Topic: Increasing Idle

[Harsh]

I have been putting off making this post for a while now trying to read the many other posts here on the subject.  However, some of the suggestions in those ranged from one end of the spectrum to the other.  I just don't know what to do and need some help.

Specs: 
1974 CB750K stock engine.
4-1 exhaust (possibly a Dick’s Cycle West) with a Cone Engineering Straight Core Cone/Reverse Cone Slip-On Muffler.
Carbs rebuilt by Digger - He said he changed the mains from 105 (stock) to 110’s because of my exhaust other than that I presume they are stock.
Valves set and triple checked
Timing is set and is spot on at idle and at 2500 rpm
Air crews are 1 turn out
Stock airbox with new stock filter.
Vacuum sync has not be completed due to idle issue

Issue:  Rising idle when the bike starts to get warm.  When I first fire up the bike idle is right at 900-1K.  It fires up with the simplest of ease with no choke.  After about 5 to 10 minutes of idling the RPM’s begin to climb as everything starts to get warmed up.  I tried to adjust the idle knob, but it did nothing.  Taking a look at it, it barely made contact with the bar before I adjusted it so obviously after I made an adjustment it wasn’t even touching the bar.  If I press down on the tops of the carbs I can get the RPM’s to drop, but as soon as I release the RPM’s rise.  Right about the 2K RPM mark I can hear it start to hunt vice the slow steady increase in RPM it had had prior.  Throttle response has been great with no noticeable bogging down.  Granted the bike is not ride able and I am just sitting in the driveway giving twisting the throttle.  I noticed last night that if I close the throttle past it’s natural stop (physically rotate the throttle closed) I can get the RPM’s to drop down to about 1100-1200 RPM.  I can see a piece on the carb linkage move away from the stop bolt/pin slightly.  I thought that maybe I had the throttle cables too tight, but a second (and third) look shows that there is slack in the lines at rest.  When the bike is still at a lower RPM and I rev it, the revs drop back quickly, but once the revs get up near the 2K mark they don’t drop back down as quickly.

I am not sure where or what to do at this point.  I don’t want to start making adjustments here or there and take a chance of making the issue worse due to lack of knowledge.  I hope I included enough info for an informed decision on where or what to do or try.

[kghost]

Sync the carbs then set the idle

You have 1 or more carbs set higher then the rest.

[Harsh]

I thought of that, but aren't you supposed to sync the carbs at idle (900 RPM) and the engine at normal operating temp?  How can I do that if by the time the enging gets up to temp the idle is at 2K plus?  Should I try and sync them as soon as I start the engine, i.e. cold?

[flybox1]

a few things to check...
throttle snap back....after a blip of the throttle, do the slides ALWAYS drop back to the same position? every time?

leaning towards what kghost said...one or more slides sit higher than the others.
take a look inside your carbs at the slides.
with the idle screw backed out all the way, do the slides sit all the way down?
I'm thinking  they dont drop all the way, and you stated "I tried to adjust the idle knob, but it did nothing.  Taking a look at it, it barely made contact with the bar before I adjusted it so obviously after I made an adjustment it wasn’t even touching the bar."
I would head right to a re-do of your bench sync....

[ekpent]

Approx how many threads do you see visible at the top of the large slide adjusting knobs at the tops of the carbs that are used to do the synch. As mentioned those may be a little out of whack and set to high. Other symptom could be a sticky slide.  These bikes do have a tendency to increase their idle once they are warmed up. Think we are all pretty good at tweaking the little idle screw when needed.

[thep1pe]

I had that problem on my 550. Turned out to be the slides were too high. This had the effect that the idle was controlled by the vacuum and not the mechanical adjuster, if that makes sense. I turned all the adjusters back until there was about 2 treads showing adjusted the idle with the screw, then balanced carbs.

[Dream750]

Have you discussed the idle issue with your carb rebuilder?

See if this Honda service bulletin can help with adjustments:

http://manuals.sohc4.net/CB750SB/750_25.pdf

(useful information for the 750 K1 to K6 owners with stock carbs)

[strynboen]

mine cbx does it often..the 550 does it also..they have been vaccum syncr..but still do it...i call it charme..and adjust it in first red light i hold ..are the return spring strong..i can adjust it very presise..vhen the bike are in running temperature..next day in kold start i have to rice it a bit..and the show are going on


you cant adjust idel in the cabels..it have to adjusted in the mekanik adjusting schrev..the cabels have to give some slock to adlau it to fall back to the adjustabel stop

[70CB750]

On the K0, the carb cable routing was touch sensitive and it would make the idle climb randomly.  Took me a while to find that one out - but K0 is 1-4 cable.

[iron_worker]

I would definitely be checking where you have your sync adjusters set to. Like others have said, there should only be 1 or 2 threads showing above the locknut on top of the sync adjuster. If they are set with too many threads showing it will not allow the slide to return to the carb's bottom.

Also, if you can start from dead cold with no choke then your bike is probably running very rich.

IW

[lucky]

You are on the right track.

Remove the air filter and look inside the carbs. You can use your fingers too.
When you pull the throttle open all the slides should start to move at the same time exactly.

IF it is running and you push down on the "bar" and the idle gets lower like you say THAT is the problem. The slides have to shut all the way.

The fact that it will start up with no choke at all means something is off.

As far as the main jets being increased to #110's, that is reasonable, although usually with a stock air box and aftermarket exhaust all that is usually needed is a pilot air screw adjustment. The 110 main jets are fine and are good insurance at WOT to make sure it does not run too lean. A good idea.

[stickman]

Harsh,  If syncing the carbs does not cure the fast idle,  try checking out your rubber manifold boots on the engine side of the carbs.  Make sure are boots are not cracked and all  clamps are snugged up tight.  Good luck.  Stickman

[Harsh]

WOW...thanks for the replies.  I honestly didn't expect this many as it has been well discussed topic.  I will try to answer the questions as they were asked.  I am at work so measurements and the like will not be able to happen until I get home, but at least I have a place to start looking.  I completely understand the rising idle as the bike gets warm, which is part of the reason I am worried.

Throttle snap back.  Yes the throttle snaps back very well, however since I installed the carbs on the bike I did not check to see if the slides moves as described.  I did look at them with the crabs off the bike and they did rise and fall at the same time.

Threads visible.  I didn't count them, but I would say there is about 2mm worth of threads showing.

No I haven't discussed it with the carb builder, but I would say he is well respected here and I do not suspect anything wrong with his build quality.  I figured it was a tweaking thing that all carbs have to go through to match the engine they are attached to.  Thanks for the SB.  That is one I have not read yet.  Looking at it the bolt.pin I mentioned in my first post was the eccentric stop pin.

Cable routing.  The cables are new (and I lubed them before installation) they have very gentle arcs to them (headlight not installed so it isn't hindering them) and I feel no resistance other than the spring on the carb.  There is a little bit of slack on each cable in the fully closed position indicating that they are not pulling one way or the either.

Lucky you mention the air screw adjustment.  With the mains that I have should the turn be increased or decreased?  If I understand how it works correctly I pressume I would want to increase the turns to allow more air in which would lean it out a little bit.

As everything sits right now should I attempt to sync the carbs?  The rubbers looked good and the clamps are tight.  I re-used the manifold boots after I soaked them in some wintergreen to soften them up a little bit.  The trumpets rubbers are new.

Thanks for the help and suggestions so far.  When I get home I will get accurate measurements and visual inspcetion results back to all of you.

[steakCB550K]

since you are not riding the bike, I would suggest a box fan in front of the bike just to help cool the engine while you are adjusting the carbs. 

[lucky]

When it is running, push down on the "bar" and see if the idle goes down.

I had that problem and fixed it by adding a spring.
I suspect it is one of the cables.

[Harsh]

I have been using a fan in front of the engine.

Yes if I push down on the bar as it sits over the carbs the idle will go down.  From what I can tell the spring is pretty good.  By that I mean it snaps the slides closed with authority.  One of the cables?  They are brand new, lubed, and are gently curved.  I guess I could tell one way or the other if I disconnected them.

[flybox1]

did you do a gauge vacuum sync, and how did it turn out?
did you calibrate your gauge to one of the carbs to ensure all dials read the same?

i re-read your original post.  gotta ask Digger if he did a bench sync.  i'm guessing no, and one or more slides are high.

[lucky]

I agree get those covers off and do a bench sync.
Maybe one of the slides is up too high .

[ekpent]

Quote--

  I tried to adjust the idle knob, but it did nothing.  Taking a look at it, it barely made contact with the bar before I adjusted it so obviously after I made an adjustment it wasn’t even touching the bar. 


 Start by finding out why you cannot use the idle adjustment screw and the problem will probably be cured. You need a little more headroom probably in the slide heights.

[Harsh]

No I have not done a vacuum sync.  From everything I have read that needs to be done on an engine that is at its normal operating temp.  I can't do that because once the engine gets to that point the idle is too high.

OK...Just got home and pulled the airbox.  I also pulled the throttle cables to make sure they weren't the cause of the problem.  I manually operated the carb and verified all slides moves simultaneously and without any of them sticking.  They move exactly as they are supposed to.  I snapped a couple pics of the threads and the slides.  A 1/16" bit slides in easily and has some slop, a 1/8" bit won't even remotely go in.  The three bits I have in between those two are all broke and not in my drill index.  Based on what everyone has said about the threads I think I am going to have to bench sync them.  Can that be done with them on the bike?

[iron_worker]

I know my bike is not exactly the same but my sync adjusters only have like 1 or 1.5 threads sticking out the top. Yours seems to have 3 or more.

The method I use for syncing ... while off the bike ...

-Unlock all the sync aduster lock nuts
-Adjust all the sync nuts and locknuts basically all the way to the top of the threads with about 1 thread sticking out
-While looking at the engine side of the carbs ... where the slide has no cutaway and is flat on the bottom ... by eye determine which slide is set lowest (ie which one rests firmly on the bottom)
-You can also confirm which one is lowest by turning the sync adjusting screw ... if it has a slight resistance to turning that means it is carrying the load of the return spring .. the others should turn freely (ie they are not touching the bottom)
-Then, 1 by 1, adjust all the other sync adjusters until they *just* start to take the weight of the return spring. You will feel the screw slightly increase in resistance as the slide hits the bottom. (You're moving the adjuster UP on the threads.) Don't go too far as you'll just raise the others off the carb throat floor.
-Visually check that as you actuate the throttle arm that all 4 slides open and close in unison ... this means when they are coming down they touch the bottom at exactly the same time.
-Lock all the locknuts in place while trying not to disturb the position you just set of the adjusters.

This is how I do it. I've tried the drill bit method and it works but seems awkward and more difficult that it needs to be. I can bench sycn my carbs in under 5min with the method I described. It just takes a keen eye and a discerning touch.

IW

[lucky]

Get #2 to be bottomed all the way FIRST. That is the non adjustable master carb.
Do not use any drill bits!!

You can use the idle screw to raise the slides after all the slides are bottomed out .

Read you workshop manual if necessary.

The slides are designed to be bottomed out at idle, and the cut away in the slide provides just enough air for the engine to idle. If it is not enough, then you have your idle screw that will help raise the idle.

[iron_worker]

K4 750 doesn't have a non adjustable "master" carb. I believe you're referring to the later 77-78 carbs.

IW

[lucky]

You are right iron worker. Ooops!

[flybox1]

here's my on the bike bench sync process...

turn your idle screw in(from just touching) 2 full turns
do as IW mentioned
Adjust all the sync nuts and locknuts basically all the way to the top of the threads with about 1 thread sticking out
now..
reverse the idle screws all the way out so that all slides 'bottom out'  plus a turn so there is a gap at the set screw tip.
at the top caps, unlock the slides from the arm on all.   press the slide of #1 down firmly so it is fully bottomed out, and re-tighten the screw to hold it in place.  now do this for the rest. 

vacuum sync.