Topic: CB550k Hanging Idle

[TwoTired]

The slide cutaways face the carb inlet/air filter.  You should be keeping each slide with its mated bore in the carb.

The hole in the slide you are fretting about, is done by the factory.  The hole's purpose is to prevent a vacuum lock impeding the slide movement when the top covers are on and sealed.  I've never noticed that vacuum balance is effected with top covers on or off.
 
These carbs worked fine from the factory.  Imo, you should be focusing on things that are different from factory configuration, like dirt, corrosion, or tampering.

The bowl vent/pressure equalizer openings allow atmospheric pressure into the top of the fuel in the bowls.  It is this pressure that pushes the fuel through the metering jets when the cylinder makes lower pressure in the carb throat.  The "vent" system is coupled to all the carb bowls at the top of the bowl cavity.  However, each of the bowl overflow stand pipes also allow atmo pressure into the bowl cavity, as long as they aren't blocked by overflowing liquid.  Therefore, plugging the top bowl vent tube won't effect your current run problem, provided any of the 4 bowl drain tubes are clear.

That is not to say the top "vent" tube should be plugged.  It should be open and have a tube long enough to discourage dust, dirt and insects from making its way into the carb bowl cavities.  Originally that tube ran back and down to a point between swing arm pivot and back of engine case.

At some point, you should review and correct the air plenum drain hose, and the air filter box drain hoses, as capping them off disables their true purpose.  However, defeating these, will not cause "hanging idle".

[DaveBarbier]

These carbs worked fine from the factory.  Imo, you should be focusing on things that are different from factory configuration, like dirt, corrosion, or tampering.

Oh, I am. I'm not questioning the design of the carbs. Just trying to understand them more.

Therefore, plugging the top bowl vent tube won't effect your current run problem, provided any of the 4 bowl drain tubes are clear.

Darn, I was hoping this was something that was obviously the issue. Nevertheless, I'm going to route it behind the swing arm with the drain tubes.

Thanks for all the help so far, everyone. I have a few things I'll try this weekend and report back.


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1978 Honda CB550K

[Maurice]

Well don't worry there's a reason for the vent being there.

[DaveBarbier]

So there may be progress. I'm not really sure. I put a hose on that air vent and routed it with the bowl drain hoses. I started it this morning and took it for a ride. Smells very rich at idle and little power until half or WOT, I'm sure that doesn't mean anything until the carbs are actually synced, just saying. It even feels out of balance. After the warm up it idles and revs alright. But if I increase the idle with the idle screw, to 1.5k or 2k it starts hanging and fluctuating. But if I bring the idle back down to 1k it's better. But weirdly, if I keep the idle screw low but use the throttle to idle it around 2k (sort of using the throttle as the idle control) it's fine. It's when I use the idle screw then it throws a fit. Not sure if that makes sense.

But either way, it needs a sync. I don't want to use the manometer I made because I've tried to sync it and it's still way off. So I'm waiting to sync with the Sync Pro that's coming on Monday.

My question is, if I can't get a good idle with the idle screw at 2k rpm, is it ok to sync it at 1k for now? What would you guys do if you couldn't get a steady idle at 2k? Just power through it while it's fluctuating or keep it low until you get it close then bring up the idle and finish the sync?

I don't know if uncapping the vent hose helped because what TT said totally makes sense about the drain tubes providing the atmospheric pressure in the interim.

I think it's a combination of many things. Air vent uncapping, IMS and flow levels all messed up and out of sync plus the possibility of my manometer being made wrong.


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1978 Honda CB550K

[TwoTired]

I always sync at the lowest RPM possible.  This is when the slides are at their smallest openings and engine needs most critical for even firing operation.
The intake pulses are at their farthest separation at idle speed, btw.  At higher revs, the pulse frequency makes all the cylinders seem more even.
If it's in sync at lowest RPM it will be a in sync at higher RPMs.

[DaveBarbier]

I always sync at the lowest RPM possible.  This is when the slides are at their smallest openings and engine needs most critical for even firing operation.
The intake pulses are at their farthest separation at idle speed, btw.  At higher revs, the pulse frequency makes all the cylinders seem more even.
If it's in sync at lowest RPM it will be a in sync at higher RPMs.

Oh ok, that makes complete sense. For some reason I remember reading that you should sync at around 2000 rpm.

Thanks TT


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1978 Honda CB550K

[DaveBarbier]

Not great. So I got my manometer yesterday evening actually and I'm out with the bike. Started it up and took it for a little ride to warm up. Seemed alright, same as yesterday. I came home shut it off and started to hook up the motion pro to calibrate the manometer and it started going crazy idle again. Then it died and now it won't start and my battery is dead from me cranking it. So I'm using the kick start and it almost starts when I have the choke on full and a little throttle. Then promptly dies.

Any ideas?


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1978 Honda CB550K

[TwoTired]

Check the gas fuel level in the tank.
Switch to reserve setting.


Didn't you shut off the engine while removing the sync port plugs?  Don't expect the engine to run properly with those plugs removed and not connected to the manometer.

[DaveBarbier]

Check the gas fuel level in the tank.
Switch to reserve setting.

Crap. Thanks.

Didn't you shut off the engine while removing the sync port plugs?  Don't expect the engine to run properly with those plugs removed and not connected to the manometer.

I'm sure you're familiar with the Sync Pro, so it has to be calibrated first before using it. I have it hooked up to the #2 carb and the other carbs vacuum ports are still closed.

But now thanks to your knowledge, or my idiocy regarding the fuel, I just calibrated and am now going to sync.


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1978 Honda CB550K

[DaveBarbier]

Ok so just synced. A lot easier with the motion pro. It nice that when you change one carb, the others don't change. So you can set #1 then tighten it down, set #3 and tighten it down, etc.

Here's the before and after. I was hoping that they'd be more out of whack to start with. The second picture you'll notice that it's a higher level of fluid, that's because I was revving it to about 3k just to take the picture because the fluid was more steady.

Before:


After:


I'm going to test ride it now.


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1978 Honda CB550K

[DaveBarbier]

Out for a ride and it seems more smooth. Can't notice a hanging idle. It's idling around 1000rpm which is good but sometimes it'll die when I come to a stop. If I try to increase he idle with the stop screw it seems like it's going to start hanging again. I need to ride a but more to be sure.

But it does hesitate off the start. That means the idle circuit is too rich, correct? And also it seems not as powerful around half throttle but very powerful at full throttle.

I'd think my next step is to adjust the IMS. What do you guys think?


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1978 Honda CB550K

[TwoTired]

When your idle speed increases during the sync, you are supposed to adjust the idle knob to bring it back down.  I don't understand why the other vacuum levels don't change while the speed changes when adjusting an individual carb's slide position.  They should, imo.  But, if you have all the vacuum levels equal ant any engine RPM, that is the goal.

If the spark plug electrodes are clean, hesitation on throttle advance is often caused by lean idle mix.
You could open the IMS more.  Probably need this with the change from stock mufflers to a lower pressure exhaust.  I wouldn't be surprised if you need 4-5 turns out from lightly seated.  May have to stretch the spring on the screw some to keep it from vibrating out.

Lean mixtures can cause idle to increase.  Check plug deposits. 

When the bike was still stock, the idle mixture screws were turned out first, then turned inward while watching a sensitive tachometer for small speed increases.  Inward turning was stopped just as max RPM was achieved.  The idle knob was reset for proper idle speed and the next carb's IMS was adjusted in the same way.

With the exhaust change, the idle mixtures are now way lean.  So, you have to find the new setting which your exhaust/engine combo demands.

First, find an over rich idle setting that allows good throttle response (one half of remaining twist range, mark your throttle).  Then turn in the screws to find the peak speeds as noted above.  You should have a brand new filter in the filter box, and all the proper hoses connected to the air plenum and the filter box, before embarking on the fine tune process.  In fact, all of the 3k tune up check list should be fresh, before any carb fine tune process.

[DaveBarbier]

When your idle speed increases during the sync, you are supposed to adjust the idle knob to bring it back down.  I don't understand why the other vacuum levels don't change while the speed changes when adjusting an individual carb's slide position. 

In my "before" picture, you can see the levels of each cylinder. Is that, in your opinion, not far off? Or is that out of whack? I didn't really have far to go as far as bringing the three cylinders to where the #2 level was. So I didn't really notice an idle increase or decrease as I was adjusting them. When I said when I adjust one carb and the others don't fluctuate, I just meant that with the manometer I made all the tubes are linked so raising the fluid in one would in turn lower the fluid in the others. The motion pro's fluid levels are segregated from each other. So bringing one fluid level up or down doesn't raise or lower the other levels. That just made it easier to sync for me.

If the spark plug electrodes are clean...

I pulled them out just now and they're all black. Not wet, just very black. I should have put new plugs in for the sync but I forgot. I'll put new one's in and see what happens.

Hesitation on throttle advance is often caused by lean idle mix.

It really smells rich when it's idling. And I see smoke out the tale pipes when I rev it. Not blue oil smoke but just exhaust smoke. Right now the IMS are 2 turns out.

Probably need this with the change from stock mufflers to a lower pressure exhaust.

Right now I have the stock 4-4 chrome mufflers on the bike.

I'll put new plugs in and go for a ride. If the plugs don't fix it then I'll adjust the IMS like you said.

And as far as fine tuning, I'll do the 3k mile tune up too even though all this stuff is fairly new. I did the tune up this winter and really only rode the bike...I'll say a total of 10 hours.


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1978 Honda CB550K

[TwoTired]

I don't know what the graduations on you sync tool signify.  My gauges are calibrated to read in inches of mercury.  Without knowing the scale, it's pretty difficult to know what "significantly off" really means.
Simply raising or lowering the crank speed will change the vacuum level at all carbs.  Therefore, if you change the slide position on one carb it changes the crank speed, and all the carbs change their indicated vacuum levels.  As long as they all behave as the master carb reacts to the crank speed, they are balanced.

The plugs will blacken with choke operations.  But, it matters where they are black on the plugs.

Unless the plugs are fouling, hesitation on throttle advance is usually a lean indication.  But, another clue is how it recovers from the hesitation.  If it burbles back to running it was too rich, if to resumes cleanly like just switched back on, it was too lean.

Plug deposits are complicated by the multiple fuel metering paths and where you predominately place the slides.  The main dominates at 3/4 to WOT.  The slide needles dominate from just off idle to 3/4 position.  So, it is possible to clean the plug at some throttle positions and dirty them in others.

Didn't you say there were holes in the mufflers?  Holes are pressure leaks.  Maybe this was another post I am thinking about.

You do claim "hesitation" when you suddenly whack the throttle full open?

Dirty plugs don't really effect carb sync unless they are bad enough to drop out cylinders.


The idea behind carb tune is to do it once when all the other tune up items are in a known "perfect" state.  At the next tune up, all the things that make it run poorer are corrected, leaving the carbs still tuned in the proper state and unnecessary to attend to them again.

If you have soot built up in the pipes, it will blow out anything loose.  Once the mixture is corrected, this will gradually subside.

[DaveBarbier]

I don't know what the graduations on you sync tool signify.  My gauges are calibrated to read in inches of mercury.  Without knowing the scale, it's pretty difficult to know what "significantly off" really means.

This tool's graduations are just for visual reference. It specifically says that it's not related to any actual measurement. So I see what you're saying. But as I rev it the fluid all stays the same so I'm satisfied.

The plugs will blacken with choke operations.  But, it matters where they are black on the plugs.

This is why it seems it's too rich...I don't have to use the choke to start it. I used to, but not anymore. It is warmer out, but I'd still think I'd need to use the choke for a minute or two.

But, another clue is how it recovers from the hesitation.  If it burbles back to running it was too rich, if to resumes cleanly like just switched back on, it was too lean.

Sorry, I don't really understand burbles back to running vs. resumes cleanly like just switched back on. If I'm starting from a stop and give it throttle (just a little throttle, nothing crazy) it goes but it seems like it stutters. Then it'll suddenly clear up and I'll get launched forwards.

Didn't you say there were holes in the mufflers?  Holes are pressure leaks.  Maybe this was another post I am thinking about.

I do have an old 4-1 header that had a bunch of holes that I got welded up which I plan on installing after I get this bike running well as is. But my stock chrome 4-4's are in pretty great shape.

You do claim "hesitation" when you suddenly whack the throttle full open?

I'll have to check. I wish I knew enough to perform all these kinds of tests and interpret results. I think when I whack to WOT it goes well. It seems to only hesitate from a stop as I give it just a little throttle.


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1978 Honda CB550K

[TwoTired]

These are mechanical slide carbs, meaning they are coupled directly to the twist grip.
When the slides are suddenly opened the vacuum drops which reduces the jet flow while simultaneously allowing more air into the carb throat.  It is normal for it to stumble if the throttle is moved too far.
To achieve acceleration the, idle mix is set purposely richer than needed to simply idle the engine, so that when jet flow reduces and the air availbility greater, there is still enough fuel in the air fuel mix for the cylinders to fire.  Later carb used an accelerator pump, so idle could be set max lean and a squirt of fuel allowed the engine to pick up.  The 550 doesn't have an accelerator pump, so the idle is set richer than optimum.  However, it should not be so rich as to soot the plugs.  It's a fine tune thing as the engine will run with a range of air/fuel ratios (see below).

One thing that WILL cause sooting at idle is a too high fuel level in the carb bowls.  This is related to the mechanical float height setting.  To eliminate this as a question, attach a clear tube to the bottom bowl drain and run the end up toward the top of the carb.  Open the drain screw at the bottom of the bowl and note how high fuel comes up the tube.  It will rise to the same level as inside the carb bowl, and should be 2-3mm below the bowl gasket surface.  (bike level on center stand)

Do all the spark plugs show the same soot deposit pattern?  Not surprised it sputter with plugs sooted like that.  The spark can follow the carbon coating rather than jump the gap whichever it finds easier at the time.  New plugs will fix that temporarily (until they too soot up).

[DaveBarbier]

Ok, clear tube, understood. Will do.

Yes, all plugs are the same. I threw some new plugs in and am going to see what happens tomorrow when I start it up. I see it's only a temporary fix, though.

So far I've set everything to factory settings. Except the IMS for the UNI filter. But seeing that these carb's float levels were set around 17mm by the PO and he had no air filter and open exhaust...not to mention a torn but electrical taped front brake hose, I wonder what else is messed up that I don't know about that's helping to cause these issues.

Thanks so much for the info, tips and help Two Tired.


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1978 Honda CB550K

[DaveBarbier]

Here are some fuel bowl levels. The bike was level and on the center stand.

Right two looked fine and were the same. Left two were too high and were also the same. I only could take pictures of the outside carbs.

Left #1 carb:

Right #4 carb:


So this seems like a bad thing, or maybe good because this could be one of the culprits. Either my float measurements were off or...I don't know. What else could be causing the level to raise higher than it's supposed to? Just to explain my float level adjusting...I have a digital vernier caliper with depth gauge. I angled the carbs so the float was barely touching the valve. I didn't do it upside down. I measured from the gasket surface to the tip of the float—opposite the hinge. It seems hard because the floats are flat so one part of the float has one measurement but the other end has another because the float isn't parallel to the gasket surface.


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1978 Honda CB550K

[TwoTired]

Fuel level too high for at least some of the carbs.  But, that doesn't reconcile with all plugs showing same deposit patterns.

Was the engine running when you checked the levels? I might try that to see if there is a difference, before ripping into the carbs.

For some theory.  The carb have to draw fuel up from the bowl into the carb throat.  The higher the fuel level the easier/faster it is to draw the fuel, the richer the mix.

Works the same way as a straw in your lemonade, btw.

If the measurement was the same for all four floats, then I would wonder about the spring pins in the float valves sticking, weak, or being travel limited.
Are these new?  Were they kept married to their original seats?  I might measure the float valves, tip to tip to see if they were equal among the four.

Did you measure both sides of each float during the height check?  They can be bent askew.  That left carb fuel level is definitely too high.  Must seek cause and correct.

[DaveBarbier]

Was the engine running when you checked the levels?

No, the bike was off. I'll check when it's running too.

Are these new?  Were they kept married to their original seats?  I might measure the float valves, tip to tip to see if they were equal among the four.

Did you measure both sides of each float during the height check?  They can be bent askew.  That left carb fuel level is definitely too high.  Must seek cause and correct.

The floats and valves and springs are all old. Yes, they were kept married to the seats. As a matter of fact, I could only remove the float on the #1 carb. The rest's float hinge pins were too hard to get off. I didn't want to risk breaking the little towers that hold the pin so I left them. But when I looked at the valve that I could remove it looked clean. But I may have spun the valve 180° when installing. I didn't make sure to keep it in the same orientation.

When you say measure tip to tip, you mean hinge side to the other side?

Yes, I made sure to measure both pontoons of the float to make sure they weren't askew. Some were, and I corrected it.

Just to clarify, #1 AND #2 are the same height. I just didn't get a picture of #2. #3 and #4 are good.

So I'll check the level when the engine is running but if it's same then I'll remove the carbs. You'd first look at float height, right? Then move to valve seat/spring? I might just get new valves, seats and springs from Harisuluv to rule that out.


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1978 Honda CB550K

[DaveBarbier]

Took the bike to work today with the new plugs and I didn't notice any hesitation from a stop. I'll pull the plugs later and see what is going on in there. I wonder how long it'll take for them to become sooty.

When I started it this morning I didn't use the choke and it started right up, revved itself to about 3500rpm and sat there for a few minutes as it warmed up. Then it slowly dropped to about 2000rpm and stayed there. I turned the idle screw out and it came down to 1000rpm then rode to work (about 15 minutes). When I got there it still idled at 1000rpm.

I'm still going to check the float levels when I get home with the bike running. It still seems too rich in my opinion. It smells rich and there's smoke out the tail pipes. But like you said that could clear up.


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1978 Honda CB550K

[iron_worker]

If you can cold start without choke ... it's far too rich.

IW

[TwoTired]

No, the bike was off. I'll check when it's running too.

In theory, it should be the same.  I was just trying to rule out an aberration in measurement.

When you say measure tip to tip, you mean hinge side to the other side?

No, I was referring to the float valves themselves.  Wondering if the spring pins still had full travel capability.

So I'll check the level when the engine is running but if it's same then I'll remove the carbs. You'd first look at float height, right?

yes

Then move to valve seat/spring? I might just get new valves, seats and springs from Harisuluv to rule that out.

I don't particularly like changing part based on hope.  But, part substitution is a common troubleshooting method.
I always want to know why.  Maybe that's just me.

If you can cold start without choke ... it's far too rich.

I agree, time to start turning the IMS inward to restrict the pilot circuit flow.  I still think the float levels are too high though.  Correct that and then turn the IMS inward.


For the fast idle start up issue, check the choke linkage adjustment to be sure the fast idle cam isn't interfering with slide position when choke is fully off.

[DaveBarbier]

Thanks both of you. I'll report back.


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1978 Honda CB550K

[DaveBarbier]

Ok, I'm out with the carbs today. Day off. I'm posting a couple videos to get some info across. Kind of hard to explain with text. I just want you guys to look it over and make sure these seems fine and there's nothing that's obvious to you but not to me. If you feel so inclined to watch a couple videos, that would be great.

But first here are some pictures.








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1978 Honda CB550K