MattI completely agree with regards to running without an air box. I wasn't suggesting removing the air box for normal operation, as you point out there is a lot that goes into such a change. I was merely suggesting that it would be fine to do for the purpose of checking the mechanical operation of the butterflies. Everything mentioned about the symptoms so far sounds like a choke-type, or mechanical interface, problem (stuck throttle/choke etc) versus a needle or mixture problem. I've seen things like this happen from simple things like not adjusting the eccentric on the throttle, and even cable position when reinstalling the carbs which puts tension on a cable.

Yup.  If it is a butterfly issue, you can see it pretty clearly with vacuum. CV carbs will make it hard to see the actual butterfly.



Looking back at this thread, I don't see anything on balancing the carbs? Particularly since the carbs were taken out, balancing should be the very first thing you do. A bad balance will be most evident at small throttle openings.

one item that often gets overlooked while cleaning motorcycle carbs is the inlet valve many have an internal spring- many people are not aware of this and they get stuck or collapsed - this spring helps dampen the float, to control the float level.


this can lead to erratic idle


if new inlet valves were used this should not be a problem.

rshaug sorry for the late reply. It's a very kind offer but I'd hate to have you to come up all this way to just turn wrenches, especially with how bad traffic is.

mothman I had thought about that, but the bike has 7,000 miles on it and is extremely clean. It's essentially unused and has been sitting inside for years. I don't know the condition of the valves. How would out of adjustment valves create these symptoms.

Matt Good points. I will try that. I can get the symptoms to reproduce through restarts. Once the bike is warm, this problem is 100% reproducible.

Matt The carbs have not been balanced but other than this sticking at 2000 rpms issue the bike runs perfectly. I still have Duncan's mercury sticks in the garage so I'll see about checking the balance.

Yermo "It's essentially unused and has been sitting inside for years" would lead me to give a shot of WD40 into all the cables at there terminus, each place there is rotating or sliding contact, and a quick blast (without straw) into each carb with the butterflies held open then twist the throttle a few times and pull the choke in/out to move it around. Also check the spring landings on the throttle bar, sometimes it can position itself (the spring) so that it isn't putting enough tension to fully close.

sorry, I was trying to establish the condition of the motor. with only 7K on the clock we normally would assume it is in good shape. doing a carb balance as matt stated would show if one cylinder is off from the others - typically if valves need to be adjusted the engine would run rough but all the time not just at one RPM.

from the pics the carbs/petcock & gas tank all were cleaned is this correct? were new gaskets and or inlet valves in the carbs used when reassembling?

mothman no. We re-used the old parts. This was kind of an ad-hoc "side of the road" repair. The jets and needles were cleaned. Carb bodies sprayed out. Float height checked. The mixture screws were richened up.

I should also point out that this "sticking at 2000 rpm on the way down" problem preceeded the carb removal and persisted afterwards. We did lubricate all the linkages and checked the action of the throttle cable. 

That's what's so strange about it. I would assume, if the butterflies were sticking, you'd see it on the throttle linkage but it seems to be closing all the way against the stop. The spring is quite strong. 

The bike is extremely clean and aside from this one issue of it sticking at 2000 rpms it runs perfectly. 

I'm going to go back out to the garage shortly and mess with it a bit.

Interesting... Use the choke behavior as the guide and follow the path for what happens when using the choke. Do the inlet valves have springs? Have you checked against the shop manual parts list to make sure any/all spacers, springs, and clips are there? If opening and closing the choke manually "fixes" it, I would put money that it is a mechanical item in the choke chain of function or a physically missing/misaligned part. Cleaning would have no bearing on such a problem (other than making it easier/cleaner to work on ). The other option is in the idle speed and mixture adjustment realm, where mechanical linkages etc come into play.

Just for our benefit because I'm a little slow sometimes. Did you try manually blocking the intake ports with the airbox removed (using a piece of cardboard or something) while the engine was running at high idle to see if it dropped back down? Also, did you check that the butterflies snap closed on their own with the bike in operation (not on the bench)?

I spent a couple of hours going through the various suggestions.

New behavior. At first when I started it up it was smoking pretty badly with the smell of extremely rich exhaust. During my experimentation, this would recur then clear up (as if the choke was sticking but at best I could tell by physically pressing the choke rail on the outside of the carb it was completely off.)

I checked the plugs and 2, 3 and 4 were blacker than they should be. (These are new plugs as of yesterday.)

I took the tank off and set it up so I could run the bike while having access to the carb.

Using the mercury sticks buffalo so kindly loaned me something like 15 years ago, I noticed the carbs were slightly out of balance but not bad. I adjusted them as best I could.  I verified that when the 2000 rpm problem is occurring, the carbs are still balanced.

The sticking at 2000 rpms behavior remained.

I did the "cover the intake port with cardboard trick" rshaug suggested. Each time regardless of cylinder it would cause the RPMs to break through the 2000 barrier and settle down to a nice idle.

As Matt predicted because these are CV carbs I can't actually see the butterflies from the airbox side.

I attempted the wd40 trick and that made no change. I tried to liberally lubricate the throttle rail (what is that called?) and springs to no avail. Since wd40 is so higly flammable I didn't want to go overboard with it.

I did notice that when it's stuck at 2000 rpms, if I physically grab under the throttle rail and push it, it "gives" a little bit and settles the idle down immediately. However, it doesn't feel like it's moving, more like I'm pressing against some metal that is flexing a little bit. 

Came across this video:

 

So I'm guessing most indications are pointing to slightly stuck butterfly valves but what I don't understand is why the choke or covering up an intake port would have an effect on the idle if the butterfly is stuck.

Sorry about the orientation. Here's some video of what we're seeing.

 

on the suzuki bandit I don't know if the inlet valves to the carbs are spring loaded - I was putting it out there as more information I know some bikes may have this type of inlet.  the manuals don't show it - duncan showed it to me on the old Suzuki GT750's and if they are screwed up they won't idle - the engine vibrates so much they need it to dampen the float. the float won't shut off the fuel at the correct height and they either run rich or flood. even though you thought you set the float height correct.

I was asking about the gaskets because I was looking at the gasket surface on the bowls to the carb body on some carbs there are fuel or air passageways between the body & the bowl and if they are not sealed they could cause an internal vacuum leak. - just trying to rule out silly things that may have been over looked.

from the pics they look like you could eat off the carbs.

overall I have never worked on a bandit but I've worked on many old multi carb engines so my opinions or suggestions will be generic not specific to the engine at hand.

I have gotten away with reusing gaskets many times to make stuff run, when your in a pinch - I've been there.

if you believe everything is set as close to factory as you can get and no vacuum leaks. I'm in agreement with Matt balance the carbs and see how it behaves - out of balance carbs will do some strange stuff. - usually popping & bad idle.

YermoThe choke closes off the airway to the carb, when you put the cardboard over the intake it effectively chockes that carb (the same as a functioning choke). When pressing the choke bar/throttle bar down it is sealing the butterfly valves, effectively the same thing as covering the ports. Both restrict airflow to what should be "normal" at idle with them closed.

If you apply force to the throttle when closing it...instead of just letting the throttle go actually turn it back to the stop and twist a little harder, does it settle down? It looked like it did in the video. If so you can independently adjust the throttle and return cables, they may be out of wack leaving the return cable to slightly push back on the throttle bar enough to break the seal on the butterflies. It doesn't take much at all, once air flows over the mains the idle speed will pick right up because it is no longer actually idling. With a very small opening (such as the butterflies just off contact) the air volume will be low but the velocity will be high across the jets. It can be REALLY sensitive in this area, even if the throttle bar is a little bit bent, it can do it. You could cheat a little and file a little bit off the throttle bar stop if it's that. It could also be an alignment thing where the cam or bar is a couple of degrees off. Visualize the throttle bar as a dial connected via a belt to the butterflies, and the throttle bar connected to the throttle via a separate two-dials-connected-by-a-belt setup. 

Yermo I can't make it up there today but could tomorrow late afternoon/early evening. I'm very confident can fix it quickly if you don't before then.

Interesting. Both sides of the throttle cable, push and pull, are loose on the arm when the throttle is closed. So my assumption is that the only thing closing the throttle is the spring. There is some free play in the throttle on the handlebar.

What you say about the return cable makes a huge amount of sense to me and I remember thinking the cables were a bit loose when we originally pulled the carbs off. I will give that a try.

Thanks for the offer to help. I sincerely appreciate it. But it's such a long drive here to the hinterlands. I will give it a try. Hopefully I can get it working now that it seems pretty clear what the issue is.

Thank you all.

(BTW, I switch cell phone carriers so my cellphone is out of service while i have the number switched over.)

No joy.

I adjusted the throttle cable such that all the slack was taken out of the closing side, started it and let it warm up. The problem recurred but I could now, by applying some extra closing pressure on the throttle, get it to drop past the 2000 rpm boundary. 

But it is still sticking at 2000 rpms.

Yermo I'll swing by tomorrow late afternoon. It's not THAT far lol We'll get it sorted in no time. 

rshaug I guess it's time I admit I might need some more experienced eyes on this problem. Maybe Ben Mendis can join us after he gets out of work.

ok I just watched the video this morning. I also would be looking for possible vacuum leak and/or linkage & cable issues.



I found this PDF manual on mikuni carbs it may help if you have to go back in them or may help someone else.

I did find their way of checking the float level interesting never seen it done this way.

on the last page of the manual it states:


"If the idle speed increases significantly when the
engine gets hot, or hangs and doesn’t drop
down quickly from higher rpm, there is likely a vac
uum leak in the system or the pilot screws
are set too lean."



http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Car....

yermo can you confirm the type of carb on the Bandit I've seen on forums that they use Keihin carbs - they are still CV type carbs similiar to the mikuni but each manufacture will have slight changes and their suggested settings.

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/t...

mothman Yes, I believe they are Keihin carbs. Thanks for the find on setting the idle mixture. That reminds me of the process on the old VW Webber carbs.