Topic: Idle Hover, any thoughts?

[srook]

I have been chasing an idle hover condition on my CB750 all summer.  Here are my findings: 

Symptoms:
Starts and rides great, pulls very strong and the plugs all look great.  When I pull the clutch to shift sometimes the revs stay right where they are or climb.  On deceleration, if I pull the clutch to brake or downshift the revs will stay at 3 - 4000 or will climb slightly.  I can get them to come down by braking with the bike in gear until about 2000 or so rpm and then if I pull the clutch the revs will drop to idle at about 1100.  The other way to get the revs to drop to idle when pulling the clutch for braking or shifting is to physically push down on the bar that lifts the slides.  One other thing to note is when I have been riding fast for more than a few minutes and come to a stop the bike will settle right back to idle on its own, therefore when running above say 6000 rpm and coming to stop their is no idle hover.  While reving the bike in my driveway above 3000 rpm I can actually see the slide linkage fall short of seating the slides.  A little pressure on one of the slides and they drop right back causing the bike to idle correctly.

Possible causes:
Slides sticking in bores due to gunk or damage.  Float level too high.  Throttle cable (note I am only using the pull cable) dry, damaged, or dirty.  Air leaks around the carb manifolds.  Valve tappets out of spec.  Return spring on carbs damaged or weak.

Findings:
Air leaks have been ruled out.  Floats have been set to 26 mm.  Throttle cable is brand new, has been lubed and has no drag.  Same for the throttle tube and rh switch.  Slides have been cleaned several times as well as the bores and jets.  Slides do not stick when the engine is not running, they move up and down with no interference.  The throttle return spring seems fine and snaps the slides closed when the engine is not running.  Valve clearances are in spec.

Belief:
For some reason I believe that the CB750 motor generates alot of vaccum right around the 3-4000 rpm mark holding the slides off their seats and pulling them open in some instances.  This is why I think the push cable was added to the carbs when the throttle linkage was changed.  The extra push cable positively pulls the slides closed overcoming any vaccum that the motor generates.  Am I crazy?  I can find no other physical interference with the slides while the motor is off.  A running motor is not allowing the slides to fall all the way down in their bores.  Why is this?

Any thoughts?
Scott

[jessezm]

I think you may have answered your own question--buy a push cable and report back.  At least I hope that's it.  seems like these high-idle threads go on forever without ever totally fixing the problem!  by the way, I have a similar issue that on my 750 that I'm procrastinating on.  Got my 400f going so she gets all the love these days!

[TwoTired]

Belief:
For some reason I believe that the CB750 motor generates alot of vaccum right around the 3-4000 rpm mark holding the slides off their seats and pulling them open in some instances. 

Any thoughts?

Not a chance. (that there is enough pressure on the slide bottoms to overcome the stock return spring pressure.)


I do notice there is no vacuum balancing listed in your correctional action items.
...And no mention of a replacement or nob-stock throttle return spring. (you didn't tell us which model of 750 you have.)

The "push" cable is to ensure throttle closure in the event that the single return spring fails when you can least afford the time to deal with that failure.
When the throttle linkage was "changed", it went from each of the four carbs having a closure spring, to just one spring to close four carb slides.  The "push" cable (actually a pull closed cable) makes certain the slides can be closed instead of remaining at full or mid throttle during riding.

Cheers,

[mrbreeze]

Lloyd is right on the $$$ about the carb synch.Thats what it took to make mine straighten up.You will simply be amazed at the difference.

[srook]

The carbs were synched  by a reputable repair shop.  They were also hot tanked and cleaned + I have had them off the bike 3 times and cleaned them thoroughly.  I don't buy that a carb synch would change anything other than running issues (the bike runs strong throughout the rev range).  I can physically watch the slide bar lift the slide linkage as I rev the bike.  When I let go of the throttle the bar and linkage drops, but not all the way down.  Here is where the revs are hanging.  There seems to be no actual physical interference because when I twist the throttle with the engine not running everything snaps closed (which also eliminates the spring as the culprit).  This happens on start-up from cold as well as when its hot, therefore I don't think heat has anything to do with it.

What could possibly cause a running engine to demand more mixture than it is given to the point of lifting or holding the slides open?

Any thoughts?
Scott

[Spanner 1]

If one of your four slides is sitting higher ( were talking just barely ) , the increased air allowed in will increase rpm on that cylinder, resulting in all four slides ' lifting' and now were 'idling' @ 3K rpm !! So, yes it is a carb sync. issue. Snapping the throttle closed with the bike not running really doesn't prove anything, what's happening under engine vacuum is whats important.... because your motor has 4 cyls. and 4 carbs you have 4 independent motors connected to one crank, all 4 fighting for dominance !!, unless finely 'balanced' by a vacuum carb sync. ( Best done by the owner... IMO  ).

[ekpent]

So when the revs hang up you can physically reach down and push the top of the rack mechanism and the revs will drop. Is that correct ??  Does this happen all the time or sporadically. Have you inspected the carb top caps well,if its 76 or older there are rods that connect to the slides that pass through rubber grommets at the top of the cap.I have seen these grommets go bad from over soaking/cleaning etc and they can bind on that rod.Other than that,if the carbs are synced fairly close it still sounds like the classic throttle slide hang up.Just a very,very small stick can increase the rpm's dramatically. Good Luck on it.

[TwoTired]

The carbs were synched  by a reputable repair shop. 

Engines don't run on faith.  If your shop mechs are a lot younger than the bike, who taught them to work on it?
I'm telling you that the vacuum sync is most critical at idle speeds.  This is where very small slide opening differences among the carbs make a big difference in which cylinder is providing the strongest power pulses at idle settings.

They were also hot tanked and cleaned + I have had them off the bike 3 times and cleaned them thoroughly. 

We've all heard this before.  And your carbs may indeed be clean.  But, until you actually prove flow through each of the pilot passageways, you cannot verify they are "thoroughly clean".  You have to not just clean them, you have to clean to right portion of them to cure maladies.

I don't buy that a carb synch would change anything other than running issues (the bike runs strong throughout the rev range). 

As Yoda would say, "that is why your fail".

A half millimeter slide opening difference at idle slide position is critical, a millimeter difference among carbs at mid throttle position is inconsequential.

What could possibly cause a running engine to demand more mixture than it is given to the point of lifting or holding the slides open?

Nothing.

Good luck on your quest!

Cheers,

[srook]

Lloyd,
Thanks for the imformative post.  BTW the shop I bought the engine and carbs from is owned by a man that used to work at Imperial Honda in WNY in the 1970s.  He has one other tech who also used to work in that same shop in the 70s.  Both gentlemen are completely schooled in CB750s.  The carbs are absolutlely clean.  I have cleaned them several times.  Pilot jets out, main jets out, holders out, slides out, all orifices and bodies cleaned.  I understand that different slide heights would effect idle and if this were the case I would never get the bike to idle it would either be too high or die altogether.  I understand the need to "know the answer" but you have created your own narrative about my carbs and haven't addressed the reasons the slides will not settle all the way down when the throttle is closed.

*****The cause of the hanging idle is the carb slides are not settling all the way down in their bores*****

I'm trying to explore why.

Ekpent,
I like this.  The only thing that I don't understand is why the slides show no interference when the motor is not running.  I will try to get some video of this happening tommorrow as my camera is at work.

Spanner,
I understand what you're saying, but if one slide were higher it would never drop down to a nice 1000-1100 idle.  The idle would jump around or always be high.  Something is preventing the slides from seating and it only happens while the motor is running.  Maybe a combination of vaccum, weak spring, and swelled grommets on the carb tops. 

I haven't failed yet Lloyd.
Scott

[scottly]

For some reason I believe that the CB750 motor generates alot of vaccum right around the 3-4000 rpm mark holding the slides off their seats and pulling them open in some instances.
 I can find no other physical interference with the slides while the motor is off.  A running motor is not allowing the slides to fall all the way down in their bores.  Why is this?

Any thoughts?
Scott

Nothing really wrong with your thinking. Any time the motor is running with the throttle closed there is a higher vacuum, which pulls the slides towards the engine side of the bore, which will increase friction. I suspect it may help to carefully polish both the bores, and the slides on the engine side. Or you might try a heavier return spring.