Topic: CB500 — hanging idle? [solved]

[sammermpc]

 Rebuilt my carbs over the winter and buttoned everything up last week. I had some issues bench-syncing the carbs, but followed Harisluv's advice, and I think they're fairly-well dialed in. I also vacuum synced, and things look good. Checked for vacuum leaks, sticking throttle cable. The Idle Adjustment Screw makes contact and the bike responds to it.

The issue is after moving the throttle, the engine tends to hang between 2-3k. If I force the throttle forward, I can sometimes bring it back down to a steady idle.

The key, actually, is that if I push down on the cap of the #2 slide holder (highlighted in the photo below), the idle immediately drops down to what I've set (and stays there). Just looking for some confirmation on what this means (a sticking slide?) and advice on how to fix it.

[Scott S]

 Did you use stock Keihin/Honda jets and brass or the parts that came in the rebuild kit?

[sammermpc]

I cleaned up the old parts. The brass was in good shape. The only replaced parts were the gaskets. Throttle response seems strong and even across the range, as far as I can tell. There may be some issues (particularly on start), where I blip the throttle and upon releasing, the engine dies. That doesn't last long, though.

A few more details:

• Stock airbox + paper filter
• 4-into-1 exhaust (don't know the brand).
• Idle air adjustment screws at 1.5 turns out.

Throttle springs seems good, and it comes back sharply. Not sure whether cable adjustment can play a role, but I don't think so. One thing that happens, when it's idling really fast, is that I can bring it back down by adjusting the idle adjustment screw — but then, at some point, it will idle too low and the engine will die.

[Deltarider]

One thing that happens, when it's idling really fast, is that I can bring it back down by adjusting the idle adjustment screw...

That's a good thing. BTW in the process of rebuilding your carbs, you didn't separate them from each other did you?

— but then, at some point, it will idle too low and the engine will die.

Could there be an airleak or airleaks? The 30,8 mm O-rings in between the enginehead and manifold are notorious for becoming hard over the years. Try with the airscrews one turn out.

[sammermpc]

BTW in the process of rebuilding your carbs, you didn't separate them from each other did you?

I did separate the carbs from the rack when rebuilding. What should I look out for? The choke butterflies seem to behave properly (and are in-sync). I am less sure of the various stop-screws and such around the throttle linkage.

Could there be an airleak or airleaks? The 30,8 mm O-rings in between the enginehead and manifold are notorious for becoming hard over the years. Try with the airscrews one turn out.

I replaced these when rebuilding the top-end last year — I am not convinced that there is an airleak (I sprayed WD-40, etc., around) — but also because the engine doesn't really sound like it's racing as air leaks in. It's more that it kind've hovers after releasing the throttle...

On my way in to the garage, now so I'll be able to answer real time soon w/ the bike in front of me.

Question re: my photo — is that a red herring? What is happening when I push down on that highlighted nut? I took it for a ride yesterday; behaved nicely. At lights and such, the idle wouldn't come down. I reached under the tank, pressed on that nut with my finger, and the idle would drop to normal. Then it'd be fine till the next light, where I'd rinse & repeat.

[DH]

subscribed. my 550 does too, and nothing Ive tried fixes it. totally at random. Suspecting like you that it's something related to
the throttle lifters in some way. My bike is all stock with new honda throttle cables and 4/4 o.e. pipes.

[sammermpc]

I've got the carbs off at the moment -- any thoughts of what I should check out? Just rechecked float levels, which look good. Took a lot at the slides, and they don't seem to be binding at all, which makes me wonder if it's in the throttle mechanism.

[sammermpc]

Yes -- I just tried that (actuating the carb throttle linkage without the cable) and the rpms hang before coming down. The cable appears to move freely in its housing.

[sammermpc]

It does settle down eventually, but appears to hover around 2.5k before drifting downward.

[OneWheelDrive]

Don't overlook a possible leak between the airbox and collector. That large rubber seal can become unseated and cause issues.

[sammermpc]

Thanks for the thoughts. I'm going to keep track of my observations and what I've tried here.

I tried turning out the air mixture screws two full turns (typically, it seems that people suggest 1 to 1.5 turns). That immediately improved the idle return. I took for a half-hour ride around the city, last night, and as the engine heated up, the problem returned. It seemed slightly different this time, though — and displayed the problem where it idles really high (3K), but when I use the idle adjustment screw to bring it down, it drops and dies (or I need to keep the throttle open to maintain it at around 1K).

A few more notes:

• Also — I did another test for vacuum leaks, and was not able to find any. Both on the engine side of the intakes, and the carb side. Tested with brake cleaner & WD-40 (not together), and noticed no changes in RPM either way.
• The bike starts up very easily (though not sure from absolute cold). When it's warm, it won't start with choke on. This suggests a rich condition, in general? Could be wrong about that. Just collecting notes...From what I've read, hanging idle is caused more easily by lean,
• Pushing the throttle forward seems to "help" it return to regular idle.

I still haven't tried running the carbs without the airbox, to observe the slide return and make sure the slides are seated correctly.

Given that the air adjustment screws had such an effect, in general, what does that tell me about the running conditions? Not sure what the next best testing process is....

[sammermpc]

Yeah, I wasn't trying to fix anything. I just hate sitting at a light in neutral with the engine gunning at 3K! Makes me hurt inside. You're right in that it certainly doesn't do me any good overall...fwiw when I do have the idle set correctly, it is steady.

[JWExperience]

I had the same issue with a different bike and I had to up the jetting. The hanging idle was caused by a lean condition.


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[flybox1]

turn the idle set screw all the way out.
when the throttle snaps closed, can light be seen through the carb throat?
(are the slides all the way closed?)

[sammermpc]

when the throttle snaps closed, can light be seen through the carb throat?
(are the slides all the way closed?)

I'll reconfirm when I re-pull them, but I believe so. Never hurts to check again, though. The more I pull these, the easier it gets! 

I had the same issue with a different bike and I had to up the jetting. The hanging idle was caused by a lean condition.

That's my new latest hunch. What does 'up the jetting' mean? Move the needle up a notch?

[flybox1]

I had the same issue with a different bike and I had to up the jetting. The hanging idle was caused by a lean condition.

That's my new latest hunch. What does 'up the jetting' mean? Move the needle up a notch?

means to increase fuel supplied/increase jet size/lower the needle clip notch, etc....but you'll have to investigate further to determine which one of those to change, and how much.
check the slide issue, first.  then post up a picture of the tips of all your plugs, and test for bowl fuel levels (clear tube test)

[sammermpc]

I've done the clear tube test. It appears that the float valves are functioning correctly, and the bowls maintain a fuel height of 2-3mm beneath the lip of the bowl.

I'll continue working down a list of suspects. This is what's on my TODO list next time I'm in the garage:

• Make sure slides are fully shut w/ idle adjustment screwed out (no light through bodies).
• Put in new plugs, bring bike up to temp, and check (new) plugs, to see if they're lean or what.
• Tinker with pilot screws (currently 2 turns out, which is lean), and see how that changes the lean/rich profile.

Any thing else?

[JWExperience]

By up the jetting I meant to richen the mix with larger jets. Sounded like you have the stock jetting but running a 4-1? That's not a huge change but could lean it a bit which is why I suggested to change jetting.
I had a Yamaha sr500 that did the same exact thing for two different reasons...1. It was low on fuel and starving and secondly it needed to be jetted for more fuel. Once I jetted the bike I only saw the high idle when fuel was getting low and time for a fill up.
Which side of the carbs is the air screw on, engine or intake? That determines which way to turn it to richen the mix.
On my cb750, I turn the idle up a little when it's cold otherwise it will keep dying out. Then the idle will raise on its own as the bike warms up. After riding for 5-10 minutes and up to operating temp my idle doesn't change so I can leave it set. If I don't want to mess with the idle when cold, I have to use the throttle to keep it running until it's warm. This is why calj suggested to leave your idle set because it sounds like your bike may also just be getting up to operating temp. Do you set the idle once up to operating temps and still get the hanging idle?
Your todo list looks pretty good, are timing and valve clearances in spec?


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[juice916]

I have the same bike with the same exact problem. I have been reading on this forum for a bit so i took some advice and put all the original brass back in the carbs and moved the clip on the needle to the 4th groove and it seemed to help a good bit.  The bike is kind of gutless but its my first 500 so i have nothing to compare it to so i can give you any more info. Sorry

[sammermpc]

Just a few updates from tonight's stint with the carbs —

when the throttle snaps closed, can light be seen through the carb throat?
(are the slides all the way closed?)

No. With the idle screw all the way out (not making contact), if I shine a flashlight straight through the carbs, I can see a speck of light on the bottom of each one. Does that mean that I benchsync'd them incorrectly. I also vacuum synced them as well (I understand I'll have to do that again). I thought I followed the correct procedure...

I also ran the carbs at idle for a few minutes — and this is what my plugs looked like. I didn't have new plugs to test, so I'm not sure how meaningful this is, but here they are:



Sorry the pictures aren't that great — but nothing jumps out at me. They do seem lean (except #3, which seems rich...). Here's a bigger version: https://i.imgsafe.org/7ac368d.jpg.


The carbs are 069A carbs (original to the CB500, I believe). The air screws are on the intake side, so I believe I screw in to enrich, and out lean?

About the Idle Adjustment Screw — if I get it up to operating temps, and then set the idle, the idle is fine. It's rock solid at 1000, or 1200, or whatever I set it to. The issue is when I then accelerate, say, to 4k — it kind've "floats" around 2.5k. If I force it down (by deliberately loosening the IAS), I can get it back down, but of course, then it's not actually "adjusted", and the engine dies unless I hold the throttle open. Does that make sense?

Appreciate any thoughts as I work through this! I'm gonna get it right, even if these forums are already filled w/ "hanging idle" threads  .

Your todo list looks pretty good, are timing and valve clearances in spec?

I just adjusted the valves, but I have not done the timing since last season. I did regrease the advancer (and put the points plate back in the same spot), as I saw in some threads that was the issue. No difference. So far, the biggest changes I've seen are in adjusting the Idle Mixture Screws, but nothing I've done has seemed "right."

[harisuluv]

069A carbs are for a 550F, supersport. 

Your symptoms just sound like classic lean hang.  Air screws would have been one of the first things I would have tried.  So for your carbs, turning them in means richer, and turning them out means leaner.

What are they set at right now?

[Deltarider]

On my cb750, I turn the idle up a little when it's cold otherwise it will keep dying out. Then the idle will raise on its own as the bike warms up. After riding for 5-10 minutes and up to operating temp my idle doesn't change so I can leave it set. If I don't want to mess with the idle when cold, I have to use the throttle to keep it running until it's warm.

This is also common practice for many CB500/550 owners. That big idle adjustment screw (aka throttle stop screw) is easy to find and can be turned with gloves on.

069A carbs are for a 550F, supersport. 

Your symptoms just sound like classic lean hang.  Air screws would have been one of the first things I would have tried.  So for your carbs, turning them in means richer, and turning them out means leaner.

Correct. That's why I suggested to experiment with the settings of these airscrews, provided there are no airleaks, ignition timing is OK and spark advancer works as it should(!). BTW, if your 069A carbs are in itself original, there are a few differences to the 627B carbs that are supposed to be on a CB500. 069A carbs (all CB550F models) have needle set 16012-390-004 with needles number 273004. 627B carbs that were on the CB500/K1/K2 had needle set 16012-323-004 with needles 272304. Not only that, needle clip position also differed.
Then there are the airscrews that differ: 16016-323-004 for the 627B carbs and 16016-390-004 for the 069A. Here's the difference: the airscrews on the CB500 were hollow and crossdrilled (see pic below), those on the CB550F were solid and had a different, 18^o taper where CB500 had 12^o). I'm not saying your engine won't run fine with the 069A carbs, but it will take some experimenting. My bet is: experiment with the airscrews setting and control RPM with the Idle Adjustment Screw. And remind that's not the RPM at idle that matters, a good pick up when accelerating is what you want. So in spite of what many think, your goal should not be to look for a highest rpm at idle by turning the airscrews. If you do that, you'll have a hanging idle guaranteed. I'd experiment with airscrew settings (warm engine) at 1, 1¹/₄, 1¹/₂, 1³/₄, 2 turns out and take notes. Than maybe refine with ¹/₈ increments. Again, spark advancer should work correctly.
BTW, I don't know if there are any other differences than this between the various types of carbs, like differently shaped venturi. I take it Harrisluv would know this.

[harisuluv]

Yeah there are a few differences.  The needles are actually different, in addition to the jets, and then I think the air screw is even a different part number.

[jonda500]

Pushing down on that brass nut pushes just carb #3's slide down which is obviously enough to return it to idle speed.

069A carbs have #38 pilot jets, needle clips at #2(second from the top) and #98 main jets - it would be interesting to know what yours have?

550F's have a very restrictive silencer and duct/snorkel air box cover - without these you probably need to move closer to 627B (CB500) settings (#40 pilot's, needles clipped #4, #100 main's).

John

[sammermpc]

Your symptoms just sound like classic lean hang.  Air screws would have been one of the first things I would have tried.  So for your carbs, turning them in means richer, and turning them out means leaner.

What are they set at right now?

When I started tuning, I had them turned to 1.5 out. I had the same hanging symptoms I've described.

I tried turning them in to 1 turn out (and at one point, all the way), and had the same issue, with the idle floating very high.

I then turned them to 2 turns out, and it started behaving well — returning sharply to normal idle when the throttle was released. However, that performance only lasted while the engine was cold. I took it for a ride, and about 10 minutes in, it started showing the same 'hang' at 2.5k or so at that same setting.

Because I have not seen consistent lean/rich results based on the screws, I have been a little confused. Granted, I don't have a clear idea of what those screws actually do.

My bet is: experiment with the airscrews setting and control RPM with the Idle Adjustment Screw. And remind that's not the RPM at idle that matters, a good pick up when accelerating is what you want. So in spite of what many think, your goal should not be to look for a highest rpm at idle by turning the airscrews. If you do that, you'll have a hanging idle guaranteed. I'd experiment with airscrew settings (warm engine) at 1, 11/4, 11/2, 13/4, 2 turns out and take notes. Than maybe refine with 1/8 increments. Again, spark advancer should work correctly.

Ok. I have not done that systematically. I don't think I realized how much of a difference 1/4 turn could make. I've been making much larger (generally .5 or full turn) adjustments.

And — also, an apology after all of the helpful information about the CB550 carbs.

I double-checked, and my bike has 627B (CB500) carbs . I got mixed up when I typed in the numbers. Apologies. But then again, I guess that simplifies the situation.

The pilots are #40, the mains #100. I am not sure how to read where the needles are clipped, but I did not change the setting.

Thanks for all of the ideas. Here are my current next steps (the carbs are currently off, so if I should do anything before the below, let me know!)

• Warm up engine
• Check performance w/ Idle Adjustment Screws at 1, 1 1/4, 1 1/2, 1  3/4, 2 and document results