Mutually exclusive carburetor issues

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kingram
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Mutually exclusive carburetor issues

Postby kingram » Wed Aug 05, 2020 7:21 pm

Hello fellow enthusiasts,

Glad to be here - long time reader, first time writer. I know there are multiple threads with similar issues, however I've never seen a definitive answer in any thread for what the problem may be. Let's get down to it:

I am having two intermittent issue with my carburetor that I believe are mutually exclusive, and, therefore, the same root cause is causing BOTH problems. They are as follows:

Issue 1) The bike idles excellent, starts good, has good acceleration, and overall, excellent abilities around town. However, when taken on the highway (or anywhere else) requiring RPMs over 6,500, it bogs down and won't go higher. The higher the gear the lower the rpm threshold: 2nd it'll go up to 6k, 3rd it will go up to 5.5k, 4th up to 5k, etc..

Issue 2) The bike idles steady and smooth at 4,000 RPM once it's warm, and won't go lower. It has good acceleration and overall good performance around town, however, when I come to a red light or stop sign and take the load off the engine, it revs up and holds at 4,000 RPM. The idle screw is ineffective when having this issue; I can turn the idle screw all the way in, or all the way out without affecting the bike maintaining 4k RPM while in Neutral. ALSO, Issue #1 is no longer applicable when the bike is having this issue. Which means I can now rev up to 10,000 RPM in all gears with no issues.

In summary, the bike has either Issue #1, OR, Issue #2. They never happen at the same time.

In all situations:
- There is very little or no black or white smoke coming out of the exhaust, even when starting cold.
- The two issues trade off - they have never happened at the same time (which makes me think they are the same issue)
- I don't know what the catalyst is that causes it to go from one issue to another (I suspect going over speed bumps could have been a contributing factor on my last ride)
- The ignition timing is good. Timing advancer is clean, rust free and working well
- New spark plugs, new spark cables, new battery with good voltage and amps.
- Air filter is new (standard K&N filter and housing, not pods)
- Throttle cables appear to functioning properly and the carb spring "snaps" back after every throttle twist
- Spark plugs are all the same after being replaced - slightly black from running rough
- The bike sat for a year before I started working on it - carbs were drained during this time
- I have dismantled and completely cleaned the carbs and installed an inline fuel filter with no changes in performance/issues

With the above information I believe that the issue is one of three things:
1) There's dirt in there, somewhere.
2) It's a vacuum line issue not providing enough air and/or partially clogged/ something biding in there
3) There's a binding in the spring that's causing the throttle to stay partially open, despite the linkage snapping shut

Any feedback on what the root cause of these issues could be and how I can address would be much appreciated! I await your wisdom, fellow contributors!

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kingram
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Re: Mutually exclusive carburetor issues

Postby kingram » Wed Aug 05, 2020 7:23 pm

I should also note that the bike is a 1981 Honda CB 650 with Keihen VB44A carbs.

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Volker_P
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Re: Mutually exclusive carburetor issues

Postby Volker_P » Thu Aug 06, 2020 1:55 am

Welcome here :D

I would suppose your bike does not run permanently on all four.
As you occasionally have full power I suspect a mechanical issue at your carbs.

First check for an air leak between engine and carbs. Not very probable but quick and easy to do with the carbs still installed. And it needs to be excluded to avoid getting crazy when you find it after you tear apart everything else. :roll:

As your black idle knob screw has no effect check your throttle spring. It should feel quite strong. Some people hang it to a weaker position. It might be too weak to close the throttle completely. Your statement sounds a bit like that:
3) There's a binding in the spring that's causing the throttle to stay partially open, despite the linkage snapping shut

Make sure the throttle cam hits the end of the idle knob thread. No way to reasonable idle adjust without that.
If the throttle spring closes the throttle as it should be, probably your carbs are heavily out of sync. Possibly some adjustment screw got loose.

Make sure your choke is all right. The choke cam is two halves connected by a spring mechanism. If one side should be loose and move randomly when you pass a speed bump, issues just as you report could happen.

If your carbs are out of sync, start with a visual syncronization: all four throttle flaps should open simultaneously to a tiny gap when you turn in the idle knob from completely closed. Vacuum syncronization is fine tuning and may be done not before everything else is all right for sure.
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kingram
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Re: Mutually exclusive carburetor issues

Postby kingram » Thu Aug 06, 2020 6:33 am

Thanks for the quick response, Volker_P!

I've recently replaced the carb boots with brand new ones from Honda (or OEM replacements). Additionally, I've misted water around the carbs while it's running trying to get it to bog and determine where/if there's a leak. No luck. Or, the water is contacting the carbs before it can make it to the linkage where a potential leak could be, buried between carbs..

The throttle spring is very tight. It hurts my hand to hold it open any longer than 20 seconds. Additionally, I have performed a static check on the throttle flaps. I used the base of a drill bit to prop one of the carbs open, and checked the gap in the remaining 3 carbs. The spacing is within 1/32nd of an inch.

I haven't been able to check the bike thoroughly while it's running at idle at 4k, as I'm worried it is going to overhead and/or irritate my neighbors. I'll dive into this more tonight.

The choke appears to work excellently. I use it to start the bike, push it back in and it operates as expected. Further, I have tried pulling the choke when the bike is having "Issue 1" where it won't rev over 6,500, driving at 80km/h. It makes the situation worse. Which I believe indicates that my issue stems from a lack of air, not a lack of fuel?

When I take the carbs out again (assuming that's necessary) I'll check to make sure no choke or throttle flaps are lose, however, everything was tight when I put them back on last week.

It should also be noted that I am not running the vent lines between banks 1&2 and 3&4. The rubber tees dry rotted and it's impossible to find replacements. From what I've read, most guys aren't running those anyways. Could that be an issue?

Thanks again!
kingram :D

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Volker_P
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Re: Mutually exclusive carburetor issues

Postby Volker_P » Fri Aug 07, 2020 2:29 am

So throttle spring and carb boots seem no problem.
My assumption is that your "issue1" means it does not run on all four. During "issue1" you should have one or two headers that are significantly colder. Worth to check. A CB650 usually runs quite fine up to 60mph on three cylinders, it even needs some experience to recognize it during riding. So I would guess that probably only two cylinders really fire permanently at full power.

"issue2" means that now the bike runs on all four as it should. Of course then the idle setting from "issue1" is too high which explains your 4k idle.
If turning out the knob does not reduce rpms, something blocks the flaps from closing completely or you have some air bypass/leak.
One closed flap with others still open might be a reason, too. Therefore the suggestion to do a visual synchronization. It is difficult to measure the tiny gap with reasonable accuracy at home.
I used the base of a drill bit to prop one of the carbs open, and checked the gap in the remaining 3 carbs.

Really hope that you did that without the load of the strong throttle spring on the throttle flap with the drill.

I cannot really help with the T-vents as I do not have own experience with the CV carbs. They seem to be a vent to the carb bowls and might impact pressure conditions there and possibly further internal pressure conditions inside the carbs that have further consequences on the movement of the pistons.
Some report severe problems with cracked T's others seem to have no issues. Possibly it depends on the movability of the carb pistons, but this is just a guess. But if you have issues and everything else with your carbs is fine, you probably will need them, too.
Some people used aquarium equipment to avoid paying Honda prices.
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kingram
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Re: Mutually exclusive carburetor issues

Postby kingram » Sun Aug 09, 2020 11:34 am

Further troubleshooting:

I believe issue #1 was something suck in a jet. I don't seem to be having this issue anymore. But, you were right. I don't believe it was running on all 4 cylinders before. Now, after cleaning, it is running on all 4 - (checked all spark plugs are brown-ish. #2 is slightly darker than the rest). Just issue #2 remaining.

I took the gas tank off and sprayed water all around the carbs and connections, top and bottom, and in-between, attempting to get it to bog down. Not a blip.

I inspected the throttle cables and choke cable again. They don't appear to be binding and I can physically see the cables tighten and slacken/ hit the stops as i move the throttle. Moving the handlebars side to side doesn't do anything either, which would indicate stuck/tight cables.

The idle screw works excellent when the bike is cold. When i first start it i can manipulate the idle from 1,400 to higher as needed.I can't seem to get it to idle lower than 1,400 though...

Even when the bike is just sitting at idle in the driveway, once it's warm it slowly creeps up to 4,000 RPM and the idle adjustment doesn't work anymore. It seemed to temporarily go away when I had the gas tank off and was troubleshooting. Could this be a float/flow issue?

I have also played around with removing the air filter cover and using penny washers to hold the K&N filter on. It doesn't make a difference in performance or idle.

Grr... I must be missing something super simple...
kingram :D

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GeorgeSweety
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Re: Mutually exclusive carburetor issues

Postby GeorgeSweety » Sun Aug 09, 2020 6:25 pm

Hi Kingram, I had a similar problem with my CB650 Custom when I first bought it. The problem turned out to be the throttle cables were from a CB650Z and were a couple of inches shorter than the Custom cables, the handlebars on the Custom are a fair bit higher than on the CB650Z, it might be worth a look?

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Volker_P
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Re: Mutually exclusive carburetor issues

Postby Volker_P » Tue Aug 11, 2020 1:01 am

kingram wrote:Further troubleshooting:

The idle screw works excellent when the bike is cold. When i first start it i can manipulate the idle from 1,400 to higher as needed.I can't seem to get it to idle lower than 1,400 though...

The engine should stop if you turn out the black idle knob completely (when choke is off) in any case.
Otherwise one of the throttle flaps or something else holds the throttle open a bit or the throttle flaps are bent or damaged or bolts are missing.
This configuration seems anyway quite all right by chance as long as the engine is cold. Are all four headers equally hot then?
However at operating temperature the same throttle gap is good for 4000rpm.
This is what I would conclude from your descriptions up to now.
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kingram
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Re: Mutually exclusive carburetor issues

Postby kingram » Tue Aug 11, 2020 6:22 am

All headers were equally hot. I was having issues with #2 cylinder again (going back to issue #1), however, I believe that is related to more gunk/rust/dirt in the carbs that I never got when cleaning last time as I was also having issues going over 100km/h again.

Another interesting tid-bit: When the bike is hot and running normally (still idling at 4,000 RPM), the choke makes the RPMs go higher. Shouldn't the choke kill the bike when used after it's warm?

I'll be taking the carbs apart on Thursday to do more troubleshooting!
kingram :D

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Re: Mutually exclusive carburetor issues

Postby Volker_P » Wed Aug 12, 2020 1:03 am

kingram wrote:All headers were equally hot.

At 1600 rpm "cold" idle this would mean it runs on all four from the beginning. Was choke still pulled then?

kingram wrote: I was having issues with #2 cylinder again (going back to issue #1), however, I believe that is related to more gunk/rust/dirt in the carbs that I never got when cleaning last time as I was also having issues going over 100km/h again.

You have a clean tank?

kingram wrote: Another interesting tid-bit: When the bike is hot and running normally (still idling at 4,000 RPM), the choke makes the RPMs go higher. Shouldn't the choke kill the bike when used after it's warm?

Depends: If you pull the choke while riding this should happen. Not that sure for idle as the choke also opens the throttle a bit, may depend on throttle lift lever settings. Have a look, the choke should start to lift/open the throttle not before it is pulled out already quite far.
And in your case it also might be a hint to strange pressure conditions in the carbs possibly due to missing T-vents and hoses, maybe in combination with residual gunk.
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kingram
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Re: Mutually exclusive carburetor issues

Postby kingram » Wed Aug 26, 2020 8:36 am

Issue resolved! There were two issues:

1) The carbs were wildly out of sync. I don't have a balance tool. The last person I paid to do it never did it right, apparently.
2) The K&N air filter won't let enough air through with the cover on for me to get over 6k in 5th gear (>110 km/h)

The carbs are now balanced and the bike idles/runs perfectly.

I took the air box cover off and installed washers around the filter to keep the cover on (see attached pictures). Runs flawless now. Plugs are golden brown. This issue has been studied and determined that at lower RPM ranges (3500) there is a slight air flow restriction with K&N filters. This would be compounded with carbureted engines running at 6k>. Check out the paper by Googling: "Effect of OEM Style and Aftermarket Performance Air Filters on Vehicle Parameters". This is my guess for what's wrong.

For all those future Honda CB650 carb troubleshooters, my lessons learned for dealing with idle/carburetor issues are:

1) Check if all headers are getting hot equally (is it running on all 4 cylinders)
2) Take the carbs off and clean very thoroughly, top and bottom. Make sure everything moves as it should
3) Make sure there are no air leaks - spray water/car cleaner around the carb boots (connecting carb to engine) while it's running to see if it bogs/idles up, respectively
4) Check the plugs to see if they're running too rich, too lean, uneven, not getting spark from center of plug, etc..
5) Check timing and advancer
6) My bike doesn't have the t-hose splitters between carbs that run into the air box, and it doesn't impact performance - the carbs need to be balanced in this configuration for them to be accurate
7) After the carbs are cleaned, make sure the tank is clean before you install it again. Install a good inline fuel filter for added security
8) Make sure throttle cables are snapping back and there's a bit of play when sitting at idle
9) Check main idle jets - Hayes manual calls for 2-1/4 turns out on my 1981 CB650 with CV carbs- this changes with pods/after market filters
10) Make sure the idle screw adjustment is manipulating the carbs evenly and they have a firm spring back to closed position

There's my wisdom. The best of luck to all those who read this far!

Thanks Volker_P and all those who posted on this thread, and others. You all make troubleshooting these beautiful old bikes much more approachable. These forums are invaluable.

Until the next issue, gentlemen...

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kingram :D

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Volker_P
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Re: Mutually exclusive carburetor issues

Postby Volker_P » Fri Aug 28, 2020 1:09 am

Thanks for the final report and the check list. I think I will include it in my technical internal link list.

From what I heard K&N filters seem typically less restrictive. It might be different in this case.
Anyway It would be interesting to see your bike running with a stock air filter.
Because I suspect that you presently compensate for the changed pressure conditions in your carbs due to the missing tees and hoses by leaving away the filter cover. This might explain why some people have trouble without the tees and others have not.

Anyway if it runs fine, plugs look all right and mpg is all right, too, nothing to worry.
Have fun! :)
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