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!
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. :-P 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.
I found this on Keihim carbs I don't know if it applies to the carbs on the bandit. "Fuel screw: With the carbs synched, the engine warmed up to operating temperature, and the idle speed set, adjust the fuel screws so that when you rev the bike in neutral and release the throttle, the revs quickly drop to exactly idle speed again. If, when you release the throttle, the revs hang up a few hundred rpm above idle speed, then drop to idle, the idle mixture is probably a bit too lean. Turn the fuel screws out. Use a minimum of half turn increments until you know you've just about nailed it. You'll drive yourself up the wall trying to tune the thing in eighth or quarter turn increments if you're a mile off. In extremely lean cases the idle will hunt between the proper speed and something above it. If, when you release the throttle, the revs drop below idle speed then pick up, the idle mixture is probably a little bit too rich. Turn the fuel screws in. In extremely rich cases the engine will die after revving the bike and releasing the throttle, unless you've turned the idle speed screw way in, in which case it may act like the idle mixture is a little bit too lean. In slightly rich cases, the engine will respond well to throttle blips when cold, but will die or dip slightly below idle speed when hot. While having an exhaust gas analyzer is almost mandatory during tuning of main and needle circuits, do not rely totally on them to set idle mixture. Use an accurate tachometer (like your ear) and adjust the fuel screws to find the highest vacuum for each cylinder or highest idle speed, which will often be the same setting." http://www.factorypro.com/tech/tech_tuning_procedures/tuning_FCR_Burns,Pat.html
Starting a thread on the Suzuki GSF600S (a.k.a. Bandit S) which Ben Mendis(https://miles-by-motorcycle.com/7576) recently bought. As mentioned on Facebook, I took it for a test ride and the bike stalled on me about 1.5 miles from the house and would not restart. I pushed the bike under a shade tree after a few minutes of trying to get it started. After a little while a man walked up and introduced himself. I started telling him what I was up when I turned away. He interrupted saying he was deaf. Interesting. He spoke and read lips near perfectly. "I'm a Suzuki mechanic and I bet your crank case is over full." We had picked up the bike from the seller but and checked the sight glass but the idea the crank could be overfull is something I would never have considered. "I see it all the time." he said, "they 'change' the oil by adding 4 quarts without draining the old out. My van is just up there, why don't you push the bike over and we'll pull some out with my pump." I did as the man suggested and we proceeded to pull over 1.5 qts of fluid out of the crank before it became visible in the sight glass. The bike started up almost immediately there after. This is something I did not know, that an overfull crank case would prevent the bike from running. Apparently it builds up too much pressure and fluid gets by the rings fouling the plugs. We ran the bike for a moment. He put his fingers on the handle bar to "hear" the motor. "Running a bit rough. There's a stumble. It's been sitting for a long time, no?" he asked. "The carbs are probably dirty. With as clean as the bike is we can probably get away with just pulling the bowls and cleaning the jets. If you've got time why don't you follow me and we'll take care of it." Absolutely. So off we went. He was between jobs so we ended up working outside his storage unit which was filled with tools. Within minutes the tank was off and the carb rail was out. (I'm going to have to buy a bunch of tools. Clearly, I don't have enough.) It was at this moment we noticed the gas tank was leaking from the petcock. We took that apart to find the seals had all hardended. No matter what we did, without new rubber, we could not get the petcock to stop leaking. It was at this moment a thought dawned on me. I had tried to smell the oil in the crankcase to see if it smelled of gasoline and there was just the slightest smell. I pulled open the milk jug we had pulled the oil into. I remember thinking it looked a bit thin. The smell of gasoline was overwhelming. The crankcase was not overfilled with oil, gasoline had leaked down into it. Damn. I had heard of this happening but have never actually seen it first hand. Obviously, we pulled the oil out of the motor immediately. Some time later, he had the carbs disassembled, cleaned and reassembled. I know very little about carbs having only owned fuel injected bikes for the last 21 years so he explained each step of the process showing me how to do it. Fantastic. We put new oil in the engine and he loaned me a little clamp to clamp the fuel line once I park the bike. Riding it back home that evening, the bike was MUCH improved but not perfect. Greg, the mechanic, sent me a few texts mentioning some thoughts on what else needs to be done. It's just crazy the nice people you'll meet on a motorcycle. Bandit S broken down(https://miles-by-motorcycle.com/136/photos/506/untitled/7870/bandit-s-broken-down) (https://miles-by-motorcycle.com/136/photos/506/untitled/7869/untitled) Carb rail out in minutes. Crazy.(https://miles-by-motorcycle.com/136/photos/506/untitled/7871/carb-rail-out-in-minutes--crazy-) (https://miles-by-motorcycle.com/136/photos/506/untitled/7874/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7872/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7873/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7875/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7878/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7876/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7877/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7879/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7880/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7881/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7885/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7882/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7886/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7883/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7884/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7887/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7888/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7889/untitled) (https://miles-by-motorcycle.com/136/photos/506/untitled/7890/untitled) This guy landed and just hung out with us for quite some time.(https://miles-by-motorcycle.com/136/photos/506/untitled/7891/this-guy-landed-and-just-hung-out-with-us-for-quite-some-time-)