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bandit 250 air fuel mix

Discussion in 'Technical and Troubleshooting Torque' started by nouxoid, Nov 30, 2010.

  1. Hi there I'm a bit new to this forum and to bikes in general, bought a gsf250 bandit 2 months ago, it sound very nice, goog condition looks wise, and a 2000 model with 15000km on d clock, bike had issues like dying at robots and when I go down to lower gears aswell. As I didn't know much about bikes I had it seviced,plugs,filter,tuned carbs etc...but still gave the same 'not idling' issue. Told the mechanic that he can't tell me he serviced the bike back serviced but it still had those issues, then he checked the fuel tap, all ok, then played around with some screws regarding the fuel/air mixture...the bike doesn't die on me anymore, but the performance is horrid, its like there's no power going through then it sounds very bad like a lawnmower, which it didn't previously, but only sometimes, other times, under 6000rpm it would sound perfect, nice deep sound from exhaust , then wen I oppen throttle it just loses power and refs go up but the speed is like 50cc. Might this be due to air/fuel mixture, and if so How do get it right....should it be 14.7. --- and what does that even mean....where do I tune it. Please help


  2. Welcome!

    Uh... Ok, the air/fuel mixture the mechanic is talking about is the pilot screw, it's under your carbs, but I wouldn't advise that you play with them unless you know what you're doing.

    No offence intended but it doesn't sound like you do...

    It's a 250cc Inline 4, so you're going to have to rev it a lot before it makes any power... Maybe 8 or 10K rpm...

    If that doesn't help, take it to another reputable mechanic.
  3. Is the clutch slipping?
  4. Maybe it's not dying and just playing dead because it's afraid of the Superior technology of the robots.
  5. 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.
  6. Great stuff, Brett. Time to have a play.

    Out of interest do those diagnostics vary much between twins and fours. I am only guessing but I am assuming that when things go wrong with twins they are more noticeable.