Topic: What now? Idle "hover"

[Scott S]

 I'm chasing my tail trying to solve an idle "sticking" or "hover" problem on my CB500.
 The bike has been fully tuned up and the carbs have been gone through at least 4 times. Floats set, rebuild kits installed, needle clip position set, linkage checked and straightened, etc. etc.
 New 5.5mm fuel line, new Honda throttle cable. Nothing binding, nothing sticking. Correct slack in cable, etc.

 The ignition has been through as well. New coils, wires, caps and plugs. I have a prototype electronic ignition in it right now, so no points (this happened with points and e.i. ). Advancer assembly checked and cleaned. One coil cut from springs. Timing has been set and is dead steady with the new ignition.

 The bike starts easily and will idle correctly for a short while. If left to idle, the idle will decay and I have to adjust the idle screw to bring it back up. When I do that, the sticking idle problem returns. It's intermittent and moving the throttle handle won't cure it. Sometimes blipping the throttle will, sometimes it won't.

 If you ride the bike and let it engine brake, bringing the revs down to ~2,000rpm and then shift, it doesn't hang up. But if you shift at ~3,000 rpm or above (keeping the bike in it's powerband, it will hang. Not a problem going down the road, but at the next stop light, the idle is at 2-3K rpm.
 When the idle hangs, the linkage stop is on the idle adjustment screw. That tells me that a slide is not sticking or the linkage is not binding.

 I can't find anything wrong with the carbs. My buddy who is building the electronic ignition is an electrical engineer. He rides and has built thousands of ignitions for the Yamaha XS650. He knows his way around the advancer, weights, timing, etc. He swears there's nothing there that's causing it.

 I've been told that carb synch can do this. Here's a pic of the vacuum gauges taken just before I rode the bike to my buddy's house for the ignition upgrade. I've checked synch at least three times and it's spot on.
 What now?

[scottly]

Sounds to me like your idle mixture screws aren't set right. Your carbs may be in sync, in that all the slides are set to the same opening at idle, but your mixture is not right. I'm guessing too rich, from your statement "If left to idle, the idle will decay"

[Scott S]

 I thought the same thing. But if I start backing out the idle mixture screws, the driveability suffers. It gets a part throttle hesitation (which is odd, because it's now at speed) and it makes it hard to leave smoothly from a stop. The bike pulls from a stop and runs in the upper RPM range best when they're at 1 turn out.
 1.5 turns out and I have to slip the clutch and rev the engine to pull away smoothly.

 The rebuild kit was aftermarket and I no longer have the stock idle screws. I'm thinking about ordering a full set of jets in genuine Honda. Mains, slow, idle mixture screws, etc.....

[Scott S]

 Well, the main and slow jets are still available. The idle mixture screw is not. I don't think I still have the genuine Honda mixture screws.
 Now what? Anyone have some parts carbs out there that I can get the mixture screws off of? I think the '74 and '75 CB550's share the same screw.

[scottly]

Try turning the idle screws in, in very small increments, blipping the throttle after each change, one carb at a time. You are looking for a very slight increase in idle speed. As you gradually approach the right mixture, keep lowering the speed screw. It's kind of like tuning a guitar; make a slight change, blip the throttle, and LISTEN. Also, what happens a lot with old carbs, is over the years someone over-tightens the mixture screws, distorting the seats in the soft carb body, which can make the stock setting of x turns out from lightly seated useless, as well as making the adjustment even more sensitive. In these cases you have to truly "tune by ear".

[TwoTired]

A lean idle mixture will cause the idel speed to increase.  I'd try turning the air bleed screws in to maybe 3/4 or so to see if that cures the hang idle.

The loss of exhaust back pressure may be the reason for non-factory setting requirements.

Cheers,

[Scott S]

 Bike is a CB500. I'm supposed to go OUT to lean the mixture and IN to richen it....right?

 I can't blip each carb individually, they're all linked. I'll keep playing with the mixture screws,,,

[KB02]

check for vacuum leaks as well. I'm fighting the same thing on my 750 (850) and that was the one suggestion (from the board) that I haven't checked out yet.

[Cqyqte]

I had a similar decaying idle as well and found it was corrected when I leaned out the mixture screws, but that too caused the major hesitation when trying to roll on the throttle at a stoplight.  I used to just deal with it as a fact of life, until I was tweaking setting looking for more mileage out the fuel tank.

I lowered the carb needles into the jets by moving the clips up one notch, low an behold the hesitation disappeared.  The hesitation was cause by the mixture richening too quickly as the needles rose.  Depending on where your clips are that may be outta the question.  Now have you done a plug chop at wide open throttle?  Tough to do with out running into trouble with the law, but I found that running in 3rd gear up a hill with the throttle cracked wide open was the best way.  You don't get up into the really high speeds and you can hold the wide open throttle longer.  You may find that you're one size of mainjet too big.  Which if you are could do the same for you as moving the clip one up.

[Spanner 1]

Good suggestion on dropping the main jet needle a notch....... engine vacuum at idle may be pulling fuel from the main jet and the idle jet and the motor getting in a 'run-away' increasing rpm mode.

[Scott S]

 I was able to locate the original jets and screws. The first, and easiest, thing I'm going to try is replacing the aftermarket mixture screws with the original Honda screws.
 I've already moved the clip on the needle one notch so far.

 If need be, I'll order some genuine Honda mains and see how that goes. No telling just how "right" the aftermarket jets are. They are marked 100 and measure just over that.
 Stay tuned. My work schedule may not allow me to work on it too much for a few more days.

[Scott S]

 Went to my buddy's house today (he has the bike so he can build the ignition. Replaced the aftermarket idle mixture screws with the Honda screws. Still have the same problem.
 Let me clarify a few things here:

 Idle hover definition  When I say the idle is hovering or sticking, I mean that it goes up, sticks at 2-3K or more and STAYS there. The only way to make it come down is to A) hit the kill switch, or B) lower the idle screw on the linkage until it drops.
 When I lower the idle screw, it will start to decay and die on the center stand. I can set the idle at 1050-1100 and it won't die on the stand (though it does seem to load up and get rich a little bit), but the throttle starts hanging up again. If I set the idle low, I can NOT make it hang up, but it won't stay running.

 Synchronization  The carbs are synched. See pic above. When I give it gas, all four gauges move in unison. When the RPM's hang/stick, all four gauges are still the same.

 Air mixture screws too lean I see what you're saying about having an idle hovering problem when the mixture screws are too far out (too lean). I can reproduce this. The farther out I go, the more the idle hovers BEFORE IT RETURNS TO IDLE. In other words, it will come down to ~1300-1500 and then return to idle. When the throttle is sticking, IT NEVER RETURNS TO IDLE, unless I hit the kill switch or basically starve the engine by backing the idle adjustment screw too far out.
 Again, at this point, it won't hang up, but it won't idle either. The idle is too low; around 500-800 rpm. If I get it to idle between 1000-1100, it will idle, but the sticking starts again.

Vacuum leaks Never say never, but I feel VERY confident that there are no vacuum leaks on this bike. I've sprayed enough carb cleaner and WD40 around this thing for a dozen bikes. Besides, if there's a vacuum leak, it's on all four carbs. See my comment above about how the vacuum gauges react when it sticks. If there was a leak, wouldn't I see one carb running away?

 I'm beginning to swear it's something in the advance. My electronic engineer buddy says he's been through it and doesn't see anything. Advancers for the CB500 are no longer available. I have a used one on the way and he has a CB750 advancer coming, just for comparisons sake.
 It's been suggested that we wire the weights closed and see if it still hangs. My buddy says that will do no good because "of course it won't...because it can't! Won't really prove anything". He seems to think there's something up with the idle circuit.

 Please, keep the suggestions coming.....

[Kevin400F]

When you say the timing is "dead steady", have you put a light on it and watched to verify it stays at the "F" mark the whole time your idle speed is wandering around? 

Second idea, when she "hangs", can you push down on any of the carb linkages and get an effect? Maybe one or more slide is sticking in the carb bore.

When the carbs are at "idle position", does each slide snap closed sharply?
 

[Scott S]

When you say the timing is "dead steady", have you put a light on it and watched to verify it stays at the "F" mark the whole time your idle speed is wandering around? 

Second idea, when she "hangs", can you push down on any of the carb linkages and get an effect? Maybe one or more slide is sticking in the carb bore.

When the carbs are at "idle position", does each slide snap closed sharply?
 

 The timing has been set and checked with a timing light. Again, the speed DOES NOT WANDER. It STICKS. When it sticks, it sticks and stays steady.
 The only time the idle "wanders" is if I back the screw out too much and the idle is below 1000 rpm. It will then slowly decay and drop off until the bike dies. Above 1000 rpm, it will idle until the tank runs dry.

 I have tried pushing down on the linkage, adjustment arms, etc. Checked the throttle cable. When it's sticking, the idle adjustment screw is touching the linkage. If a slid was sticking, wouldn't it be holding the linkage/slides open and creating a gap at the idle adjustment screw? It is not.

 Carbs snap closed sharply and positively from any throttle position.

[Kevin400F]

By wandering around, I was referring to the scenario where you said, "the more the idle hovers BEFORE IT RETURNS TO IDLE. In other words, it will come down to ~1300-1500 and then return to idle."  

I was wanting to know if you had a timing light on it when it came down to 1300-1500 and then returned to idle.  If, in fact, the advancer was acting up, you might see the timing shifting, driving an idle speed change.  

Ok, here's another dumb question---did you reverse the slides between #1 and #2 carb, and between #3 and #4?  Is the big cutaway on the slide on the engine side, or the airbox side?

[TwoTired]

My WAG is that your bent actuating arm isn't really fixed and there is still some binding or some alignment issues preventing full travel ease.

Cheers,

[Scott S]

 Kevin, you're right...I did say that. But that only happens when I back the air mixture screws out too far.
 I did not have a timing light on it when it was dropping. Something to experiment with....

 What would reversing the slides between 1 and 2 and between 3 and 4 do? What would that tell me? Are the slides different?
  I'm going on memory here, because the carbs are on the bike with the airbox on it, but the cutaway is on the engine side.

[Scott S]

My WAG is that your bent actuating arm isn't really fixed and there is still some binding or some alignment issues preventing full travel ease.

Cheers,

 I wish you were right...just so I could find it and fix it. But again, let's say that an arm is bent, the linkage is sticking, a slide is sticking, etc.
 Wouldn't I see the idle stop screw OFF of it's stop when it's sticking? Would that cause all four slides to stick open DEAD EVENLY, which is what I show on the vacuum gauges when it happens?

[Kevin400F]

I'm going on memory here, because the carbs are on the bike with the airbox on it, but the cutaway is on the engine side.

Cutaway on the engine side = BIG TROUBLE!  If you have a good memory, you have a problem!

[TwoTired]

My WAG is that your bent actuating arm isn't really fixed and there is still some binding or some alignment issues preventing full travel ease.

Cheers,

 I wish you were right...just so I could find it and fix it. But again, let's say that an arm is bent, the linkage is sticking, a slide is sticking, etc.
 Wouldn't I see the idle stop screw OFF of it's stop when it's sticking?

The RPM difference you are noting is a very miniscule change in slide position.
Go ahead and move the throttle just enough to move the stop off the screw tip, or put a .010 feeler in there.  How much extra RPM do you get?

Actually, see if you can slip ANY feeler gauge in the stop when you have the throttle hang.

Would that cause all four slides to stick open DEAD EVENLY, which is what I show on the vacuum gauges when it happens?

They are all linked together, hold one open and all can be held open a smidge.  And the difference in idle you are noting is only a smidge in slide position.


Is the throttle return spring the strong stock one?  Or, has it been replaced with a weaker spring?  Have you backed off the throttle friction adjust?

Cheers,

[TwoTired]

I'm going on memory here, because the carbs are on the bike with the airbox on it, but the cutaway is on the engine side.

Cutaway on the engine side = BIG TROUBLE!  If you have a good memory, you have a problem!

+1 !

[Spanner 1]

With the cutaway ' on the engine side' all bets are off and the engine vacuum will lift the slides/ pull from the main jets....... sorry but sounds like your slides are indeed in backwards.

[Scott S]

I'm going on memory here, because the carbs are on the bike with the airbox on it, but the cutaway is on the engine side.

Cutaway on the engine side = BIG TROUBLE!  If you have a good memory, you have a problem!

+1 !

 No, no, no....My mistake. I looked at the pictures I took while the carbs were off the bike. There's not much of a cut away on these carbs, but it's on the air box side. See the pic.

[Scott S]

 And here is the engine side.

[Scott S]

 Some better pics. After they were cleaned and rebuilt. These are my actual carbs.