Topic: Now that the idle hover is fixed....help me tune the carbs

[Scott S]

 Really, I just want you to confirm or discourage what I THINK I should do next.

 '71 CB500, Mac 4-1, Uni filter in stock airbox.

 I recently set the floats at 22mm (to the best of my ability, anyway!    ). That pretty much stopped the idle hang-up at 2-3K rpm.

 I'm now experiencing what I believe is a rich condition in the low and very early mid-range. The bike starts fine and seems to run better when cold. It even likes a little choke now, which it did not with the floats set too high.

 What I'm experiencing sometimes is the idle staying a little high for a few seconds and then returning to idle (which is set ~1,000-1,100 rpm). If it stays for more than a few seconds, I can blip the throttle and make it return.

 ONLY when it's fully warm, and ONLY a couple of times, has it not returned by blipping the throttle.

 I tried to lean it out with the mixture screws (going OUT on a CB500, with the screws on the air box side), but this doesn't make a very big difference and then it really gets sluggish right off idle. Low end response gets much worse. It feels like nothing happens when I twist the throttle...at least for the first bit of travel.
 Going back in makes the throttle more responsive.

 Top end is fine...pulls hard. Plugs look OK, a little on the rich side, but the porcelain isn't black or fouled.

 All of my carb literature says to go leaner on the pilot (slow) jets or raise the needle clip to lean it out. I've never seen smaller slow jets for these carbs...are they even available?
 I'm 90% sure that the needle clip is in the #4 position...next to last from the pointy end. If I were to raise it, that would put it in the middle (of 5) positions.

 Am I on the right track?  

[Kevin400F]

Did you mean to say you tried to "lean it out" by opening the mixture screws (going "out", turning counter-clockwise)

Since these screws control the amount of air bled into the idle circuit, opening them leans the mixture, closing them richens the mixture.

Further, a old factory Honda "pocket-size" service spec book I have here says the standard needle position is "4th groove from top".  Sounds like yours is in the second groove from the top.

[Scott S]

Did you mean to say you tried to "lean it out" by opening the mixture screws (going "out", turning counter-clockwise)

 Yes, that is what I meant to say. That is what I tried. I will correct the original post.

[Scott S]

Further, a old factory Honda "pocket-size" service spec book I have here says the standard needle position is "4th groove from top".  Sounds like yours is in the second groove from the top.

 I'm 90% sure that is the position they are in. It gets confusing...people count from different directions....but I had the carbs gone through by a local guy and he said they were in the bottom (richest...closest to the point) and he raised them up one notch (the clips.....effectively dropping the needles). They should be in the 4th, or next to richest, position. I did not see this with my own eyes, which is why I'm only 90% certain. I have spoken with him and he confirmed the "4th" position.

  Are we all confused now? 

[Kevin400F]

Sounds like your local carb guy did the right thing regarding the needle clip position.

The Honda spec of "4th groove from the top" would be the "next to richest" position.

[Scott S]

 So what do I need to do about being a little too rich on the bottom end?

[Old Scrambler]

You still have to deal with the mixed original and aftermarket parts. I would raise the needle clip position to be sure that the needles are completely closing the main jets.

[Scott S]

 I was afraid of that. MAN, I don't want to pull the carbs again.

 I wonder if getting genuine Honda #100 mains to go with the Honda needles would help? I can change them on the bike.

[Kevin400F]

From your first post, your mentioned it isn't running as well as you would like in the low and very early mid-range, but you also said it gets much worse if you lean out the mixture using the idle air screws.  How far open (from seated) are your idle mixture screws now?  Maybe it is already too lean, and closing the screws further would help.  Factory setting per a very early original copy of the service manual is 1 turn, +/- 1/8.  Later references, and the pocket specification booklet, says 2 turns.  Try 1-1/2 turns as a baseline.



 

[TwoTired]

You still have to deal with the mixed original and aftermarket parts. I would raise the needle clip position to be sure that the needles are completely closing the main jets.

The slide needles have their own jet and never completely close the needle jet.
The Main jet is physically located far from the slide needle tip.
The slide needle never touches either of these jets.

I'll not post this again for you Scott S after this last time:
The carbs on these bikes MUST idle rich as they have no accelerator pump for fuel enrichment when the slides are opened suddenly.
This is why the engine stumbles when you lean the pilot mixture.  The more you open the pilot screw, the worse it will get.  Turn the pilot screw in till you can get smooth acceleration from a throttle twist of up to 1/2 total travel and declare victory.

From your description,  I don't understand what you think is wrong with the bike.  Sounds normal, to me.

Good luck!

[Scott S]

 Idle hanging up and dropping when you blip the throttle is normal for these bikes? Idle hanging up when warm and NOT dropping is normal? Stumble and hesitation just off idle/below ~3,000 is normal?

 Maybe I'll just sell the damn thing and stick with XS650's then.

 The idle mixture screws are at 1 1/4 turns out. If I go out (counterclockwise) further, it gets soggier.
 

 Maybe I'll take TwoTired's recommendation and keep going IN on the screws and see what happens.

 Sorry, I'm just frustrated and feel like it could be better. Maybe if there were a local SOHC guru who could ride it and tell me if it's OK or not, I'd feel better about it.

 (BTW, I filled up today and only got 28 mpg. Granted, that's with some tuning and a few red-line blasts, but I still feel like it should get better than that. I could get 50mpg easy on the XS.)

[Deltarider]

The idle mixture screws are at 1 1/4 turns out.

Why? The book says 1⁺/_- ¹/₈. Don't you think 1 turn out should be a good starting point?
I know some schedules mention 1¹/₂. I've pointed out several times before this information is corrupt. No one seems to be responsible and feel he should correct this.
What source tells you needles should be in 4^th position? If it derives from p. 165 of the Honda Shop Manual, you should know that data presented there are also corrupt. That Carburetor setting table means to compare CB550F to earlier 550(K) models but unfortunately writes CB500. That's a mistake, it should be 550. The 022A carbs mentioned there were found only on 550K models.
The 500 came with 627B or 649A carbs.

On my 500 (649Acarbs) needles originally were in the middle (3^rd) of five possible positions. Genuine Honda parts.

[Scott S]

 I started at one turn and have been experimenting with going out. There's not much difference in performance between 1 and 1.25 turns.

 A well respected forum member told me the 4th position was correct. I have been unable to find ANY literature that tells me ANY clip position for the CB500's 627B carbs.

 If you think that putting the clip on the 3rd position will help, I will certainly try it. Would that make the mixture screws act more like they should?

[Deltarider]

I started at one turn and have been experimenting with going out. There's not much difference in performance between 1 and 1.25 turns.

Not or very little whilst idling. Under acceleration might be different. I agree from experience with what TT said on the subject.

A well respected forum member told me the 4th position was correct. I have been unable to find ANY literature that tells me ANY clip position for the CB500's 627B carbs.

The Honda Shop Manual is well respected too. Yet a mistake is made on p.165 where Honda ment to compare 550(K) models with the new 550F, but accidentally typed 500.
Nevertheless I've asked this forum before to produce a documented source that prescribes 4^th position and a screwopening of 1¹/₂. I mean, Honda America could have prescribed other settings maybe. So far no reaction and now you yourself have found no evidence, which leads me to the conclusion that forsaid information (4^th position and 1¹/₂ resp.) is wrong (for the 500 that is) and has it's origin in false information Honda has given on p. 165 of the manual.  

 

If you think that putting the clip on the 3rd position will help, I will certainly try it. Would that make the mixture screws act more like they should?

For me is hard to say. All I know that according to the CB500 Parts List your and my needles have the same partnumber and that mine originally were in 3rd position (of five). Stamped number is 272304 by the way.

[Scott S]

 Thanks guys. I'm not trying to be hard headed. I swear.

 I'm a bit of a perfectionist and I just want these things as good as I can get. This is my 1st Honda and I've still got a lot to learn.

 Even my "general" carb info has indicated that the needles are too high. I was hoping for a fix that didn't involve going into the carbs again, but....

 Has anyone ever changed the clip position without pulling the carbs? If I remove the tank, is there enough room to work on them through the tops of the carbs?

[Deltarider]

Could be that for your bike with modified exhaust and/or intake 4^th position is OK. In France like in USA they had the 627B Keihins and they had needles in 3^rd position. This is all I know so far. Maybe somebody has other information, but I can't trust information in schedules on the internet, nor in Haynes', Clymers and what not when even... the Honda manual is not always right.

[Mavryk03]

have you tried syncornizing your carbs yet with a carb syncornizer just a thought

[TwoTired]

Have you over-oiled the UNI foam filter?  A little oil is good.  A lot of oil is NOT better.

If your fuel mileage is bad, and your air filter is free flowing, raising the slide needles (yes, it can be done with carbs mounted on engine) is NOT going to improve MPG.

Are the plugs showing the same deposits across the cylinder bank?

[Scott S]

 To answer a few questions:

 -Yes, I synched the carbs with vacuum gauges

 -No, I don't believe the filter to be over oiled. I've handled it recently and it seems fine. Even a little dry.

 -Haven't done a "proper" plug chop yet. They seem a tad on the rich side, but not horrible. The porcelain is clean/not fouled.

 Here's what I did today:

Went back to 1 on the mixture screws. If you twisted my arm and MADE me choose, I'd say it was a little better, but still hanging. Rode it like that for a bit and then closed them some more....effectively going RICHER, which is contrary to what it SEEMS I should be doing. I'd say I closed them by about the width of the slot in the screw head; I guess you could say they're at 9/10 of a turn from closed.

 Rode the rest of the afternoon and the throttle never hung again. I hate to declare victory, but this is the best it's been so far. Idle is pretty consistent and taking off from a stop is much improved.

 One thing I've been battling is a sticky/notchy clutch cable. I ordered a genuine Honda cable today and I can't help but think that's going to help the off idle progression even more. It's tough battling a carb issue AND a grabby clutch at the same time!

 I'd like to thank ALL of you for your help so far, especially you, TwoTired. Like I said, I'm not declaring victory yet....but it sure looks promising so far.

[Scott S]

 I'm about ready to burn this *%#%#!)* too the ground!

 I think I still have float level and/or needle/seat problems. More info soon....

[gregk]

I'm about ready to burn this *%#%#!)* too the ground!

 I think I still have float level and/or needle/seat problems. More info soon....

Scott,  The float setting in the manual indicates a 22mm space.  Now I have assumed that this is the measurement from the bowl flange to the bottom of the float , when the float tab is just touching the needle. 
Other printed specs claim a measurement of 14.5mm which i again assume is the distance from the bowl flange to the bottom of the float when the needle valve is completely closed.  I have used the 22mm setting on  my 550 but didn't check the "needle closed" measurement. 
I am having a similiar problem with my bike with the idle hovering or in fact climbing from an idle to 3000 or more. 
Can any body clear up this question?

[Scott S]

 I guess my problem is all related to fuel level issue(s). Got home last night and there was a puddle of fuel under the bike. I've been smelling fuel in the garage a little bit, but I have two cars, three motorcycles, lawn mowers, weed eaters, gas cans, etc., in there.
  I left the petcock in the ON position and at least one carb overflowed. This seems to indicate to me that they (possibly) could be overflowing while the bike is running, causing my issues.

 The float levels on the two outside carbs were set on the bike and that seemed to make a difference. I pulled the carbs and set the two inside carbs myself. I didn't re-check the #1 and 4 carbs while I had them off. Mistake on my part. I did check to make sure the floats weren't binding on the shaft. They were not.

 I measured from the gasket surface of the carb to the round part of the float, with the float barely touching the needle valve. Similar to what is shown here:

 http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm

 The only difference is that the CB500 carbs don't have that lip, so it's easy to get right on the gasket surface. I leaned the carbs over slightly. Not vertical, but not fully horizontal.
  I have one holder for the main jets that has a little corrosion around the end, but the o-ring goes well past the corrosion and up into the holder. I don't think that is a problem I will get pics next time they're off.

 What gets me is that the problem is intermittent. The bike ran great Monday afternoon. Yesterday, it didn't want to idle when I first cranked it. Full choke, half choke, no choke...made no difference. The idle problem is different, too. At one stop, it may hang at 2-3K rpm, the next stop, it's fine. The next it will hang a little high but then drop to normal idle.
 Sometimes, blipping the throttle helps. Other times not. Sometimes, using the clutch to "grab" the engine will stop it and return it to idle (I guess it's changing the vacuum on the engine??).

 The carbs are coming off again this morning.

[Scott S]

 Here are some pics of the main jet holder I was talking about. Is it possible that this is part of the problem? I haven't checked float height again, yet.

 This is the #2 carb. Everything looks fine.


 This is the #3 carb. See the corrosion on the jet holder? The o-ring is past that, but could this be an issue?

[Scott S]

 And here's an interesting find!

 I picked up a set of 627B carbs from the local scrapyard yesterday (they were on a CB500 engine). I pulled the bowls off today and immediately noticed that the slow jets are VERY different.
 From left to right: parts carb, carbs that I'm running and aftermarket slow jet.
 The parts jets are marked with a slanted star looking emblem and "40". No marks on the middle jets. Just "40" on the aftermarket jets.


 The jets from the parts carbs are much longer, ~34mm overall with the majority of that being the part that sticks into the bowl. The other two are ~21mm overall. The part that sticks in the bowl on the longer jets is approx. ~10mm longer than the other two.

 Besides that, the holes in them are much larger (I can see it with my naked eye) and there are more of them; four vs. three. Notice, too, that the two smaller jets not only have less and smaller holes, but the way they're laid out is different in respect to each other.

 I have the larger jets soaking in a can of Berryman's. What do you make of this? What difference would the length of the body and the lay out of the holes make? Which ones would you use?

[Deltarider]

Comment on pics.
 

This is the #2 carb. Everything looks fine.

I agree, that is as long as the jet's O-ring seals well.

This is the #3 carb. See the corrosion on the jet holder? The o-ring is past that, but could this be an issue?

Again, as long as the jet's O-ring seals well.
BTW, the tab of your float looks new, as if nobody tampered with it. Keep it like that. To much float adjusting going on in this forum, if you ask me. A tab in original state runs parallel and is unbent. Yours look beautifull. Slow jet is NOT genuine.
You could have a problem with sticking float needles (intermittent maybe). An additive to the fuel can help. I believe you guys use Seafoam for this? But I'm sure there are other additives too.
Pic of slow jets. I would use the one on the left. That's the genuine #40 slow jet.