Topic: Hanging idle....I'm done

[Alan F.]

And again.... would I be lucky enough to have an oddball issue like that on two bikes?

Nope. That's why I take stock in where Hondaman's thought process goes.

For this issue to carry over from one bike to another it's time to draw parallels and start thinking outside the box.

Did you deviate from your normal procedure or try any new products or parts on these 2 bikes?

Perhaps fuel, perhaps plugs or plug type.
Perhaps air is being introduced at butterfly shaft ends.

It's got to be an unseen and discounted cause, if you're this knowledgeable and have been looking this hard, perhaps think more toward the realm of the less likely?

Please tell me it's not something dumb like fuel line inner diameter or a clogged vent line.

[Scott S]

 Fuel line is brand new 5.5mm. Vent lines are new.

 Going on the "it's lean" suggestions, I dug out a paper EMGO filter and installed it. Tweaked the mixture screws and idle and, at first, it seemed like I had found the cure. Bike runs really strong and clean throughout the rev range.

 Tried a few different mixture screw settings to see what it liked best and, after about 10 minutes of riding...enough to get it good and hot.....guess what happened?

 I tried te see if I could get a visual on any of the slides sticking up but...no. I can push down on them and affect it, but that would be the case on any bike.

 And, I PROMISE YOU, there are no air leaks present. I've checked and checked and checked.

[Deltarider]

Of topic, but is that a 500 or a 550 engine. The clutch seems to be 550.

[Scott S]

Of topic, but is that a 500 or a 550 engine. The clutch seems to be 550.

  '78 CB550 engine in a '73 CB500 frame.

[Scott S]

 I guess, at this point, the carbs will come off, but I have no idea what I would check or change.

 These are rebuilt and set up just like every single previous set. I know a forum member that is running two bikes that were done the same way and isn't having any issues.

 Eliminated the ignition, because I've tried a Pamco, Tytronic and points. The mechanical advance is fine.

 Tried foam and paper filters.
 
 No air leaks. None.

[Scott S]

 627B carbs with correct manifolds. 100% OEM Keihin jets, needles, emulsion tubes, settings, etc.
 New O-rings at the head. Good intake boots with brand new, correct size clamps. 1973 air box, so no EPA stuff in the bottom. The only drain is on the plenum and it has the proper Honda hose on it.
 I tried it without the air filter cover, without a filter, and with a UNI and a paper filter.

[maxheadflow]

If you blip the throttle and let the slides snap shut, does it come back to idle better?

[Deltarider]

Sorry I just removed my previous post which was:
OK, I take it is a CB550K3 or K4 engine in a CB500 frame. Now about the carbs and the intake manifold. Can you inform us with the stamped in carb number which are oldstyle and... what intake manifold, the old style or the one the K3s had? Or is it an F2 engine after all? Below the carb setting data sheet I have compiled. You may have a look at it.

[maxheadflow]

Here is another suggestion.  Put a washer on the advance mechanism to lock it up at idle timing.  Does the problem go away?  Throttle will be sluggish but it should return to idle.

[Deltarider]

Scott, are you sure that, before the synching, you had the correct position to start from?
For some reason p.8 is missing in almost all internet copies of the Shop Manual Honda CB500-550.
The page deals with the carburetor adjustment. Recently somebody on the German forum has clarified what was somewhat confusing in that particular page and left many of us puzzling. Although the text is in German, the modified pic makes clear what Honda ment.

[Scott S]

 Can anyone translate that?

[Deltarider]

Only what is decribed at 2. matters. It says:
2. Adjust the big idle adjuster screw thus, that the lever (as indicated by the bottom red line) is at a distance of 49 +/- 1,5mm from the stay (as indicated by the top red line).
3. (just describes how to connect the 4 adapters for synching)

[Scott S]

If you blip the throttle and let the slides snap shut, does it come back to idle better?

 No. Once it hangs, I can either slip the clutch (if it's in gear) or quickly hit the kill switch (off/back on) and it will drop.

Here is another suggestion.  Put a washer on the advance mechanism to lock it up at idle timing.  Does the problem go away?  Throttle will be sluggish but it should return to idle.

Put a washer where? I'm not sure I follow you.

 I mentioned it before, but the mechanism has been disassembled and cleaned. I hit the shaft with emory cloth, as well as inside the cam, to make sure there was no binding. It also did it with the electronic ignition "cam".
 The weights swing smoothly and freely when I take off the springs. The springs have each had one coil trimmed off and they snap back nicely when assembled.

[Scott S]

Only what is decribed at 2. matters. It says:
2. Adjust the big idle adjuster screw thus, that the lever (as indicated by the bottom red line) is at a distance of 49 +/- 1,5mm from the stay (as indicated by the top red line).
3. (just describes how to connect the 4 adapters for synching)

 So, in other words, the end of the "bell crank" should be 49mm from the end of the stay where the cable end attaches, right?

[maxheadflow]

If you blip the throttle and let the slides snap shut, does it come back to idle better?

 No. Once it hangs, I can either slip the clutch (if it's in gear) or quickly hit the kill switch (off/back on) and it will drop.

Here is another suggestion.  Put a washer on the advance mechanism to lock it up at idle timing.  Does the problem go away?  Throttle will be sluggish but it should return to idle.

Put a washer where? I'm not sure I follow you.

 I mentioned it before, but the mechanism has been disassembled and cleaned. I hit the shaft with emory cloth, as well as inside the cam, to make sure there was no binding. It also did it with the electronic ignition "cam".
 The weights swing smoothly and freely when I take off the springs. The springs have each had one coil trimmed off and they snap back nicely when assembled.

Remove the nut on the end if the crank that covers the advance mechanism. Place a washer big enough to clamp the advance cam against the advance plate so the cam can't move. Do the same test.

Is this the carb you are using?
How did carb sync look when the issue occurred?

[Scott S]

 Those are the same style carbs. I haven't put gauges on it while it was stocking yet. I did vacuum sync the carbs when I first built the bike. Nothing unusual, maybe very minor adjustment to one or two carbs. Synced up evenly.

[Scott S]

Only what is decribed at 2. matters. It says:
2. Adjust the big idle adjuster screw thus, that the lever (as indicated by the bottom red line) is at a distance of 49 +/- 1,5mm from the stay (as indicated by the top red line).
3. (just describes how to connect the 4 adapters for synching)

 Hard to measure it exactly with the carbs in and gas tank on, but I'm at about 46mm. The main idle screw is nearly exactly halfway in its mount on the rack. Meaning, I have equal adjustment in and out.

[HondaMan]

I've had 2 of these in recent years. The first one was a [dying] Dyna S ignition that was making weak spark. It caused the intake tract to be a little bit 'wet' from unburned fuel, and when warmed up it would hang about 2200 RPM until I quick-killed it with the Run-Off switch (off and back on real quick-like), then it would idle until the next time.

The second one was spookier: in the end it turned out to be the too-skinny O-rings on the mainjets, which came in the carb kits. They were letting fuel sneak past them each time the throttle was open enough to start the mainjet circuit (about 3500 RPM or so), after which it would hang at 2000-2500 RPM, wandering up and down. If left long enough, it would finally use up the wetness and the idle would drop (maybe 2 minutes of fast idle time). Since then, this has happened on a CB350 (similar mainjets).

The O-rings in the kits are 1.2x4mm, too skinny: they should be 1.4x3.9mm. This prevents them from blocking fuel: it sneaks past the O-rings, unmetered, and ends up in the venturi when it's not supposed to be there.

[Maraakate]

The later CB550K carbs don't have main jets with o-rings.  They used the needle jet holders.

[Deltarider]

So, in other words, the end of the "bell crank" should be 49mm from the end of the stay where the cable end attaches, right?

Right. This is according to Gruzzel in the German forum. He has published several elaborate videos (Youtube) about the meticulous job he did on his CB500 carbs (below a pic). He concluded that the pic in p.8 of the Shop Manual Honda CB500-550 could not possibly be right and published what he thought is the correct one. He had also experienced a hanging idle. I've seen all his videos. I must say they're the best I have seen so far.

[Scott S]

The later CB550K carbs don't have main jets with o-rings.  They used the needle jet holders.

 These are 627B carbs and use O-rings on the main jets.

[Scott S]

Right. This is according to Gruzzel in the German forum. He has published several elaborate videos (Youtube) about the meticulous job he did on his CB500 carbs (below a pic). He concluded that the pic in p.8 of the Shop Manual Honda CB500-550 could not possibly be right and published what he thought is the correct one. He had also experienced a hanging idle. I've seen all his videos. I must say they're the best I have seen so far.

 Well, since I'm running out of things to check, it looks like the carbs will be coming off just to confirm everything is correct.

 If I'm understanding the "49 m.m." thing, just using the main idle screw will get the 49 m.m. setting, but then I will have to re-set the slides on the bench since to get the proper starting gap.
 I can also double check that the main jets are sealing tight and the float levels are correct.

[BenelliSEI]

Following with interest...... something you responded to ages ago: why don’t you run a push throttle cable? Is that the common denominator on the three bikes.....? They do help to truly close a throttle.

[Scott S]

 I've only had one bike that used it. The main spring is incredibly strong (ever had your finger caught in the linkage/bell crank). It's never been an issue before. I do think I have the push cable and maybe I'll add it when the carbs go back on.

[Little_Phil]

Can you try it with no cable. Opening and closing by pressure with hands.