Topic: Idles fine after quick throttle, stay high after turning throttle slowly.

[Gorms]

77 cb550.

Bike starts up and idles fine.  When I snap the throttle open quickly and let go, it returns to idle quickly.  When I slowly turn the throttle a bit, the RPM's stay high until I turn the idle adjustment screw a fraction of a degree.  Sometimes I can get it to go back down by forcing the throttle handle backwards a bit but not always.  Throttle cables are not tight and not binding.  I took the carbs off last night and found a good ammount of gunk in the idle jets that I missed when I rebuilt the carbs a few weeks ago. Squeaky clean now.  Thought that would do it but it didn't.

Is this a sign that I need to sync the carbs or could it be something else?  If a sync is the definite solution, does anyone have a carb sync in the central Vermont area?  A long shot but you never know...

Or...is this a symptom of leaking rubber carb "isolators".  I want to check that with starting fluid or carb cleaner but I dont want to spray that on my freshly painted engine/intake tubes as it takes the paint right off.

Thanks in advance.

[BVCB650]

Sounds to me like dirty or binding cables/linkage.

[TwoTired]

77 cb550.
Is this a sign that I need to sync the carbs or could it be something else?  If a sync is the definite solution, does anyone have a carb sync in the central Vermont area?  A long shot but you never know...

It's not a long shot.  Carbs out of vacuum balance will often show those symptoms as the cylinders fight for dominance of the common crankshaft.

Can't say it is a definite solution.  But, if the cables are truly allowing the carb slides to return to full idle position, the uneven position of the slides makes different cylinders want to run at different crank speeds.

[Gorms]

By long shot I meant having someone in central VT with a carb sync.  I have read that syncing them can do a world of good.

[w1sa]

You should of course vacuum sinc the carbs.......... I had a problem where, although the carbs were in sinc, at idle and low speed running the engine speed would occasionally stick at 2-3k rpm........ When I checked cables and such all seemed fine. The problem persisted and I eventually found the carb body wall had some slight roughness down near the rear low point where the slide seats at idle/low rpm.
With engine turned off there was no indication of drag or binding between the slide and the carb wall. I polished as much roughness away as possible. Still felt smooth thru the throttle............. Engine now runs without sticking at higher rpm.

I assume the vacuum through the carbs when running was enough to allow the slides to drag/bind due the localised wall roughness.

[russouno]

after a few months of having the exact same issue....and failing the AZ emissions test 6 times !($20 each trip), I needed to find out what the problem was just to keep my sanity !
The engine was rebuilt about a year ago.. completely... head to toe, everything rebuilt.
Within the last six months I started to have the same problem with surging at idle..and fixing it by bliping the throttle. This USUALLY worked.
I went through the entire bike from carbs to points to coil etc... making sure everything was within spec.  WhileI was cleaning out the carbs while still on the bike and getting ready to sync them with a drill bit, I noticed something that seemed out of place while bliping the throttle.  I had all of the top covers off of the carbs and was going to loosen the adjusting nut getting ready to sync.  While I made sure nothing was gummed up or sticking... I noticed a weird 'looseness sound' while snapping the throttle shut.  I did this a few time and witnessed that a few of the slides were 'bouncing or vibrating' while it was snapped shut.  This is what the problem was and for the life of me ... would probally NEVER would of found this unless I completely rebuilt the carbs from the ground up.  The shaft that goes through all of the carbs and is set in place to the slides via one screw.  All of the screws were loose thus causing the slides to somewhat stick not completely returning to the idle position.  I had adjusted the air/fuel mixture along with everything possible for emission but could never pass.... any time I made an adjustment, it seemed that the readings were all over the place, which didn't make sense to me. I thought I would pass this along to anyone else who is having the same issues with a sticking throttle.  If you have gone throught all else... linkage, cables, adjustments for air etc...... check your set screws that hold the slides in place under the top carb covers.  My bike now runs brand new with idle at about 900rpms and runs like a Swiss watch !!!!!!!!

[D]

thanks for the advice, I am having the same problem right now

[fmctm1sw]

Mine did exactly this after I didn't tighten the linkage that holds the throttle slides in place.  I neglected to tihten them becaise I was going to tweak the jet needles and forgot all about it.

*edit* here:  http://forums.sohc4.net/index.php?topic=57160.msg617801#msg617801

[Deltarider]

Tip. Do not separate carbs from each other unless you really have to (personally I can't think of a reason why I would want to separate them, but maybe I am dumb). When you think of separating, think again.

[gregk]

I also am very interested in this problem as i have similar conditions with a 76- 550.  I have cleaned the carbs etc and still have the rising rpm problem.  It seems that if I place some pressure on the # 1 carb link set ( the part that lifts the slide) the bike will return to an idle from 3000 rpm. 

greg