EDIT: Sorry for all the spelling mistakes!
Hello people of Kiwibiker. Its your guy, that annoying guy who asks stupid questions and doesn't take his own advice, back again to follow up on my earlier "how make bandit work" post (https://www.kiwibiker.co.nz/forums/s...post1131202076), which described steps to take if your little pos just refuses to run, despite all reasonable steps having been taken to achieve that goal.
Today I want to talk about how to get it running 'correctly' once your at the point it starts, idles on its own, and generally gets you from a to be without actively breaking down (refer to previous post if that's happening to you).
Im writing this, because I have a comparatively rare, GSF250"S" model bandit, the later generation, but without variable value timing (the 'V' range), and there's absolutely no English translated parts schematics or original manuals (at least that I could find/source). So getting the jetting the bike correctly has been a real trial by fire, or as Brett from BRM Dyno would say; "a real bastard".
First of all, you really need a baseline of "running without assistance" before you can start this process. If you bike is leaking petrol (or air), revs aren't returning to idle, or it wont idle correctly, pull your carbs and replace all the seals with a set of Liteks (https://litetek.co/Carb_Kit_Suzuki_GSF250_GJ77A.html). There's a few things to watch out for while you have your carbs apart;
- check the float values for wear on the rubber seats, and importantly check they are the correct float values for the bike. The little metal spring at the top should only stick out about ~0.9mm, if yours sticks out way further, or the little metal frame to hold it to the float tang is more then ~2mm height, you probably have the wrong float needle values and you should source a replacement set otherwise your not going to be able to set your float height correctly (which on this bike makes a HUGE difference).
- check the float value seats aren't damaged, and are again the correct seats for the bike. The ones I have, which Im fairly sure are original, are flat at the base of the seat (inside), but have an ever so slightly burred hole for the value to seat into. The float values should extend ~1 - 1.5mm out of the seat when you drop them in. Ive already mentioned replacing all the orings with Liteck ones, but if you insist on not doing that, make SURE the seats are securely suctions in by the existing oring. You do NOT want fuel leaking into the float bowl, it will make turning the bike neigh on impossible.
- floats themselves, Ive head some people say you should replace these, but as far as Im concerned as long as they float and aren't damaged you probably dont need to. Just check them for damage and that they sill float and function as expected (bounce off the value spring etc). The float "heigh" should be set at around 8mm, measure with the pointy end of a micrometer, the lip of the float at the bouncy end (towards the front of the carbs). Set your micrometer to the desired height, place the end of the tang on the flat frame of the carb where the float bowl meets the carb, and adjust the tang to change the height. Make sure your tang is straight, if it's not use some needle nosed pliers to flatten it, then use the end of your nail to push / pull it until your float is at the correct height. I suggest you put the carbs on their side, ever so slightly rocked backwards, so the floats rest, but don't compress, the spring on the float valve and use that to set your heights.
- Pilot jets, just make sure these are clean and you can see the little round hole at the end when looking through them pointed towards the sun. They should be size 30. You could probably get away with slightly smaller, but larger and you're going to have a problem (if your mixture screws are functioning as intended).
- Main jets, this is where it gets interesting. For some reason my bike came with a 112.5 main jet. This is WAY too big, but because of the impact the float height has on this bike, and the fact it has a non-standard air filter, it look me awhile to work out what was going on. From what I can find this bike came with a 97.5, but having tried that I can confidently say Im not sure that's right. I think it "should" be a 102.5, but Ive got it running nicely with 105. If you don't want to change whatever is in there for now (understandable, they are expensive and its hard to know what to get if your way off, I went through a few $100 worth of jets).
- Mixture screws, if your can use anything LESS then 2.5 turns out, chances are the little rubber orings on the end aren't sealing correctly. Take advise RE Liteck kit, they're cheap and very well made and will fix a bunch of variable "fuzz" when turning. The orings should hold the washer (check this for rust which could rip the oring btw), and spring on the mixture screw. If they fall off, your oring is bad and you need to replace it/them. You will also get a good sense of this if you set your mixture to 1 full turn out and the bike runs without displacing a hugely lean condition (with a 30 sized main jet). This is important, your bike will run like shit if the idle isn't sorted out first. When your seals are correctly, the bike should idle nicely at 2.5 turns out, and should immediately return to idle after revving. It should also start nice and easy (sometimes with a crack of throttle), no matter the time of year, and when hot and cold (although if your having hot start issues, chance are its ether very rich OR one/both of your coil packs are dying).
- needles, unfortunately I dont have too much info re the needles, other then to say the needles, are different between the s and v model of the bandit. I settled on 1 click from the top on mine. General rule of thumb is set it somewhere, and dont adjust while your change jets and float heights otherwise its going to be hard to know what's making the difference. As far as I know the needles control the fuelling just off idle to about 10k-rpm. After that it's all main jet (discussed later). There is a small amount of overlap here, but in general if you're having "top end" issues, the jets wont help. If your mid range isn't feeling very "crisp" but your top end is good, try raising the clip up one. In almost no case is a good idea to lower the clip below the third position, your've got much bigger problems if you feel you need more fuel in the mid range, and your top end is likely going to be non-existent.
- needle/needjet wear, I honestly didn't find this as much as an issue as I have been lead to believe. I have two sets of needles and needle jets, one with 20k miles and one with almost 80k and I honestly couldn't tell that much of a difference between them.
- diaphragms/springs, just check these for damage, whole, and clean any buildup on the outsider of the slides (wash out with carb clean or crc). If your carbs haven't been cleaned in awhile it's probably worth pulling the needle jet (emulsion tube) out and giving that a visual inspection too. The easiest way of doing this I found, was to get a long m6 bolt (im talking like 10cm long), take the main jet off, screw it in where the main jet screws in, then gentle tap the end of the bolt with a hammer until the needle jet is pushed out. Note that your'll need to remove the slides anyway to replace one of the orings at the base of the slide housing if you get one of the Litek kits.
- balance, make sure that all the butterfly valves of the carbs close at the same point. That is, if you open the throttle slightly, so the valves are JUST open, they should all be open the same amount. Use the adjustment screws between the carb bodies to adjust this. I haven't honestly found carb balance to be a huge issue with these bikes, provided your compression readings aren't wildly out, so a "bench sync" is probably good enough.
- choke, I don't have much info on this and mine seem to work alight. Litek does sell a kit for the seals in this too, if you suspect this is an issue for you.
- airbox, use the original airbox. Don't try and turn these bikes with pod filters, it's just going to be painful. Also the original air filters (although impossible to find), do work ever so slightly better then aftermarket ones. If yours is dirty his it with an air compressor. If you have a plastic "GS500 esk" one, expect the bike is going to struggle to breath a bit up top, you may need to account for that when turning.
- HT leads, Im not sure how much of an impact this will have, but mine were short so I bought some replacements from Classic British Bikes (https://www.britishmotorcycleparts.c...20Results&gp=5) in Auckland. I then quickly discovered they weren't insulated enough for my ignition system after being shocked multiple times, so I wrapped them in electrical tape a few times.
So to summarise, before you begin;
pilot jet: 30
main jet: 97.5-105 (you can tune by adjusting the float height and needle clip, but in general a bit larger is going to give you more flexibility)
float height: 10mm
Now take your loveable pos out for a ride and find a really big hill (I find North Shore onramps to work quite well), and starting from 1st gear, wind the bike out all the way until it ether hits the limiter (this sounds bad but its extremely unlikely this will actually happen), or it refuses to rev any more (more likely). Note how it feels in the 10 - 16k range, as this is critical;
- like it was surging, power on, power off, fairly consistently in the same places regardless of gear, OR when you CHANGE GEAR the bike experiences a momently loss/lag of power at high rpm
-- this happens because the mixture is too lean once the needle is fully removed from the seat (aka the slides are fully extended).
-- if your main jet is a 97.5 and your at 10mm, you can try changing the float heights 1mm at a time (yes, pulling the carbs 20x times is going to suck), until you hit 8mm. Anything below this isn't ideal; if the bike is still surging, your going to need a bigger main jet otherwise the float height will cause problems throughout the rest of the rev range (rich condition, even with raised clips). If you go larger on the main jet, move the floats 2mm down and repeat.
- flat, low there was no power
-- Your in no mans lands between rich and lean, go leaner, move the float heights up 1mm until you hit 10mms, if you need to go above this consider going down a main jet size as your'll likely introduce more issues if the floats are too hight. If you need to go smaller, move the float heights up 2mm and repeat the process.
- runs good until it hits a wall at a certain rpm
-- You're slightly too rich, and the bike is choking. You need to move the float heights up slightly and if needed choose a leaner main jet (refer to previous examples).
When you've wasted a few good afternoons doing this, your should end up with the floats somewhere between 7-11mm (don't be afraid to use 0.5mm increments if your almost there, float heights make a massive difference in this case), main jet between 100-105.
You can tell it's correct because it should pull harder and harder until almost 16k rpm, and when you change gear the torque should instantly increase again. It shouldn't at any point feel like its surging, and it shouldn't hit a point (before the limiter) when you hit a wall of powerless and are unable to rev thought it.
Once you're happy, you can focus on the mid range. The bike should rev crisply off idle, if its sort of "gurgles" off idle and doesn't feel very crisp you can tryyyy turning the mixture screws in half a turn, but the changes are your mixture screw orings are fucked and you need to replace them. If the entire mid range feels flat and not like the bike is "pulling" to get to higher RPM (a good test is crack the throttle open ~20% on a flat road, the bike should pull harder and harder until fairly high rpm even if you dont change the throttle input because its "chasing" for more air / fuel), try raising the needle clip by one. This should have quite a dramatic impact, if it has no impact and the bike still feels flat and like its making lots of noise without any power, consider if your entire rev range isn't too rich (how does it feel up top?). If you need to rase the needle to the top position, the bike is probably too rich overall and you will get TERRIBLE fuel consumption (counterintuitively).
Hopefully once your've reached this point you will have a super crisp and revvy bandit again, best of luck! My next mission is to tackle my partners Across which has a completely flat torque above 10k rpm, FUN.
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Originally Posted by Spuds1234
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