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which jets to put in my bike

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by coasta007, Apr 13, 2012.

  1. Hi all,

    I've got a 2008 GT650R and the only modification so far is the straight through exhaust. I ordered some jets off sigmajet (Ebay) and he sent me 2 sizes. He sent me 2 x 140 and 2 x 150 jets and some other bits and pieces to carry out a jet modification. He also included instructions on how to modify the airbox.

    My question is should i put the 140 jets in or the 150? i dont want to sit there mucking around and retuning. i just want to put the jets in which will give me the best solution.

    Any help appreciated.


  2. U would need someone who has done it to theirs. And live relatively at the same atmospheric pressure.
    I would go a leaner pilot and richer main. As a free-er flowing exhaust and air box will cause it to be rich and lean across the rev range. If that makes sense.
    If it splutters or has a big flat spot...which it will till you get the balance right. You will have to play around.
    So do the pipe first.
    Then ride it. If it's not broken .. don't fix it.
    If the plug is blk and sooty = rich. open the air box up. (it use to be always needed. Now.. mmm not so much. A good filter is a better choice than opening it up if needed.
    It's al about the suck, bang and blow. = Timing, air and fuel ratio's.
    Jets, air box and pipe are air and fuel ratio's.
    God bless EFI, a tuneable ECU and a laptop
  3. Hey Bretto, am I right in assuming that at the end of the day all ECU's are tuneable?
    As in, you may need to buy the $5000 diagnostic computer and harness etc, but you could theoretically do it?
    Just something I've wondered since my intro to the wonderful world of EFI...
  4. No. They run solid state. So you have to piggy back them. Trick them with an Powerjet or Powercommander.
    That is unless yes you have a system that can overwrite the whole ECU. My friend in Spain can do that. But he charges as much as a PC so yeah lol.
    MY PC5 has low mid and high air/fuel adjustments. Which don't work that well with the VCT on the Viffer. When you star modding VCT sucks. I should say once you increase the rev limit to be more precise.
  5. Thinking of getting a solid state hard drive for my laptop. Huge ramp in speed
  6. Sorry, mate... almost the impossible dream.

    You might get close with suggested sized jets, if you stick to the other mod's that are listed with them and the suggester has enough experience of the exact model to make a good suggestion. And then.. you'll change fuel, or ride up/down a hill and the engine will run like shite because it was too close to lean or rich.

    Pay attention to how you take the carbies out and apart; chances are you will be doing it more than once before you find the right combination. You'll be stoked when you do, and will have really earned the right to say "I did it".
  7. Fuel is denser and different here in Aus than in USA, I'm assuming that's where your kit came from.
    put in the 140's, if they're too lean then put in the 150's, if they're too rich you'll need to order the in between size (145 probably) and work from there.
    Here's how to tell if it's too rich:
    Find a nice, open, straight flat road with no cops and not too many people around. Put the bike in the second highest gear from top, and wind the throttle out to wide friggen open (WFO), in other words against the stop. Back off about 1/8 throttle. If the power picks up when you roll the throttle of 1/8th and drops off when you wind the throttle back open, you are too lean on the main jet. Go one richer.
    If you are too rich, well, with an open straight through exhaust you will hear it - the sound will be dull and a little too bassy, it should be nice and crisp.
    You might also want to put a shim under each of your needles to raise the needles slightly to make the needle clip position slightly richer, although I've no idea what size shim.
  8. Pilot jet/mixture screw:

    If you can start it cold with no choke, the pilot circuit is too rich.
    If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

    The best way to set a pilot circuit is with an RPM gauge. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.

    On a 2-stroke - You should be able to ride in 3rd gear, throttle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.

    On a 4-stroke - If it stalls in corners, flames out, and is really darn tough to start, the pilot is too lean. If it feels a little dead until you wind it out, the pilot is too rich. Another test it to rev it out a little in 2nd and then let the throttle snap shut. As it's decelerating, there should be very little backfire or popping - if it pops the whole way down, the pilot circuit is lean.

    Needle Clip:

    On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the other hand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.

    On a 4-stroke - Riding in 3rd with the throttle barely cracked open and roll the throttle open to 1/2. The engine should pull smoothly... if it hesitates and threatens to stall, then the needle is too lean. If it feels 'dead' and won't pick up RPM quickly, then the needle is too rich. An overheating thumper that doesn't have a radiator problem typically is an indicator of a lean needle.

    Main jet:

    On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.

    On a 4-stroke. If the engine feels like its run into a wall and won't pull full throttle - the engine just sounds dead - then the main is too rich. If, on the other hand, it surges, the main is lean. A lean condition will also give you some 'pinging' and a pure white plug.

    Advanced Topics:

    The needle regulates the mixture from around 1/4 - 3/4 throttle. Most people are familiar with the clip position, as it's the most common adjustment, but there's much more to the needle. The jet needle is a long rod that fits into the needle jet. On most carbs, both are replaceable with different sized components. As the throttle is opened, the jet needle is retracted from the needle jet and this creates space between the two for gas to flow through. The more you open the throttle, the more the jet needle is pulled out of the needle jet, and consequently the more gas can pass through the increasing space between them. Below I'll outline the various parts of the jet needle.

    Length - The relative length of the needle is adjustable by raising or lowering the clip. If you lower the needle (by raising the clip), then the needle sits deeper in the needle jet. This leans out the mixture across the range of the needle. Conversely, if you raise the needle (by lowering the clip), then the needle is further retracted from the needle jet, and this richens the mixture across the needle's range. Needles are offered in various lengths. If you have a needle which is still too rich, even though it's in clip position 1, then you need to order a longer needle. For example, needle 'A' in clip position 1 is the exact same relative length as needle 'B' in clip position 3. If you had needle A in your bike, and it was still rich - even though you had the clip in position 1, then you could change to needle 'B' and lean things out by going to clip position 2.

    Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the other hand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).

    Needle taper - Needles taper from top to bottom. As with all principles regarding the needle, the taper is relative to the diameter of the hole in the needle jet. Tapers are rarely changed, but here's a condition which warrants a taper change. Let's say the jetting is perfect at 1/4 throttle, but becomes increasingly leaner as you approach 3/4 throttle. In that case, you would want a needle with a shallower taper. Conversely, if the mixture is great at 1/4 throttle, but getting richer and richer as you approach 3/4 throttle, then the needle taper needs to be steeper. In my experience, needle taper only needs to be changed when the factory mis-spec'd it to begin with. Under very rare circumstances, big modifications to the motor - such as an overbore kit - will require a change in needle taper.

    Remember that jetting needs to be adjusted for every 2000' elevation change and every 15 degree temperature change. If it was jetted right this summer, it's sure to be too lean during the winter. If you rejet it now, when it's cold out, make sure to lean it out a bit in the spring.
  9. The name itself comes from the first carbide lamps that used a small hole to
    drip water on carbide rocks. This produced a gas that was fed through another
    small hole and was burned. The use of precise holes to deliver a burnable fuel
    is similar to the motorcycle carburetor, thus the name.
    The motorcycle carburetor is a very touchy device that can cause all kinds of
    trouble if mishandled or neglected. It is VERY expensive to have a dealer
    overhaul an old carburetor but is well within the skill level of most garage
    Honda used two basic types of carburetors, CV and slide both made by the
    Keihin company. The CV type, Constant Velocity or Constant Vacuum, uses
    pressure to raise the throttle slide where a cable is used in the slide model.
    A carburetor is a device that controls both the amount of air and fuel that is
    allowed into the cylinders. The amount of air is controlled by a butterfly valve
    (CV) or a vertically moving throttle slide. The amount of fuel is controlled by
    the drop in pressure in the throat of the carburetor and the size of the holes
    that allow the fuel in.
    Things you need:
    l clean work surface that will not be disturbed
    l carb cleaner, one can should do
    l clean rags
    l oil catch pan
    l toothbrush or similar brush
    l wood dowel rod and toothpicks
    l standard set of tools
    l compressed air source
    l typical manual such as Clymers, Haynes or Chilton
    Typical carburetor overhaul:
    1) Turn the tank petcock to off and drain the fuel from the lower bowl of the
    carburetor. Caution: the bowl screw may be fused to the bowl from neglect so
    dont force it. The carburetor is made of aluminum and will not take rough
    treatment very well and will strip, mar or even break if too much force is
    used. If the fuel has been left in the bowl for a long time it will give off a very
    strong characteristic odor. Discard the fuel properly. Small amounts of old
    fuel may be burned, which in itself may take some effort to light it!
    2) Disconnect the carb from the hard rubber engine sleeve and the air box.
    Disconnect all cables and fuel lines. If your carburetor is a slide type you may
    need to return to the slide you just removed at a later time.
    NOTE: If you are just starting out in the wide world of motorcycle repair, it
    may be wise to attach labeled tape to all things removed for future reference. I
    didnt for my first carburetor rebuild and really boobed it up! A small
    notebook will also help out if you make notes.
    NOTE: If your bike has 2 or 4 carbs that are ganged together, do not separate
    them but leave them in a group.
    3) Set the carburetor on your work surface with a shop towel underneath.
    Remove the 4 bowl screws and tap on the bowl with the handle of your
    screwdriver until it drops off, it may stick. Remove the bowl drain screw and
    check its washer. Many times this washer is damaged and will need replacing.
    Clean the bowl.
    4) Now you get your first look at the damage! If this carb, like so many, has
    been sitting in the bike for years without use, it will be all gummed up or even
    corroded! The lighter components of gasoline tend to vaporize off leaving the
    heavier ones, also known as varnish.
    5) FLOAT:
    Remove the float pin. If it does not want to come out, you
    need to heat it up. I use a heat gun but a small torch or even
    matches will work. Caution: remember that this IS gas and it
    is flammable! I dont recommend that you beat on the small
    pin to remove it since the two posts that hold it in place are
    aluminum and will break! The float will be brass or plastic
    depending on the age of the bike. If brass, inspect for holes or major kinks in
    the hollow float and replace if found. The plastic float does not suffer from
    these problems but may be cracked and need replacing.
    NOTE: carburetor rebuild kits can be purchased from a dealer and will usually
    have all the parts needed to replace worn or damaged items. Although most of
    the time I find such a kit is not needed, I would hold off until you know for
    6) FLOAT VALVE and SEAT:
    The small torpedo shaped piece of metal hanging from the float is
    the float valve. Clean it up and check if the nipple-like pin in the top
    (closest to the float) moves in and out freely. If it doesnt, soak it in carb
    cleaner until it moves freely. This spring driven nipple serves as a shock
    absorber to hold the valve in place. Check the beveled end of the valve for any
    grooves from wear. If you can see one clearly you may want to replace it.
    Later valves have a rubber beveled end and normally are OK.
    The float valve seat is the round brass hole that the float valve sits in. Clean
    and check it for wear. The seat can be screwed in, fitted with a washer or even
    pressed in (no removal) so check before removing.
    7) FUEL JETS:
    The term jet refers to any device, normally brass on motorcycles, that has
    a small hole. Your carb can have 2, 3 or even 4 jets depending on the
    model and year. Remove the small brass jet(s). They may be screwed in (they
    have a hex shape top) or pressed in (round top shown above). The pressed in
    type normally has a small leaf spring holding it in. Caution: all brass parts can
    be damaged easily with force! Soak these jets in carb cleaner and make sure
    the jet is clear. Forcing something hard through the small jet hole can cause
    damage to the hole. If needed use a soft material like a toothpick. Each jet will
    be marked with a number, check it against the manual.
    NOTE:check your manual to see if the jets are of the right number
    8) JET HOLDER:
    Underneath the jets are brass tubes called jet holders.
    They make sure the jet is in the proper position and
    allow air to mix with the fuel. Flip over the carb and press on the two holder
    ends with a wood dowel rod to remove them. Soak these in carb cleaner and
    make sure all openings are clear.
    9) PILOT (SLOW) JET:
    There will be one final jet in this area, the pilot or slow jet. This jet
    controls the fuel while the bike is idling and is normally in the worst
    condition. It can be found under a rubber plug and has a slotted head that can
    be screwed out. This baby has to be super clean! Soak it and blow compressed
    air through it until clear. PROBLEM: the pilot jet wont open up! This happens
    a lot and can be troublesome. You have two options; 1) buy a new one or 2)
    buy a carb jet cleaning set (offered in Dennis Kirks Catalog). The cleaning set
    is actually a set of small wires that can be used to push through the jet
    carefully. The other thing that can be used is a guitar string, sorry I dont
    remember which one.
    10) AIR SCREW:
    On the side of the carb, sometimes the bottom, the air screw can be
    found. Gently screw the air screw in and note how many turns it takes
    to gently bottom out. This information may be helpful later.The air
    screw works with the pilot jet to supply low idle mixture to the engine.
    Remove it and watch out for a spring, metal washer and rubber washer. Make
    sure it is clean.
    The throttle slide is the main device to determine the
    amount of fuel the engine receives. If you have a CV
    carb, remove the 4 screws on the top of the carb and
    remove the throttle slide. Inspect the rubber
    diaphragm for tears and clean the slide with carb cleaner. Any tears in the
    rubber warrants a new slide. Clean and inspect the needle extending from the
    bottom of the slide. It should show no grooves or nicks. This needle fits into
    the main jet holder sometimes called the needle jet found in the bottom of the
    throat of the carb.
    If you have a slide carb, go back to the bike and inspect the throttle slide still
    hanging from the cable. Clean it and the needle. Place both back in the carb
    and check to see that the slide moves freely and has no hangups. If it does,
    clean again with 0000 steel wool. Clean and check the inner wall of the carb
    12) MAIN BODY:
    You now have to clear out all of the fuel and air passeges that tunnel through
    the main body of the carb. Spray carb cleaner into all holes and passegeways
    that you can find (place the oil drip pan under the carb to catch the overflow).
    Now blow compressed air into the same holes to ensure that they are clear. If
    one does not blow clear it may be a dead end, there are several. If one is
    clogged there is little to do but soak and blow since most of the channels
    make right angle turns and pushing through a wire is not possible.
    NOTE: some people will soak the entire main body in a solvent at this time.
    Caution: if you do soak the unit make sure the solvent will not damage the
    aluminum body.
    Clean the outside of the carb body with carb cleaner and a toothbrush. If all
    passeges are clear, all jets, valves, holders and slide are clean, reassemble the
    Points of interest:
    a) make sure the gasget for the CV slide diaphragm is in its proper groove
    b) the jet holders are pushed fully in and seated
    c) the jets are firmly in place and the pilot jet is screwed in
    d) spring, washer and rubber washer for the air screw are in place
    e) the bowl gasket is in place
    If you have a brass float, set it to the specs in the manual. Set the air screw to
    the number of turns also stated in the manual. If the air screw turn value is not
    available, use the number you noted when taking it out. On the slide carbs
    there is an idle set screw on the side of the carb. Set it so it just touches the
    Theory of use:
    When the pistons of the engine
    move downward, they draw in air.
    This air is pulled through the throat
    of the carb and causes a drop in
    pressure. This drop causes the fuel
    to be drawn upward through the jet
    like a straw. When at idle, the main
    jet is closed by the needle hanging
    from the slide and supplies no fuel.
    Instead the drop in pressure causes the fuel to be drawn up through the tiny
    pilot jet. Since the jet is a set diameter it will only allow a set amount of fuel
    and thus a set idle. The air screw allows air to be mixed with this fuel and can
    change the idle speed slightly. The final idle adjustment comes from the
    butterfly valve (CV) or throttle slide position. Adjusting these devices
    controls how much air passes and thus the position of the throttle slide and
    main jet opening (because of the needle position). When the throttle is
    opened, the slide moves up and the main jet opens allowing more fuel to enter
    the throat. By the way, the pilot jet always supplies its fuel no matter what the
    main jet is doing. Some bikes have a secondary jet that supplies fuel in an
    intermediate range between idle and higher throttle settings.
    The float does just that and floats on a bowl of fuel. If too much fuel enters
    the bowl it starts to push on the float valve that closes off the fuels entrance.
    When enough fuel is used the float drops enough to allow the valve to open
    and allow in more fuel.
    Possible problems:
    l If too much fuel is entering the engine, the float valve may be set too
    high allowing fuel to enter the throat of the carb. Another sign of this
    may be fuel running from the drain tube. Sputtering and misfires may
    l If not enough fuel is getting to the engine the opposite may be true.
    Typical symptoms are poor running and no power when throttle is
    opened. Also check the fuel petcock and make sure there is no
    l If fuel slowly leaks from the bowl lip, the bowl gasket is probably torn
    and needs replacing.
    l If the bike will not idle, the pilot jet most likely needs cleaning. Also
    check the number of turns the air screw was turned out, usually about 1
    turn out is average.
    l If poor engine running presists check the connection of the carb to the
    rubber sleeve on the engine. This connection must be tight.
    l the engine races at a very high speed and will not idle down. This is
    usually caused by the throttle slide (slide type of carb) being put in
    l pilot jet keeps cloging up time after time. This may result in rust or
    sediment in the fuel tank. Install an in line fuel filter
  10. Sorry bretto, but I'm going to have to argue with you on the decel popping.
    On a big bore thumper (possibly other single cylinder engine types), you are always going to have some deceleration popping, because the engines create a high-vacuum lean condition on deceleration - this is a completely normal phenomenon. Many riders of DRZ-400's tend to over-richen the pilot circuit thinking they are too lean because of the decel popping they are getting - no. If you're on a big bore thumper with a straight through exhaust and you're not getting a fair whack of decel popping, unless you're running a carburettor with an active ACV (air cut valve), your pilot is too rich. Obviously, if your thumper sounds like a massive cannon or a gattling gun on deceleration (both? :D), you're too lean.
  11. Then pay someone with a Dyno to do it, even then it's still trial & error as to which is better.

    Sorry, that's the way it is.

    That said you only have a choice of three main jets... plus your needle height, plus your pilot screw, plus your float height, so pretty simple really ;)

    If you do want to tackle it yourself, it's not hard, just make notes & change only 1 thing at a time.

    This is a good guide: http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

    There's a good technique for setting the pilot jets which I've mostly forgotten, but IIRC, you raise the idle a few hundred RPM (makes sure you carbs are synced properly first) then wind then pilot screw in until the RPMs drop, then the other way until they drop again & set the screw exactly between those two points.

    Good luck, whichever way you go.
  12. Or...
    Raise the idle a few hundred RPM (making sure carbs are synced), wind the pilot screw in until the RPMs surge, then wind it back out until the idle just smooths out, plus 1/8th of a turn.
    Make sure the bike is at full operational temperature of course, you want to jet the bike for running at optimum engine conditions, or at least as optimum as you're willing to get them. Sync your carbs, go for a nice long ride with a couple of mates, and impress them with your knowledge of how to properly set a motorcycle carburettor's pilot screw. Buy an extended fuel screw first though :D
  13. No worries Kernal. Not mine. I copied and pasted it.
    Yup they all do but they don't call it backfiring as it's not happening in the pipe or header.
    No idea why ??? it's still not called backfiring.
    It is a time consuming game jetting. Trial and error.
    I opened up my box on the viffer and it got worse. But I like it so not worse for me. Gives the mild mannered gay bike a bit of character for mine.
  14. Thanks for your reply.

    I carried out the jet mod yesterday (including the airbox mod) for better airflow) and put 145 as the main jets and 22.5 idle jets. The screws are 2.5 turns out like the instructions from sigmajet suggests. as part of the mod, it said to take to take the diaphram out where the slider is and it said to put another washer underneath the c clip and to drill out the hole above where the needle is. the smaller one out of the two is the one i drilled out.

    I havent had a chance to give it a good/proper run but from the test run i did yesterday, there seems to be a tiny flat spot at about 5500rpm.

    i'll give it a good run tomorrow and see how it goes.
  15. Omg, I think I stumbled across a foreign language here.
    I am so glad I can just get on my bike, turn the key and it starts first go, turn the throttle and it goes forward... And fast.
    Absolutely none of the above made any sence to me at all. I am amazed I read through the whole thread....all I got was blah-blah-blah.....lol
  16. lol. its a male thing. We like to tinker with our toys.

    if there are a few spare kw left in the engine, we might as well put them to use...
  17. That might be your different jets, but it also might well be the result of the airbox modding. On modern bikes the airbox is carefully tuned/designed, and messing with it rarely gives any benefit, or very little while causing issues that can be greater than any gains.

    You can actually be getting less airflow at some or most rpm because of turbulence at the throttle bodies caused by the changes made, as well as losing a designed-in benefit (like at your 5500 rpm) where the tuned resonance of the volume and length of the intake charge actually gives better (and smoother) airflow, usually in the midrange. The bass reflex (Google that for a handy explanation) design of speakers works in much the same way, where the length and diameter of the tube is precisely designed with the air volume in the speaker cabinet and the free air resonance of the speaker itself to boost the output.

    I've seen dyno charts where 10hp has been lost in the midrange for about 1hp at peak power rpm, resulting in a crap outcome, despite some modest gains elsewhere in the rev range. Sometimes the rider thinks it's better, because the drop in power at lower and mid rpm makes it feel like there's more of a rush towards the peak, despite it only being the same or just a little more.

    Airbox filter designs used to be little more than an undersized box with a restrictive filter to quiet the intake noise to meet emissions testing, and modding would help noticably, but with the occasional exception, that's pretty much a thing of the past now.
  18. Well in that case I will leave it for Wilco if I ever feel the need to do the jet thingy....lol
  19. Midrange and top end is definitely a major improvement. Once i sort out the tiny flat spot i'll be happy.
  20. Back firing is when a decent amount a fuel gets pumped out by the engine instead of being ignited by the spark plugs, and (this part I'm not sure about) as soon as you get spark again the fuel gets ignited and causes a bang. This is why you can be cruising along, and if you're bored, you can hit the kill switch, twist the throttle a few times, hit the kill switch again to get ignition going again, and you'll hear a nice satisfying bang.
    I'm not really sure where decel popping occurs... but the sound certainly travels through the header out the pipe or else you would barely hear it.