Intermittent idle speed problems

[hondaman160mph]

I'm having an intermittent problem with my idle speed. If I set the idle speed adjustment screw to give an idle speed of 850 to 900 rpm's after warm up the bike runs great and idle speed instantly returns to 850 to 900 rpm's when I close the throttle. On cooler days 850 to 900 rpm's is a little low until I have ridden 10 miles or so. With this idle speed setting it always instantly returns to 900 rpms when I close the throttle.
If I set the idle speed screw to give an idle speed of 1100 to 1200 rpm's when warmed up occasionally the idle speed will be slow to return to 1100. It will idle at about 1800 or 2000 and if I let the clutch out a bit it instantly drops down to 1100. It only does this once in a while.
Choke cable has the correct amount of slack, throttle cables are in good condition and properly routed. Carbs were redone and benched synced this spring, idle screws 2 1/2 turns out. Any thoughts on what's going on?

[spencer]

First guess is that you need a carburetor synchronization. A real vacuum balancing synchronization, not just bench sync.

[hondaman160mph]

I have considered that but I also wonder why it only does this occasionally. Also just curious, If you bought a new CBX back in the day were the carbs vacuum synchronized or just benched synchronized?

[daves79x]

As far as I know, whatever Honda did at the factory was how you got it. Until I rebuilt mine @45,000 miles, the sync had only been checked once, and it was fine. After complete rebuild, they need carefully bench synced AND gauge synced.

I've recently concluded that a slow return to idle, if all else checks out, can be caused by a too-rich condition at idle. Possibly the mixture screw is too far out (I recently had to close to 1 turn a set I worked on that some unnamed rebuilder did), or the accelerator pump is shooting too well, or a possible air leak between the throttle plates and the body from wear or too zealous a soak while cleaning (felt rings damaged).

A throttle plate miss-aligned can be a likely cause as well. But most often it is just plain clogged or partially clogged idle and slow-speed circuitry.

Dave

[hondaman160mph]

Thanks for the info Dave. You went through these carbs last winter when you changed the fuel tube o rings for me so I am sure the internals and bench sync are perfect. I only use non ethanol fuel and don't think idle circuits would be dirty. Before this last rebuild the idle mixture screws were at 1 1/2 turns out and didn't have any problem with a slow return to idle speed. They now are 2 1/2 turns out so I think I will lean out the idle mixture and see if that makes a difference. Can the idle mixture screws all be reached with the carbs on the bike?
Thanks, Jeff

[daves79x]

Just trying to cover all the bases. I would really gauge sync the carbs first. That will tell the story. But you can reach all the screws, the center 2 are tough. I'd try going in 1 turn on each. If you can't reach #3 and 4, just do the other 4 and see how it works. Let me know what you find.

Dave

[hondaman160mph]

OK, thanks Dave.
Jeff

[hondaman160mph]

Dave, I turned the idle mixture screws in 1 turn and it runs and idles perfect. No more intermittent high idle problems.
I just ordered the long and short carb adjustment tools and will gauge sync the carbs but I can tell by how perfect it idles they are right on.
The idle speed adjustment screw also works correctly now, with the idle mixture screws at 2 1/2 turns out turning the idle speed screw just a few degrees would raise the idle speed from 900 rpm's to 1500 or 1600 rpm's.
I used the Motion Pro idle mixture screw adjustment tool and it makes the job easy,even reaching the number 3 and 4 carb screws.
I also highlighted the index marks on the tool making it very easy to see.
Nothing makes me smile like a perfect running CBX.
Thanks for the help Dave.

[daves79x]

Thanks for the update Jeff. That's exactly what I've found with the last couple of bikes I've done. That is why I think the #35 slow jets should never be exchanged for larger ones (some rebuilders do). Too much fuel at idle will 'afterburn' and cause the idle problems, I think. However, there are still many other reasons they do that, and those need explored, but this problem you experienced does happen.

Dave

[steve murdoch icoa #5322]

Jeff, did you order the long and short carb adjustment tools from Honda or did you find another source?

[hondaman160mph]

I ordered mine from CMSNL.I don't know if they are still available from Honda. I like CMSNL,they might be a little more expensive but I have ordered other OEM Honda parts from them that are not available from Honda here in the U.S.A.