0-100%. In most cases,increasing by just #2.5 will cover the specific gravity change in stockcarburetors (ie.
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Going from a #25 to a #27.5). In some areas of the countrythat are blending higher percentages of ethanol, increasing by #5 may beneeded. Remember that after the pilot jet change, you will likely need toreview the air screw setting to assure a perfect idle.About Running 91-Octane Fuel- Once you have finished your high-speed tuning on race gas, you are safe toconvert back to 91-octane pump gas with limitations. The 91-octane can workfine for most recreational and casual types of riding. However, if you want todo extended high-rpm operation, you will need to make some non-carburetionrelated adjustments.
The fuel/air mixture that offered the best rpm on racegas, will virtually always yield the best peak rpm on 91-octane pump gas.Unfortunately, the 91-octane pump gas does not have enough octane to resistdetonation during an extended peak rpm, high gear run. And installing richerjetting WILL NOT fix the problem. Installing a richer main- jet can “slightly”reduce temperatures. However it will also reduce peak rpm ability as well asincrease the occurrence of plug fouling. The truth is that a perfectly jettedvintage engine, being run at peak load, will experience dangerous levels ofdetonation if it is run on 91-octane fuel. This underlines that perfectjetting, and detonation risk are two completely separate technical issues. Andover-rich jetting cannot fix detonation risk.Itbears noting that early 1970’s gasoline was 100+ research octane, and most2-strokes of the day were intended to run on that fuel.
Most vintage 2-strokesof that era had combustion chambers that operated detonation free on 100 octane,however those same combustion chamber designs are much more detonation pronewhen run on today’s 91-octane fuel. Catastrophic percentage rates ofdetonation can easily blow a hole in a piston crown. However the more commonlow/medium percentage levels of detonation, will simply overheat the pistonuntil a piston scoring event takes place. Such piston scoring is often referredto as a “four corner seizure” because there is piston scoring in a squarepattern on both sides of the wrist pin ends. After such a seizure, some ownersassume that lean jetting is the cause, when in fact the true cause is the poorcombustion chamber design, combined with the low octane fuel.Thehigh detonation risk of extended high-rpm operation can be addressed by eitherrunning 100+ octane fuel, or by redesigning the shape of the combustion chamberto a less detonation-prone 91-Octane updated design. This 91-Octane updating isa modification that Klemm Vintage offers for many vintage 2-strokes. You canlearn more at www.