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Suzuki Bandit stalls when Revved Quickly. Adjusting/shimming needles?

Discussion in 'Technical and Troubleshooting Torque' started by JamesE30, Mar 13, 2011.

  1. Hey guys,
    sorry this is my 1st post and already asking questions haha

    I recently bought a 1990 Suzuki GSF250 Bandit with a slip-on Viper pipe
    It's been sitting for about a year. So a mate and I changed oil and filter, put in new sparkplugs and battery the other day with fresh fuel. It blew a little smoke for a few revs when started up, but starts and idles perfectly now. Except...

    When at idle after warmed up,if revved quickly (throttle turned to full quickly) the bike will stall instantly. A little gentler and it will sputter a little then rev, and gentler still and it revs fine. (btw I'm not talking about redlining it, just taking it to like 6k)
    The old sparkplugs were fouled when removed with black oil stains.
    I have read that these all sound like symptoms of running too rich..
    (Btw it was run for a bit with the stale fuel and may still have some in it)

    This guy had the exact same problem by the sounds of it:

    Shimming the needles was suggested in the above thread.
    They suggested raising the needles
    Seems to have solved the problem i have:

    However i read that raising the needles will make the bike run richer, and lowering will run more lean.

    I have adjustable circlips on my needles so i plan on moving them, my question is move needles up (richer), or down (leaner)?

    Thanks for any help, sorry if that was confusing, and yes I've searched.
  2. Just a question: when do you ever actually need to rev it to full quickly? :-s
  3. I don't...

    But it shouldn't stall when I do, right? It also shouldnt sputter and lag when i rev it normaly. (im talking after the engine has warmed up)
    So, I would like to diagnose and solve the problem...
  4. Someone here will correct me if wrong but sounds like a symptom of running rich. I suppose if it was lean you might get a similar effect (that is opening throttle will increase air and if already lean too much air could kill the engine0 but usually if lean it won't start/idle readily.

    Usually raising the needles (ie putting the circlip on a lower notch) makes for richer.

    Before you play with needles is the bike properly tuned (timing, plug gap, mixture)? Also are needles and needle seats worn? Otherwise you may be just tampering with symptoms.
  5. I think you have blocked carbs. Here is what you should do. Warm engine up, then slowly (like adding 1000 rpm every couple of seconds) increase throttle. When you get to an rpm range that causes sputtering, misfiring or similar, get back to us with that rpm range. You can also do this test when riding in say 3rd gear.
    I personally think your transition or main jets are partially blocked.
    Do not adjust the needles until the engine runs correctly! Adjusting needles is only for on road performance, the needles and slides are load sensitive, when revving the engine on the stand, there is no load.
  6. Thanks for the replys guys
    GreyBM thats what I thought. I pulled the new plugs i put in the other day for a look and they are already black (btw I havent riden the bike since i got it since it has a broken clutch lever and im waiting for a new one, just run in neutral)
    The bike also spits out little bit of fuel from exhaust when revved.

    From this I concluded it was rich and moved the circlips higher one notch, This appeared to make no noticable difference once running again.
    I also drained all the fuel from the tank as there was still some stale fuel in from before and replaced it with Premium unleaded.
    (Keep in mind all ive done is run in neutral for a few mins, the bike hasnt moved)

    I then saw your post Typhoon and promptly replaced the circlips where they were before. I conducted the test you suggested and could not notice any sputtering when throttling slowly. (I only went up to 8-9k so as not to piss of the neighbours though)
    Infact the bike seems to rev better from idle now after moving the clips up and down, and swapping fuel again.. ??
    But there is still some lag with quick throttle (though it wont stall now), and the bike spits a little fuel and blows a little white smoke.
    Any ideas?

    Btw i thought the air filter might be blocked since its old, and may be causing this so i started the bike for a bit with no filter (open airbox) to let more air in, and still had the same problem.
  7. I believe there is a carbie problem than is unique to the bandit. There are people on this forum that know.
  8. First rule of any suspect new-to-you bike... do a maintenance workup on it. I'd suggest the following:

    Sparkplugs (tick)
    Carby inspection (tick)
    New fuel if its been sitting (tick)
    Oil & filter change
    Air filter change (usually replace mine with a Unifilter or similar aftermarket reusable filter)

    Only then can you really know whats going on with the bike. Usually I'd do the oil/filters/plugs straight up and go from there, as then you know its got spark, lubrication and airflow.

    With your carbies if its gotten better after moving the circlips up then back down it sounds like you need a carby clean. Go get yourself some pins, cotton-tips, pipe cleaner wire (local craft store... $2 a pack, will last you forever). Oh, and some metho as well. Now take off your carbies and clean like hell! You'll be surprised what can build up after even a relatively short amount of time. Use the pipecleaners to get at passanges and pins to unblock totally clogged jets. Fine thread may also help clean jets by pulling it through gently to polish the inner surfaces.

    Now put them back together and start your bike again. See if theres a difference. Oh yes, you will want to tune the mixture screws all the way in and then out 2 turns from lightly seated, or until it runs best at idle. Generally 2 turns out is in the ballpark, if not spot on.

    The big test will be when you get it on the road - performance under load is a lot different than idling/revving in neutral. Lastly, if in doubt whack some choke on. If the choke makes it worse you're rich. If it makes it better you're lean.

    Hope that helps - boingk
  9. Thanks boingk that helped a lot.
    I have done all the generall maintenance you sugested except for air filter. I though the one in there looked pertty clean, but its kind of hard to tell since its the white paper type. Perhaps ill change that next.

    Im getting the clutch lever later today, so ill take it for a ride before i do anything else and see how that goes.

    Btw i did wipe out a bit of grit and muck from the carbs when i changed the needles, so maybe that's why it ran slightly better..
    Thanks for your advice on the carbs and adjusting the mixture, Ill take them apart and give them a good clean this weekend then see how it runs after that.

    Currently the bike does stumble and idle much worse with choke on so im positive it's rich atm.

    Thanks for the feedback from everyone, sounds like ive got some carb cleaning to do!
  10. Have you checked the rubber couplings between the airbox and the carb inlets?
    On the older grey import bandits like yours these tend to get quite brittle and crack creating leaks which can be intermittent depending on the amount of manifold vacuum.

    I remember my old 250 Katana (which used the Bandit engine) ran like crap till I noticed the cracks and sealed them up with some gasket goo. From then on it ran just fine.