1991 Suzuki Bandit Project; A Phoenix from the ashes?

I did burp the coolant, but only once when I first refilled it, so I can try it again.

I have an aftermarket set of needles with multiple clip positions, so I can play around with it when I work up the gumption to mess around with it more. I have to admit that the hassle of tearing it down until I can get to the carbs and then getting it all back together to test is wearing on me. Especially without further warping or damaging the plastic airbox that has already seen a few bazillion heat cycles and previous removals. It seems like being thorough about this stuff (without unlimited funds or your own personal dyno/exhaust gas analyser/EGT/wideband) involves a ton of tearing the tank, airbox, carbs off/apart and putting them back together and back on. Which leads me to ask myself, "How many carb teardowns does it take to get to a point of more hassles than converting the bike to EFI?"

The current issues are just complex enough that I don't feel like the way forward is one or two obvious simple steps.

It runs well wide open and under smaller throttle openings high in the rev range. It also runs pretty well just off idle. Carbs have been synced, spark plugs are new, float heights are within spec and within a few tenths of a mm of each other. The current small issues are:

-Sticky choke that is hard to get all four choke circuits evenly closed once you open it. Short of finding a pristine used set of carbs or a stash of NLA NOS carb parts, not sure what else can be done for this one, as most the parts involved are just a little worn, just a little bent, just a tiny bit mushroomed/peened...not enough to have one obivous culprit, but they all move OK (not great) individually, but when assembled they stick...sometimes.

-Erratic idle with a finicky adjuster. Turn it down a little, 600rpm idle and stall. Turn it up a little, 3000rpm idle, and not much in between. Racing idle gets worse at it heats up, which has me thinking lean at idle. I want to double check all the hardware associated with the adjuster to eliminate it as a variable, but am waiting until next time I have the tank/airbox off anyway. Going one step smaller on the pilot air jet didn't seem to help much, issue is about the same as before. I've continued to move the idle mixture screws out to their current 3.5 turns, which also points towards lean. Any less than about 2.75 turns with the current jets basically won't idle. I also tried 4 turns with the current jets and it wasn't better than 3.5. It is a high idle, but also an erratic one that when idling stays at one speed, but may not return to that same speed after being revved or loaded. Stop at a light and it will be idling at 2700rpm, then next stoplight it is at 3200rpm, then a few minutes later back to 2800rpm, etc. Blipping the throttle can get it to change to a new idle speed, sometime with a very slight hang for a couple seconds, but not a consistent hang, and not a consistent speed. Very occasionally (maybe 10% of the time) a blip or two of the throttle will get it to settle all the way down to an unstable 600-800rpm idle that might eventually stall.

-Bike seems to run hot, but what do I know? It is my first liquid cooled bike, it is summer, there is traffic in the city, and we have had some 90+ degree days here lately. I will try another burp of the cooling system and see if it helps.

-Rough running around 5000-6000rpm. Not terrible, but not great. Got a little better with a really good sync of the four carbs, but still present afterwards. Hard to describe, but it isn't as happy to rev as elsewhere in the rev range, sounds a little sour, almost like a slight occasional misfire. Happens when cold and when warm, and worst under steady light throttle. Also happens both with small increasing throttle openings and decreasing throttle; in other words, lightly rolling off and/or rolling on doesn't result in much of a change. Large handfuls of throttle seem to cure it, but you accelerate so quickly I don't have much time to assess. I haven't had enough time and empty road to do repeated top gear tests to see how it responds to large throttle openings in this rev range. Taking a look at the plugs just after running in the rough midrange are sooty and point towards rich mixture. Maybe I should trying dropping the needles clips one step?

General plug readings don't raise any major red flags, and fuel mileage seems to about on par (33-35mpg) with other Bandit 400 owners, and about the same as my dad got with his Honda CB-1.

Since this is my first bike of this type and I didn't ride it before putting it back together, I don't have a great yardstick. I am trying to get one my very experienced riding buddies over to take it for a spin and tell me if it seems a bit off to him or if I am chasing an impossible perfection that 25-year-old CV carbs and a junkyard engine just won't be able to deliver.

Question: did you remove, and confirm the idle jets (slow jets) were clear? I've seen idle issues like you describe from some I thought were clean(pushed a wire through). After much heartache and one of those welding tip drill cleaning things, it was cured-note, be careful to only remove solidified old fuel, not metal.

Also, are the CV boots in good condition? Do the slides move freely? Float levels dead on?

I apologize if any of this is too rudimentary to be helpful. No Ill will intended, but as long as I'm at it, almost all 4 cyl Japanese bikes I have ridden have a midrange stumble. They don't pull great, until the really come to life in the mid/upper RPM range, but they don't fall flat on their faces either. Gotta be something-good luck,You Can Do Eeet!

In reply to wheelsmithy:

Thanks for the words of encouragement! Last couple carb cleanings I pulled the idle jets, mechanically cleaned them with a tiny brush, then dropped them in the ultrasonic parts cleaner for a couple rounds. CV boots are in good shape, and the slides move freely (I've watched them move under vacuum with the bike running, airbox off.) The float levels are as close as I can practically make them, maybe 0.2mm total variation? I'm not sure I can bend the little tabs any more finally than a tenth! I measured the first two as even (let's say at 15.0mm as an example-don't have actual value in front of me) went to the third and it measured 14.8mm or so, try adjusting, end up at 15.1mm, try again, back to 14.8mm, and so on until I decided that I was probably close enough. Is that close enough?

The midrange stumble might be normal; as I said, it isn't terrible, and this is my first experience with a four-cylinder bike. I'm hoping for a more experienced friend to test ride and give me his opinion. My cars don't have such a stumble, but my cars also don't have individual carbs and aren't expected to operate over a 1,300rpm-14,000rpm range!

ae86andkp61 wrote: ... try adjusting, end up at 15.1mm, try again, back to 14.8mm, and so on until I decided that I was probably close enough. Is that close enough?

Surely to all that is holy, that is close enough! Good on Ya, getting a second opinion.

I started posting, and can't seem to go back and check, but did you ride it, holding at the rough spot, kill it, coast to a stop, clutch in, THEN try reading the plugs? Royal PITA, especially in your case, but seems like either that, or dynamite.

I did some plug chops that were inconclusive at best. Since my last post, I have installed a wideband oxygen sensor in the collector, bought a Microsquirt, ordered a spare set of GK7AA (second-gen JDM Bandit 400) carbs which include a factory TPS which I will eventually convert to throttle bodies, and formed both a short-term and long-term plan of attack.

Short-term:

-Break out the multimeter and shop manual, go back to basics, and test plug wires, plug caps, coils, and CDI.

-Do a compression test on the junkyard engine.

-Check valve clearances on the junkyard engine.

-Wire in the wideband plus the little digital gauge I got to put on the triple clamp.

-Test ride with carbs and wideband to see what I can learn.

Long term:

-Get my spare SV650 EFI fuel pump into my spare Bandit 400 tank.

-Get my IAT sensor in the airbox, my coolant temp sensor in the coolant block thingie, and modify my spare stock crank angle wheel as needed for the Microsquirt.

-Decide on an injector/fuel rail setup. I have some early EFI CBR 600 injectors I will examine, others have used ZX6R injectors, which I will also consider. Both bikes are 600cc and use dual injector setups, so one set may not be too far off for my little 400.

-Upgrade the electrical system as needed to deal with the added electrical load.

-Modify the petcock as needed.

-Convert the carbs to throttle bodies, add a MAP sensor, mount injectors and rail.

-Build/configure Idle air setup.

-Figure out how the heck the ignition plays with this fuel injection nonsense and modify as needed.

-Package it all on the bike.

-Tune as needed.

-Ride off into the sunset!

It's just that easy, folks!

Spray the carb area with carb cleaner as its running. The boots may llok fine but still have hairline cracks and the orings go bad too. Make sure the diaphragms on the slides are in good shape too. There are little o rings under the cap where the vacuum ports are. make sure those are in place too.

A lot of these little things can be done with the carbs in place.

While you are in there checking the slides and such is a good time to throw the washers under the needles. The clips are a full step change, the washers are a half step. If you dont have any on hand you can go to radio shack and get a bag of their assorted washers and there is one size in there that is perfect.

MulletTruck, thanks for the tips! I can confirm the diaphragms are good, I've checked them several times recently. I am aware of the little o-rings, and not only replaced them with new, but took extra care to make sure they stayed in place while installing the caps. I will double check the boots and try playing around with washers and report back on the results.

Well, not much progress on the Bandit as my AE86 project has been sucking up my time, but I did get a package from Japan containing these babies:

There was a JDM Bandit 400 with variable valve timing, one of the first production vehicles to offer it if I am not mistaken. These Variable valve Bandits had a few minor differences. These carbs are the same as we got in the U.S. with the small addition of a TPS! I snagged some off Yahoo Auctuons Japan. The purchase price plus all the fees, plus shipping was in the same ballpark as a good bank of used Bandit 400 carbs in the States, provided you can find some.

My long-term plan is to pull the float bowls off these, blank them off with an aluminum plate, and machine ports for slightly upward-angled injectors in that plate. I will have to plug some additional internal passages, and I think I will leave the CV slides intact. Doing so is appealing for two reasons; I can keep the rebuilt carbs I have to reverse the EFI project if needed, or to sell if successful. The second hassle saved is fitting a TPS to the existing carbs...it has been done before, but it involves more machining and fabrication.

The TPS uses the same three-wire connector as the Hayabusa and many GSXRs.

Why not just use some EFI throttle bodies off some other bike? Great question! The Bandit carbs are small (32mm or 33mm depending on year/spec) and most EFI throttles are 38mm or bigger...which will lose velocity, and won't help the low-to-mid range I am trying to improve. The other consideration is width. The stock carbs just nestle between the frame tubes, and are among the narrowest bank of four I've ever seen. I tried measuring both CBR600 and ZX6 throttle bodies, and both are quite a bit too wide. Triumph 600TT are also wide, but a bit more modular, but the injectors are In the head instead of the throttle body, so I would have the fab work of respacing linkages and brackets plus adding an injector location. A guy on the Bandit forums also tried ordering CB400 throttle bodies from Japan and found them too wide. I think it will be easier to make the JDM carbs into throttle bodies than it will be to modify the frame, or track down something that fits.