SprintKS, the throttle linkage is connected hard to the butterflies. The butterflies are not visible due to the slides being in the way but I can tell you the throttle linkage does not move when the rpm's jump. From that I conclude the butterflies are not moving either. At that RPM, however, as closely as we can guess the secondary bypass ports are uncovered so for the longest time we thought maybe they were clogged causing a momentary lean condition. I've run water through them and visually inspected them. As far as I can tell they are clean but it's still possible there's some obstruction, I guess.
It's a real head scratcher.

Hmm, not sure why this didn't occur to me before: how long were the carb bodies in the ultrasonic?  It may be that a second extended run through might be necessary to fully clear the passages. Are the stock idle jets still in place, or are the larger ones still installed?

buffalo I dipped each carb body in the chemical dip noxiousness for 24 hours. Then I ran each carb body through the Ultrasound cleaner at max heat for an hour. 

We have tried every combination of jets and floats. We are currently running the stock size idle jets but when we went up to the oversized idle jets it made no difference.

But I was thinking along the same lines, actually. What I would really like to find is a way to verify that each of the idle circuit bypass ports are actually clear. I dripped water through each carb body and tried to see if I could notice water coming out each port, and to my eye it did look like water was coming out each port, but such a test isn't exactly conclusive.

1996 Suzuki GSF600S (Bandit 600S) dyno results for Sitwon Ben Mendis:

So if you need a filter and oil for an oil change, but you're too lazy to visit a shop...

...through the magic of The Internet you can get same-day delivery from Amazon.

The OEM fuel line is garbage. A kink in the tube was causing me trouble today. The replacement we picked up at Advanced Auto Parts is a better quality line with thicker walls to prevent kinks.

Yea, I had that problem riding your bike once. Are you still running the in-line fuel filter?

Yea, the fuel filter looks clean and fuel is passing through it. After replacing the kinked fuel line the problems went away.

Hey Yermo,
I have a 1996 GSF600 with the exact same idle jumping problem at the same exact RPM as yours.

Were you ever able to suss out the problem?

I've made it a little more bearable by putting larger idle jets in and leaning up the needle a tad so I don't stall off the line. That seems to have solved the idle hanging at 2200 forever at least.

That said, it still is a bear to drive in parking lots at low speeds.

Thanks!

These carbs are ridiculously sensitive to balance issues. So make sure they are as perfectly balanced as you can get them.

But to answer your questions, we got it to be slightly better but were never able to correct the problem. We went through so many possible solutions it's hard to list them all. Two sets of carbs, completely cleaned through an ultra sound cleaner,w with new jets, seals, gaskets, etc. We checked for vacuum leaks everywhere. We checked valve clearances. We replaced the ignition computer. We checked the alternator. It went on and on and we were never able to get it to stop jumping.

I talked to one very experienced mechanic, Ed Huey of Knoxville, who said that some riders have been successful in compensating for that problem by adjusting the lengths of the breather tubes coming off the fuel rail. We did notice that if you kink those tubes off the bike runs like hell. We did not experiment with lengthening the tubes because, frankly, we got to a point where we were tired of trying to track it down.

So Ben Mendis just learned to compensate for the jumping by slipping the clutch and now hardly notices it. 

Thanks for the quick reply!

I tried to track down the problem all of last summer to no avail. I had thought of getting an entirely different set of carbs, but perhaps i won't bother anymore based on your experience.

I'll definitely try looking at the breather tube length when it starts to warm up again (Minnesota...) though. It seems a heck of a lot easier than tearing down the carbs for the 5th time.

BTW, when I tore the carbs apart last summer I did notice that the tabs on the side of the slides were a little worn. I wonder if some extra air sneaks around them? it could explain an "unfindable" vacuum leak.

Either way, I've gotten pretty good at slipping the clutch as well!

Yea, we bought a second set of carbs and they both had the same problem in the same place. The breather tubes are currently the best lead we have, and I expect we'll do some more experimentation there, but I'm honestly skeptical.

The most telling thing to me is that I've seen this problem across at least 3 bandits and 4 sets of carbs and it always happens at the exact same point in the RPMs. On my bike we measured it with a digital tachometer and no matter which set of carbs, or how they were tuned and tweaked, we could never change the point at which the jump happens.

On my bike, it always happens precisely at 1600RPM. And it appears to be the same on every other Bandit. That says to me that it's probably not a problem in the carbs at all. If it were a fuel-air mixture problem, then by changing the jets or tuning the carbs to be fat or lean it should cause the jump to happen either sooner or later in the RPMs.

I strongly suspect that the problem is a design flaw in the ECU (the only digital component on the bike). The ECU controls the timing of the spark plugs firing in relation to RPM of the engine. If at precisely 1600RPM the timing were to change slightly, it could result in the jumping behavior we have observed.

If the ECU can be opened and the EEPROM chip removed, it may be possible to dump the code and inspect the OEM programming. If not, we could always hook it up to a digital oscilloscope and signal generator to try and reverse engineer the timings. Either way, it's a project I have the resources for, but have yet to tackle. Once I started riding the bike I was simply having too much fun to worry about it. 

Ben, I share your suspicions about the ECU. And I certainly understand the "carb fatigue" you guys might be feeling at this point. I went through a bit of carb fatigue myself when trying to tune a set of Mikunis for my KH500 after I had it ported. I probably had those carbs off and apart 20+ time before I finally felt like the jetting was right.

But IMO, it's probably still worth the time and trouble (assuming you're up for it) to try Ed's experiment with the breather tubes.

IIRC, he suggested starting with them long (perhaps as long as a foot) and then gradually trimming them down a bit at a time to see if they make any difference. He was of the opinion that if you tried that, there'd be a good chance you find a "sweet spot" where the stumble was greatly diminished if not eliminated.

Having said all this, vent tubing is cheap, but if you've simply got accustomed to riding with the bike as it is, you may (understandably) not feel the need to bother with the experiment at this point.

The ECU is an interesting suspect. The symptoms certainly point in that direction, especially the commonality in the idle jump speed that our bikes share.

So, what could go wrong in the ECU?

- the bike couldn't have come from the factory like this.
- the ECU code is unlikely to age
- digital logic is also unlikely the age
- analog stuff could certainly age, and it's worth cracking the box for a look

I'm of the opinion that the bike may have been tuned right on the hairy edge and after some wear in the air/fuel system it triggers this problem.

Maybe being able to tune the timing in the ECU could actually "fix" the problem.

Perhaps I'm only reiterating what Ben said, but I'm certainly curious to find the root cause.

Thanks!

I've put a lot of miles on this Bandit since this thread was last updated, and made quite a few modifications as well. It's a well loved bike at this point, and mechanically it's still running great. Still not leaking oil, nor burning it at any noticeable rate.

What I am having trouble with, however, is the lock cylinders. They've been a bit sticky since I bought the bike, but recently they've gotten even stickier. This evening I ended up bending the key a bit trying to get my tank open... which meant that after filling up I spent another half hour fussing with the ignition to get it to turn to the ON position again.

Where is the best place to find a replacement lock set for a bike like this? New OEM replacements are looking kind of pricey (at the few places that even list them as available).

Before you go replacing stuff, try some oil in the lock mechanism.  Might be all it needs.

I've done that on my bikes more than once and it made a huge difference.

I concur with Ian.

I've had luck with something similar to this:

Thanks, Ian.

I tried some WD-40 on the two locks and they both seem to be turning smoothly now.

I am feeling motivated to finally get a spare key made. Even though the locks are turning now, I don't trust that they won't seize up again. Getting stuck at a gas station for 30 minutes because a key won't turn is not fun. 

Yea, I would strongly recommend getting a couple of spares made. 

Typically when we travel at least one other person has a key to each bike.