GSXR 400 Bandit 400 rough low running or won't idle

[moforockband]

I found this gem of information on a gsxr forum. And it may help explain why our little bikes run like a bag of poop from time to time. Indeed mine is suffering from conking out when it's hot atm. There is more to the BST carb than the simple idle jet/air jets/needle and main jet thinking I sometimes get stuck in.
I'm hoping this can stay up, I think this guy hits the nail on the head.

Pilot screw O-rings on Mikuni BST's

These are very frequently overlooked due to owners not wanting to disturb settings and/or Suzuki placing brass caps over the pilot fuel screws, but are critical to the correct operation of these carbs.

Symptoms : unstable idle (OK one minute, not OK the next), having to set pilot screws differently to stock settings and each other; bogging, stumbling or stalling on opening throttle, up to around 5-7,000rpm; hanging-up (revs staying high for an extended period) after closing the throttle; a general sense that the carbs are so inconsistent in behaviour that they are either completely worn out or subject to demonic possession.

These problems are usually worse when warmed up, and in extreme cases the bike can become impossible to ride although fine when cold. This might look like a 'rich' problem, in fact it's the opposite. Very frequently owners then start fiddling with float height, which just adds to the difficulty of finding the cause.

Typically these problems appear after carbs have been removed for a while and dried out, say during an engine repair. After reinstatement they no longer work properly, and attempts at adjustment do not help even after strip and cleaning of the carbs. including the idle jets, with carb cleaner. Often this makes things even worse. Factory-sealed pilot screws seldom give trouble whilst the bike remains in use and the screws are undisturbed.

Explanation: The idle circuit on BST's is critical because it also controls fuel flow to the progression jets. The progression jets are an array of tiny orifices in the carb body just below where the throttle butterfly seats when closed. You probably won't even know they're there unless you look for them.

Some carbs have accelerator pumps but the BST uses these progression jets to similar effect, with venturi vacuum drawing the necessary surge of fuel as the butterfly uncovers them. Extra fuel released into the increased airflow causes the revs to begin to rise which generates differential vacuum that begins to lift the slide. At that point the slide, needle and emulsion tube begin to take over. However the progression jets continue to flow fuel and contribute significantly to the overall mixture well into the midrange.

This mechanism makes the BST idle circuit crucial to throttle pickup to around ~5k-6k rpm, far higher revs than is usual with other designs. If you can get beyond that and the motor pulls well, but below 5k throttle response stumbles, bogs, cuts out, or hangs up once the throttle is closed etc, it's almost certain to be an idle circuit fault - blocked idle jets and/or progression jets and/or O-rings leaking.

Blocked idle jets are common thanks to dirt, but symptoms are consistent - no or bad idle, bad pickup. Progression jets seldom if ever block. It's easy to overlook the O-rings because they appear OK. If they aren't perfect, you get completely f*cked up throttle response : instead of extra fuel being drawn in as vacuum increases, the engine sucks extra air past the pilot screws. If that happens, the mixture suddenly turns lean and vacuum to lift the slide is not developed.

Bizarre inconsistency is a hallmark of pilot screw O-ring leaks as usually they don't leak some of the time, only when there's enough vacuum; behaviour varies with engine speed, temperature and even how fast you open the throttle.

Solution:
- Remove the idle adjustment screws. Off the bike is easier but with care and patience (and a suitable screwdriver, also a mirror helps) this can be done with carbs in situ, but have a tray positioned to catch parts because you will drop something.
- Make sure you fish the old O-ring out of the carb body if they don't come out on the pilot screw, but don't scratch the seat they seal against, it's soft metal. A mirror is handy for checking the seat is clean.
- Replace these O-rings. The old ones will likely look and feel fine, but will have hardened with heat and age and now allow air into the idle circuit unpredictably creating the bogging and hanging. This happens especially if the carbs have been allowed to dry out for a month or two, then serviced with carb cleaner, then the screws adjusted.
New O-rings from a hardware store (ordinary nitrile rubber) are fine in this location as they are an air seal, not fuel. Other O-rings in the BST (eg float perch) must be Viton else hot fuel will quickly destroy them.
- Make sure each new O-ring is backed with its plain washer, then the lockspring.
- Set the idle screws to 2 turns out or as recommended for your model
- Balance your carbs using a manometer like Morgan Carbtune or vacuum gauges.
- Set idle speed to 1,100rpm or as recommended for your model, using the large central throttle stop adjuster.
- Be amazed that such tiny, cheap inconsequential parts can cause so much damn trouble.

Where to get these O-rings

First, check we are talking about the same size. If your pilot screws measure 3mm diameter across the plain shank (where the spring, plain washer and O-ring fit), then the O-rings you need are 4.5mm OD x 1mm. These are a nice tight fit around the screws.

For some reason they seem to be much easier to get in UK where I am, than in USA where you probably are. I haven't been able to find a US seller - O-rings Inc, an Amazon seller lists them, but currently unavailable - see http://www.amazon.com/O-Ring-4-5-BN7...0423583&sr=1-9

However any Suzuki dealer should be able to supply them. Any bike with Mikuni BST's uses them. One Suzuki pt. no. is 13295-29900 (GSXR400 - 33mm or 35mm BST's), I don't have a parts listing for GSXR750/1100 but should be the same. Suzuki will of course charge you a ridiculous amount. BST's were also fitted to some Yamahas (Diversion 900 I think) and Triumphs (several) so their dealers may be worth trying.

[Variablevalves suck]

They are not actually 4.5 x1mm but a weird 4 38 mm or something stupid.

Best place to get your orings is Blair , don't know if he's still on here but use to frequent the fzr section and runs a replacement oring business.

[moforockband]

The size from Mikuni/Suzuki is 1.18 x 2.78mm id I think I read somewhere. Anyway these little 1mm nitrile suckers are doing the job quite well at the moment. No more stalling at traffic lights, and the bike picks up much better.