Starting a thread on the Suzuki GSF600S (a.k.a. Bandit S) which  Ben Mendis 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.

 

A question for those wise in the ways of carbs, namely buffalo, Matt, mothman, rshaug

As was previously mentioned, Ben Mendis's 1997 Suzuki Bandit 600 S had it's carbs cleaned by a road-size Suzuki mechanic. 

The bike runs significantly better, smoother, faster, etc. Any any speed over 2000 rpms, it seems like the bike is perfect. It pulls strong. It's smooth and well behaved. It wants to rev.

However, it's original problem remains, namely that after the bike is warmed up when it is revved past 2000 RPM and the throttle is closed, RPM's get stuck at 2000 RPM more times than not. i.e. revs will fall to 2000 but not down to idle.

Here is what we have observed:

1. The bike starts, idles and revs up without issue.


2. When the bike is cold and revved, it will pause momentarily at 2000 rpms as it drops back down to idle.

3. As it warms up, it progressively "pauses" at 2000 rpms for longer intervals.

4. Eventually, once completely warmed up, once revved past 2000 rpms it will get stuck at 2000 rpms on the way back down.

5. /Sometimes/ blipping the throttle causes it to break through 2000 and settle at 1200. (When at idle, it idle's perfectly smoothly.)

6. Interestingly, if, when it's stuck at 2000 rpms, I blip the choke momentarily it will settle back down to a normal idle every time.

7. Also, interestingly, if I'm riding along let's say at 3000 rpms and I close the throttle and pull in the clutch, it will, as expected, get stuck at 2000 rpms and won't go any lower. However, if, with the throttle still closed, I let out the clutch and drag the brake forcing the RPMs below 2000 and then pull in the clutch (again, without having touched the throttle through this process) the idle will settle back down.

8. Taking it on a 20 mile ride, on rare occasions the idle would settle back down on it's own.

9. We induced a vacuum leak as an experiment. The behavior did not track with the vacuum leak. Spraying carb cleaner over the carbs at the boots, vacuum lines, etc did not alter RPMs so I strongly suspect it is not due to a vacuum leak.

10. The spark plugs have been replaced.

11. The bike has fresh gas.

It feels like a mechanical issue to me. Like something is sticking, but when the carbs were off the bike everything looked cleans and the butterfly valves worked smoothly without any binding.

I'm at a loss to come up with an explanation of what might cause this. Any thoughts?

I would look at the following first:
- Idle adjustment, try adjusting the idle down to a lower than normal point and see behavior
- remember, there are separate jets for low and high speed. It could be the low speed jetting, check the low speed jets and that they are seated properly and operating properly. Also make sure the right jets are in the right positions
- Return cable sticking (assuming a 2-cable setup, also check the throttle return spring)
-Check to see if anything is impeding the throttle bar, pully, or lever arms. Could be a hose/cable/etc catching on one of the mechanical throttle elements
-Airbox/airfilter, how do they look?

Sounds mechanical to me, like maybe the butterflies sticking, or a needle issue.

Oh... could also just be the choke. Check the choke cable and mechanism to make sure everything is lose, works smoothly, and closes/opens properly. the choke will cause these symptoms.

See the first couple of posts in this thread (just the ones applicable to this exact problem). This tracks with what I thought re: the idels adjustment and mixture controls (but check the choke first...easy stuff first): 

Yea, what is really strange is all of the above look ok, but maybe I am missing something.

Interestingly, when we took off the carbs and cleaned them we turned the screws out to 2 3/4. Greg said that was a typical change because the factory settings were too lean. Maybe they need to be turned a bit more.

The throttle cable is not sticking as far as I can tell.

I would follow the steps in the post above first. See if dropping the idle speed induces better behavior.

Interestingly enough, the factories that make the bike tend to know quite a bit about them. I would return them to the factory 1 3/4

Yea, same behavior at the factory settings. That's what makes this so strange.

We'll go through everything carefully and report back.

Go to the factory settings, then use the idle set screw to drop the idle speed. If that doesn't do it check to make sure the right needles are in the right place on all of the carbs and that they were put in correctly and the slides in the carbs are moving smoothly. It just really sounds like either a choke issue (almost always mechanical), or a jetting transition thing. That it clears when you choke it makes me think its just something sticking...like the butterflies. that can be lots of things...springs, something touching where it shouldnt, rubbing parts, cables too tight. also check both throttle cables to make sure they aren't too tight and that the cable moves freely.

The weird thing is that when it drops to idle the idle is perfect. I mean fuel injection perfect. 

Checked the throttle cables. I can see clearly that they are not binding. 

Yea, feels like something is sticking. I guess the thing to do is to pull the carbs off again and carefully inspect them again with a much more careful eye. I'm thinking a carb rebuild kit is probably in order. 

Make sure that the butterflys close COMPLETELY when at idle or when closing the throttle fully. Check the return springs on the throttle cables. Really check the operation of the butterflies though and make sure they're closing completely. If not just start going backwards from them and check cables, springs, anywhere where something moves.

Thanks for all the hints. Will do and will report back.

I would not pull the carbs again. you should be able to see the butterflies without pulling them and you want to troubleshoot this with them on the bike. It certainly sounds like a mechanical thing, and really sounds like the butterflies not closing completely (which is usually a cable/spring type of thing). Theres nothing that indicates a complete carb rebuild is in order and the miles and years of the bike certainly don't warrent it. I've had bikes go 20 years without doing any major work to the carbs. It's almost always something simple.

Hmmm. good point. I guess I should be able to see the butterfly valve from the airbox side. There'll be enough fuel in the bowls to run the engine for a bit.

Funny, I had almost exactly the same problem with my fuel-injected Monster.  Turns out I had the cable for the faux choke adjusted a little bit too tight.  No slack at all.  So when I closed it, it was ALMOST closed all the way, but not quite.  I could drop the revs by letting out the clutch lever a bit while in gear and the revs would drop and stay down, but the warmer the engine got, the worse the problem got. 

Does the choke on this bike operate by a cable?  I guess all carbureted bikes since the '80s do...  Check the adjustment of the cable and make sure it's not too tight.

Ian interesting point. I've looked at it a dozen times but I will check it again. It's a cable. Hmmm.

When the engine is at high idle, try whacking the side of the carbs- could be a stuck float or butterfly.  It is pretty easy to get things slightly bound so that it won't return. If you can get individual carb vacuum gauges on it, you'll be able to see if it is just one carb sticking open.  I wouldn't recommend actually running with mercury gauges, though.

If not mercury gauges then what would you recommend?

You can run the engine without the airbox connected...it has nothing to do with fuel delivery (or velocity stacks with K&Ns wouldn't work ). I think you may be overthinking/overengineering this a bit. I can come by later tomorrow afternoon and take a look at it if you want, we can probably fix it quickly.

if you haven't yet, I would check the vacuum hoses up to the petcock & check the petcock for a possible vacuum leak.

There are mechanical gauges. You run the (very slight) risk of sucking the mercury out. 

If one of the butterflies is stuck open, a vacuum difference will be obvious. If the high idle persists across starts, you can see the it, quick connect the sticks, and then diagnose- just don't rev the engine high and quickly close the throttle, or do a big engine brake.

While you *can* run without an airbox, the engine won't run right, particularly at small throttle inputs- the carbs are designed to meter fuel across a certain vacuum differential, and particularly with carbs with a diaphragm (CV- most everything modern is CV from what I'm aware), the slide won't move at the right rate, and the needle won't do it's job of metering fuel right. Running without an airbox for any length of time (or with stacks or K&Ns) on a carb will cause the engine to run very rich, so that's why you have to re-jet in those circumstances. The real magic of a good carb is smoothness at small throttle inputs and transitions.

Idle air screw settings from the factory are usually a bit lean to make emissions better- opening them up a little is usually harmless and can help with the off-idle response.

I know this is going away from the carbs, but being recently purchased when was the valve lash last checked or adjusted?