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Banshee Jetting FAQ by: BenBB
Everything you ever wanted to know about Banshee jetting! While the specifics listed are based on the Banshee's stock carbs, the principals remain the same and are similar for other carbs (and anything else with a carb). I'd like to thank all the past and present members of BansheeHQ.com for all their valuable help and input. I've tried to cover as many carb issues as possible, from the most basic to the more complex, so some of you may want to skip ahead (go straight to Q#12 if you want to dial in your mains), and others are welcome to add anything that I've missed or have yet to learn.
Here's the carb specs for a bone stock '87 and up Banshee:
Mikuni VM26SS roundslide
Pilot jet size:25
Needle clip:Middle (clip #3 of 5)
Main jet size:200 (some dealers will provide or install 190, 210 or 220 mains)
Main jets are standard Mikuni small hex mains, available from most bike shops, and come in increments of 10 (210 is one size larger than 200, etc.)
Pilot jets are Banshee-specific, only available from a dealer or a knowledgable shop, and are available in increments of 2.5.
Yamaha pilot jet part numbers:
Now some basic q & a:
Q#1:Why is proper jetting important?
A#1:Depending on your priorities, proper jetting will get the most power out of your motor, and prevent premature wear (which can quickly lead to failure on a high-revving 2-stroke like the Banshee).
Q#2:What happens when my Banshee isn't jetted properly?
A#2:If the jetting is slightly lean or rich, the most you'll suffer is a loss of performance. However, if the jetting is way too lean or rich, your Banshee will overheat rapidly resulting in melted or seized pistons (which is the ultimate "bummer" for the 2-stroke motor).
Q#3:What does "rich" and "lean" mean?
A#3:Rich refers to having more fuel per volume of air in the intake charge entering the motor than the perfect combustion ratio. Lean refers to having less fuel per volume of air than optimum.
Q#4:So how do I know if I'm too rich or lean?
A#4:Your best, most definitive means of identifying a rich or lean condition is by reading your spark plugs (see Q#37). Eventually you'll get a good "feel" for the jetting of your own bike, at least if you ride it often enough and hard enough.
Q#5:How do I correct a rich or lean condition?
A#5:Jetting, of course. Jetting in itself refers to the process (mostly trial & error) of adjusting your carbs to get as close as possible to the perfect air/fuel ratio. Typically this consists of changing the main jet to a larger (or smaller) size, changing the pilot jet size, adjusting the airscrews, and adjusting the needle clip position. Sometimes it can involve adjusting the idle speed, synchronizing the carbs, replacing the needles, and adjusting the float height. Don't worry, it's not as hard as it sounds!!! Most importantly, don't get frustrated, take your time, and don't be afraid to ask for help. Carbs can be confusing, and nobody was born with the knowledge required to tune a single carb, let alone two of them. If you have some basic problem-solving and screwdriver-turning skills there's no reason to let anyone else touch your carbs, EVER!
Q#6:I think my jetting is lean or rich, what do I do?
A#6:First, make sure jetting is the problem and not something else; a Banshee can exhibit symptoms of rich jetting without that being the actual cause. For example, a dirty or clogged air filter will restrict airflow, resulting in a rich air/fuel condition. Rejetting in this case could solve the immediate problem (rich condition; poor performance), but could result in a lean condition if you cleaned the air filter and didn't rejet back to where it was (plus that's twice as much work as just cleaning the air filter in the first place). ANY mechanical/electrical problems need to be addressed or ruled out first before altering the jetting (air leaks, torn boots, clogged filters, shorts in the wiring, TORS system faults, bad grounds, water in the fuel, bad fuel, bad bearings, worn rings/pistons, etc.). This is because some mechanical and electrical faults can mimic or cause poor running which could otherwise be contributed to bad jetting. If you're certain that the jetting is the problem, the process would be:
1.determine if the condition is rich or lean
2.identify which carb circuit(s) is the problem
3.correct the problem circuit(s)
4.verify that rejetting solved the problem
Q#7:What are some indications that my jetting is rich?
A#7:A slightly rich condition will result in noticeably reduced performance, or "bog". Say your main jets are one size too rich, it starts and runs fine, but whenever you have the throttle pinned (or Wide Open Throttle=WOT), the motor doesn't continue building RPM; it bogs down because there's too much fuel per volume of air and it cannot burn efficiently. Other indications of being rich are fouled plugs, thicker than normal exhaust smoke when the motor is fully warmed up, and the dreaded overheating. Be careful of these symptoms though, as any of them alone may point to a non-jetting related problem (thick white smoke is more likely a head gasket leak, fouled plugs could be a weak stator). While overheating due to a rich condition is not common on Banshees (typically the plug will foul and not fire before it gets too hot), it's still not a good idea to run for extended periods with the jetting too rich. Periodically reading your spark plugs is a good way to get an idea of your overall jetting; if the center electrode (the part that's snow white when brand new) and base ring are light chocolate brown, you're jetting is perfect (slightly rich); if it's black and wet, you're too rich. Doing a plug chop will tell you for sure if your mains are too rich or lean (more on this later). Another trick is to warm up the motor, pop the choke out to the first notch, and ride it; if it bogs worse with the choke out you know you're too rich. Some motor mods will cause a rich condition, as well as changes in temperature or elevation (see Q#13, 14, & 15).
Q#8:What are some indications that my jetting is lean?
A#8:A slighly lean condition can result in a hesitation, missfire, or revving high & fast with no power. Say your pilot circuit is lean, when you start it up with the choke "on" it's fine, but when you turn the choke off it idles very high; there's more air per volume of fuel so it burns hotter (in this case airscrew adjustment may solve the problem). Other indications of being lean are a backfire and rapid overheating. Overheating alone will cause serious problems, so even a slighly lean condition is usually much worse than being too rich. Like rich conditions, lean indications may not necessarily mean that jetting is the problem; an air leak between the carbs and the cylinder can exhibit the same symptoms as lean jetting, often if this is the case only one cylinder will act lean. Never take a lean condition lightly, as it's the quickest way to damage your motor. Again, spark plug color is a good indication of overall jetting; if the center electrode remains white or light gray and the base ring shows not coloration after riding for more than half an hour (starting with fresh plugs-if the plugs were already brown for correct jetting they will not turn back to white if it's now lean), it's too lean. A plug chop will tell you for sure. The choke trick is an easy way to tell also; when the motor is warmed up, pop the choke out to the first notch, and ride it; if it runs better (no hesitation or revving high & fast with no power) then you know you're too lean. Most motor mods that improve airflow will cause a lean condition (remember more air per volume of fuel=lean), and will require rejetting richer. Temp and elevation changes can also cause a lean condition (see Q#14 & 15).
Q#9:OK, I'm sure it's rich or lean, how do I know what to change?
A#9:For a breif recap of the Carb Theory 101 link mentioned earlier; the carb delivers fuel depending on the throttle position:
0 to about 1/8 throttle is controlled by the Pilot jet size, and fine tuned by the airscrews
1/4 to 3/4 throttle is controlled by the needle taper & length, fine tuned by the clip position
3/4 to WOT is controlled by the main jet size
Once you know which circuit or circuits are rich or lean, you can begin to dial in the jetting.
Q#10:I have a bog or hesitation right off idle, OR an erratic idle, OR a very high or low idle...
A#10:Since the problem is between 0 and 1/8 throttle, the pilot/airscrew circuit may be rich or lean, and you may need to also adjust the idle speed screws and check the carb synchronization (see Q#23 & 24). Start by adjusting the airscrews; on each carb turn them in (clockwise; which is richer) a half turn (180 degrees); adjust both airscrews the same amount. If you have the motor idling allow about 20 seconds for the motor to react to the new setting. If the condition gets worse, try going out (counterclockwise; which is leaner) on the airscrews and see if it improves. If going in on the airscrews helps but the problem persists after you turn the airscrews all the way in, or to within a half turn out from seated (don't torque the airscrews! the tip is pointed and overtightening them can cause damage!), try the next size larger pilot jet, and start over with the airscrews 1.5 turns out; tune the airscrews from there to get a clean idle and off-idle response. Conversely, if going out on the airscrews helps but the problem persists after you get to about 3 turns out from seated, try the next size smaller pilot jet and start over with the airscrews 1.5 turns out; tune the airscrews from there to get a clean idle and off-idle response. Once you acheive a clean idle and crisp off-idle response (no bog or hesitation right off idle), you may need to adjust the idle speed up or down by adjusting the idle screws. If airscrew adjustment does not have any affect on the bog or hesitation, and you are certain the problem is below about 1/4 throttle, check the carb sync, make sure the pilots aren't clogged (it only takes a speck of dirt to block the tiny passage), and insure that both airscrews are set the same number of turns out from seated. Be advised that the stock Banshee carbs have a pilot jet that is specific to Banshees; a standard Mikuni pilot jet will not work (see next section for pilot jet details). To verify your pilot/airscrew circuit jetting, start the motor and let it warm up fully; install fresh spark plugs, start it (without using the choke) and let the motor idle for about 10 minutes; pull the plugs and look at the center electrode and base ring; they should be a light chocolate brown color if you're dialed in. As a side note, since the pilot jets are flowing fuel throughout the throttle range (they are solely responsible for mixture at idle to about 1/8 throttle but continue to deliver fuel above 1/8 throttle) changes made to the pilot jet size MAY have an affect on the main jet jetting; if you change pilot jet sizes it's a good idea to verify your mains as well, and to a lesser extent the needle clip position.
Q#11:I have a bog or hesitation at about half throttle...
A#11:Since the problem is between about 1/4 and 3/4 throttle, your needle clip position may be rich or lean. If you know the jetting in this range is too lean, move the needle clip down one clip position (richer-moving the clip towards the pointy end of the needle). If you're sure it's rich, move the clip up one clip position (leaner-moving the clip towards the blunt end of the needle). If you're not sure if you're rich or lean, try the choke trick; with the motor fully warmed up pop the choke out to the first notch and see if the problem gets better or worse; if better then try going a slot richer on the needles; if worse try going a slot leaner on the needles. Remember to check the carb sync whenever the tops are off the carbs, and don't get the slides reversed; the cutout on the bottom of each slide should face the airbox.
Q#12:I have a bog at WOT, OR it revs high & fast with no power at WOT...
A#12:Since the problem is at WOT, your main jets may be rich or lean. Since the motor is spinning so fast (usually) when the throttle is pinned, the main jets are the most critical circuit on a Banshee; running too rich or lean on the mains can be extremely hazardous to your motor's health: rapid overheating combined with high RPM's are a recipe for disaster. If you have done mods that affect airflow, start with the manufacturer's recommendations or those shown below to get a ballpark number on your main jet size. Similarly, if the temperature or elevation has changed use the below guidelines to get your mains close (see Q#14 & Q#15). Once you think you are close to the right size on the mains, one method of dialing in the main jets is to start with large mains; if it bogs at WOT drop a size until it revs out clean; if it doesn't bog at WOT go up a size until it does and then drop one size. The bog at WOT is usually an indication that it's rich on the mains, however since running lean is usually worse than being a little rich, the choke trick can be used to make sure before you go leaner on the mains: with the motor warmed up pop the choke out to the first notch; if the bog at WOT gets worse you know you're rich and can safely drop a size on the mains; if it gets better with the choke out you're lean and should start going up on the mains until it bogs at WOT, then drop a size. Another method is doing a plug chop (see Q#33), but since you will be revving the motor out in 6th gear it's best done to verify the mains after you have gone down one size from bogging at WOT as detailed above. When dialing in the mains it's best to err on the side of rich than lean to avoid any damage to your motor (rich symptoms are fairly obvious in the form of decreased performance, and can be remedied before engine failure-lean indications may not be apparent until it's too late). Don't be afraid to go big on the mains, as long as you work your way down to the point that the motor revs out clean all the way to WOT you'll be less inclined to risk damage from running lean.
Q#13:My jetting was perfect a couple months ago and now it won't run right; why?
A#13:If the outside air temperature has changed by more than 10 or 20 degrees Farenheit since it was jetted perfectly, you'll need to rejet to compensate. If this is the case, typically you will need to rejet the mains a size smaller (if it was dialed in for colder weather) or a size larger (if it was dialed in for warmer weather), and possibly adjust the airscrews. If the temp has NOT changed, and you have not added any other mods or made any changes that could have affected the jetting, your problem could be as simple as cleaning the air filter to needing a fresh topend, before you alter the jetting rule out any other possibilities.
Q#14:Why do temperature changes affect my jetting?
A#14:Since the carbs deliver fuel dependent on throttle position, they cannot compensate for changes in air density (only volume). As air gets colder, the molecules become closer together in a given volume (higher density); cold air contains more oxygen than the same volume of warm air. More oxygen in a volume of air means that the same jet sizes will not deliver enough fuel to burn the oxygen at the highest efficiency, and what results is a lean condition (just the same as if you decreased a jet size). On the flip side, as the air gets warmer the molecules are able to expand (lower density); so there's less oxygen in the same volume of air. Less oxygen in a volume of air means that the same jet sizes will deliver too much fuel, and the net result is that you'll be running rich. What that all translates to is that colder weather will require jetting larger (usually just the mains), and warmer weather will require jetting smaller. Going from colder weather to warmer weather will usually result in a bog at WOT (see Q#12), and when this happens you can simply go a size smaller on the mains (and possibly need to adjust the airscrews, see Q#10). Going from warmer weather to colder weather may or may not give you any indications that you're too lean (it may even run better; the highest performance is on the ragged edge of too lean, however this is also where you can risk damage from overheating rapidly). Mikuni says that a main jet size is good for approximately a 30 degree F temp range. While true for a stock machine, as you add mods the temp range that a main jet size works best for may be as little as 20 degrees F (for example: 310 mains for 40-60 degrees F, 300 mains for 60-80 degrees F, etc.). If there's any doubt (especially if there are any indication of lean jetting; see Q#8), dial in the mains again (see Q#12) or do a plug chop.
Q#15:Why do elevation changes affect my jetting?
A#15:In the same way air temperature changes the density of oxygen in a given air volume, so do elevation changes. As the elevation above sea level (0') increases, there is less oxygen per volume of air (lower density). Typically you'll need to rejet your mains one size for every 1500'-2000' elevation difference; jet smaller when going to a higher elevation and jet larger for going to a lower elevation (for example: 300 mains at 3000', 320 mains for 0' or sea level IF the temp is the same). Bear in mind that you'll need to compensate for BOTH temperature AND elevation changes, so going from a hot low elevation to a cold high elevation may net no jetting change. Much like temp changes, going from a low elevation to a higher one of the same temp will exhibit a bog at WOT because it's rich (dial in the mains-see Q#12); going from a high elevation to a lower one may or may not give you any symptoms of being lean before damage occurs. While there are other atmospheric factors that can affect jetting (like humidity, barometric pressure, etc.), temperature and elevation changes will have the most drastic affect.
Q#16:Why do premix ratio changes affect my jetting?
A#16:Just like temperature and elevation changes, altering the amount of fuel delivered by the carbs in a given volume of air will result in a rich or lean condition. By changing how much oil is in the premix, you also change how much fuel is available to burn. Since the oil in the premix is intended to lubricate the crank, it is burned off in the combustion process; however the oil that's in the oil/fuel mixture displaces the amount of fuel that is available for the air to burn; more oil means less fuel and less oil means more fuel, from an air/fuel ratio standpoint... For example, you change your premix ratio from 32:1 to 100:1, every gallon of fuel will contain less oil, so more fuel; from an air/fuel ratio standpoint there is more fuel to be burned so your jetting will be too rich and you'll need to jet leaner. In contrast, going from 40:1 to 20:1 means that every gallon of fuel will contain more oil, so less fuel; the air/fuel ratio will be too lean and you'll need to jet richer. Basically a higher ratio will require leaner jetting and a lower ratio will require richer jetting. On a side note, changing your fuel octane should not affect jetting.
Q#17:Why do some motor mods require rejetting and others don't?
A#17:Whenever you alter the amount of air flowing in or out of your motor, you'll need to rejet to compensate. More air flowing in from a more efficient air filter will result in a leaner air/fuel ratio; more air flowing out from more efficient pipes will do the same thing; in both cases you'll need to rejet to compensate (typically larger mains and sometimes larger pilots as well, depending on the mod). In addition, mods that affect how efficiently the air/fuel ratio can be burned will also require rejetting (some ignition system mods for example). Changes to compression ratio typically don't require rejetting, however octane requirements often will change and are even more critical to preventing motor damage.
Q#18:Specifically what mods will require rejetting?
A#18:Pipes & silencers, air filter(s), removing the airbox snorkel, removing the airbox lid, reeds, reed cages, reed spacers, ignition advance, aftermarket coil(s), aftermarket CDI, porting, some aftermarket pistons that alter the port timing, altering the premix ratio (see Q#16), and obviously changing carbs.
Q#19:How much do I need to change my jetting for a mod?
A#19:It depends. With the multitude of combinations of mods, along with the many variables from machine to machine and location to location, it's impossible to know anyone's exact jetting. However, it is possible to get close; your first resource is contacting the manufacturer (pipes, for example) or machinist (porting, for example) to get a rough idea or baseline for your jetting; next is to inquire about the jetting other people have for the same or similar mods, if possible (keep in mind that other mods, temp, and elevation differences will be a factor); post in the jetting forum at bansheehq.com; or use the rough estimates in the next section.
Q#20:I need to rejet my mains, how do I do it?
A#20:Start by making sure your Banshee is clean, specifically around the carbs. Turn the petcock "off" at the tank, and disconnect the fuel line at the first carb you're rejetting (do one carb at a time so that the bowls and/or slides don't get reversed). Remove the choke hose, located between both carbs (just on the other side of the choke knob on the left carb). Disconnect the bowl overflow line at the bottom rear of the carb. Using a small phillips-head screwdriver, loosen the hose clamp on the black rubber boot at the front & rear of the carb. Push the body of the carb rearwards, compressing the airbox boot, until the front of the carb clears the front reed cage boot, then twist the front of the carb outwards and pull the carb free of the airbox boot. Stick a rag on the top of the motor right under the carb to catch any fuel that's in the float bowl. Using the same small phillips-head screwdriver, remove the four screws that hold the float bowl on (don't lose the two little metal guides for the overflow hoses). Gently remove the float bowl straight off the carb body; if it doesn't come off easily tap the sides with the end of a screwdriver, just don't pry on the gasket mating surface or pull it off at an angle. Once off, check the bowl gasket, it will usually stay stuck to the carb body but it doesn't hurt to check it for tears, or remove the gasket to clean it and the upper mating surface. After the bowl is removed, don't rest the carb on the motor, as you may bend the floats (the two roundish black plastic peices that were inside the float bowl). Right in the center of the carb body (below and between the floats) you'll see a hex-shaped brass thing: that's the main jet. Sitting around the main jet is a white plastic splash sheild, slide it off the main and note how part of it goes into the carb body; that passage is where the pilot jet sits. Use a 6mm nut driver to remove the main jet, be careful not to lose the brass washer under it. Now install the new main jet with the old washer, being careful not to overtighten the main jet. Slide the shield back over the main and into the pilot jet passage. Put the float bowl back on, tighten the four screws, make sure there's no dirt in the carb or boots, and put the carb back in the boots just opposite of how you took it out. Now do the other carb the same way, replace the choke tube and connect the fuel and overflow hoses, turn on the fuel and make sure fuel doesn't flow straight out the overflow hoses. If it does, the float valve is stuck open, just tap on the float bowls with a large screwdriver and they should close.
Q#21:I need to rejet my pilots, how do I do it?
A#21:Follow the instructions above (A#20), and find the pilot jet instead of removing the main jet. Use a small flathead screwdriver to remove the pilot jet and install the new one. Make sure that the small pilot jet passage isn't clogged, it only takes a speck of dirt to block the pilot jet. Also make sure you have the pilot jets specific to Banshees, a standard Mikuni pilot jet is longer and will not fit (see below for more info).
Q#22:I need to adjust my needle clip position OR change my needles, how do I do it?
A#22:Start by making sure your Banshee is clean, specifically around the carb tops. If you have stock TORS carb caps, remove the side retaining screw and unscrew the carb top off the carb body; if you have aftermarket Mikuni carb caps, simply unscrew them (do one carb at a time if possible to prevent reversing the carb slides, if you are unsure which slide goes into which carb, look at the bottom of the slide; the cutout or bevel at the base of the slide should face the airbox). Once the top is loose, lift it straight up and the carb slide below the cap will come out of the carb body. With one hand, hold the carb cap and bunch up the spring, holding the slide in the other hand. When you have the spring compressed, look at the top of the slide and notice how the gold-colored cable retainer sits. Now turn the slide upside down and the cable retainer will fall out. You can now push the cable from the carb cap into the slide slightly and move it to one side, freeing the cable from the slide. Let the spring decompress and leave the carb cap resting on the pipe or frame rail. Use a small phillips-head screwdriver to remove the two screws on the top inside of the slide and remove the screws and retaining plate, then push the needle up and out of the slide. To change the needle clip position, look at what clip position it is in, remove the e-clip with a pair of needlenose pliers (you might want to do this part over a clean bench so you don't lose the clip), and then place the clip in the desired slot. Remember that moving the clip up towards the top or blunt end will make it sit lower in the slide, so the fuel flowing past the taper of the needle will decrease; so the 1/4 to 3/4 throttle jetting will be leaner. Conversely, moving the clip down towards the bottom or pointy end of the needle will make it sit higher in the slide, so fuel flowing past the taper of the needle will increase; making the jetting richer from