Topic: CB550k Hanging Idle

[DaveBarbier]

Regarding the needle position, the FAQ states 2nd groove for the PD46C. So I take it that means 2nd from the bottom?

Needle clips are specified and referenced from the top of the needle.

I guess the FAQ entry is old.  Honda specifies 3rd groove.

I assume that since the '78's needle position isn't mentioned then it's the same as the '77. But ok great, thanks. I'll set them all to factory. 

I'll mention that if the IMS carb body orifice hole is plugged with a broken off needle, putting a good IMS in there will bend its tip.

I'd imagine so, haha. It'll be clean and clear and under control.


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1978 Honda CB550K

[DaveBarbier]

Ok so today I put the IMS to two turns out from closed and raised the needle one notch. So it's the third from the top (center notch). Just to be sure, the slide valves have a raised portion on the bottom. That faces the airbox, right? I also REALLY checked for air leaks and found nothing. I got a new can of carb cleaner and really sprayed in all connections around all carbs and intake manifold... even on the IMS in case air was leaking in through them. Nothing. I'm certain I have no air leaks now.

I was going to check the compression but I didn't have the right adapter. 12mm, right? I think I saw that TT said the D in D7EA means 12mm. But I warmed the bike up before this and it seemed to idle a lot better. I tried to make it idle high but failed. And I noticed very few fluctuations, if any. But it seemed to run rather rich. Smelled of fuel and smoky tail pipes. I'll check compression when I get the adapter but tomorrow I'm going to try and sync them again and see how that goes.


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1978 Honda CB550K

[DaveBarbier]

I'm out...again...trying to sync these carbs. I can't check the compression until early next week, nor do I have the unbent IMS replacement so maybe this is all for nothing but I'm trying anyway. The idle is still racing up and down and not returning when I let go of the throttle. Right now it's impossible to sync then because it varies on all RPMs. 2k rpm the #2 carb is jumping up on my manometer, but around 3k it drops significantly. Should I just wait until I get the new IMS and check compression before trying this?

I'm also questioning the manometer I made. All the connections are air tight. But I did make little restrictors to calm the fluid. They're just little solid plastic inserts that I poked holes into. What if the holes aren't exactly the same size? But they're suuuper close.

Thanks for the help guys. I really appreciate it.


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1978 Honda CB550K

[DaveBarbier]

It can be difficult to determine compression by feel, it may be normal, but its best to know for sure when troubleshooting, so I would recommend buying or borrowing a gauge to check.  Uneven or low compression will affect carburetor function and efficiency, so its a good parameter to know.

Ok so I finally got a gauge and checked the compression just now. I warmed up the bike then pulled the plugs out. No choke and full throttle. Made the engine turn over about 6-8 times per cylinder.

#1: 140psi
#2: 125psi
#3: 130psi
#4: 130psi

The Clymer manual doesn't have a psi rating but says there shouldn't be less than a 10% difference. If my math is right then this is slightly less than spec. 140-10%=126

Do you think this could be contributing to my problem or is this a nominal difference?


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1978 Honda CB550K

[DaveBarbier]

Just tried to sync them again after the compression test. I got my new IMS the other day and installed it. First I checked out the hole that it goes into and it seems completely normal to my eye.

Also, and I'm stupid, but I forgot that the airbox and plastic plenum has little orifices  that usually have tubes connected to. The bike never came with any and I got little rubber caps to seal them up. I was thinking that would definitely be the vacuum leak but no...didn't notice a difference while syncing. It's raining now so I can't do much testing but my next idea is to go from 2 turns out to 2.5 turns out as Two Tired suggested.

Please help!!! Am I forgetting anything obvious? Am I going too fast and not thoroughly testing? Can I fly one of you guys out to southern Connecticut!? (Joke...I have no money.)


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1978 Honda CB550K

[Lostboy Steve]

Are you sure you dont have a bad carb? I had a very similar issue and it was from the PO trying to force open the carb with a screw driver. Once I rebuilt it, the slide still had some gouges as did the venturi which ruined my idle. I wound up buying another seized set on ebay, and doing the work to clean each one (after having just done that with the bad set) and never looked back.

[DaveBarbier]

Are you sure you dont have a bad carb? I had a very similar issue and it was from the PO trying to force open the carb with a screw driver. Once I rebuilt it, the slide still had some gouges as did the venturi which ruined my idle. I wound up buying another seized set on ebay, and doing the work to clean each one (after having just done that with the bad set) and never looked back.

I'm pretty sure I don't. When I had them apart I didn't notice any real damage except for the IMS being bent and one main jet flat head slot being a little stripped. Slides work well and are nice and clean. The cylinders where the slides travel are pretty clean too.

And the bike worked fine about a month ago...maybe 3 weeks. So it seems to me that something changed since then.

This is really strange. And I just bought a real Motion Pro Syncpro off EBay, new for $71 free shipping. I want to be absolutely sure my manometer I made isn't giving me a false reading.

Thanks


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1978 Honda CB550K

[TwoTired]

#1: 140psi
#2: 125psi
#3: 130psi
#4: 130psi

Do you think this could be contributing to my problem or is this a nominal difference?

Probably not a problem, particularly if the bike hasn't been in normal operation for a while.

[DaveBarbier]

#1: 140psi
#2: 125psi
#3: 130psi
#4: 130psi

Do you think this could be contributing to my problem or is this a nominal difference?

Probably not a problem, particularly if the bike hasn't been in normal operation for a while.

Ok good to hear.

Any other ideas? From reading around this problem is usually related to vacuum leaks or carbs out of sync as you mentioned.

I really can't find any leaks but I don't know if I have tried all areas. I tried all the rubber boots and seams between head and intake manifold and the exposed IMS before and after I installed the new one from Harisuluv.

I just bought a new Motion Pro manometer, as I stated earlier, which should come in next week. Just in case my manometer has issues or the restrictors I made don't work well, i.e some holes larger than others, which might throw off results (that's my thinking at least). I think it's fine but I want to be sure.

I take Occam's razor seriously so I'm not dismissing these two things.


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1978 Honda CB550K

[TwoTired]

Compression good, and stable spark, pretty much leaves carburetion as the issue.

Lean mixtures or carb imbalance causing the cylinders to fight domination of the crank speed.

Would help to know if some head pipes are cooler than others.

[DaveBarbier]

Would help to know if some head pipes are cooler than others.

If I touch them with a wet finger they sizzle just the same. Do you need accuracy like with an infrared thermometer? Or is the touch test fine? I did note that the muffler on the #4 was hotter than the others (it'll burn your finger even at the very tip) I'd imagine then that the #4 header is hotter than the others then, too. But none are cold, cool or warm. All hot.

Maybe it'll help, maybe not, but this started happening before I opened the carbs. It was a Thursday and it seemed to manifest itself after the second highway ride I've ever done.


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1978 Honda CB550K

[TwoTired]

Modern engines use exhaust gas temperatures to adjust mixtures through computer control.  As you have four independent carbs, each making mixture in an independent fashion, know the exhaust gas temperature would aid in determining which carb isn't making as happy a mixture than the others.

I'm just looking for clues regarding how to help you.  Lack of information deters that.

Maybe a good carb sync is all you need.  Maybe not.  But, nailing down things that are known good one by one isolates the variable which is causing issues.

[DaveBarbier]

And I appreciate it all vastly. This is an incredible forum.

Today before work I'm going to try and test for more air leaks.

Yesterday I noticed that now when I turn the fuel on sometimes one overflow tube starts pouring fuel. But I rap on the bowl with a screwdriver handle or snap the throttle back a couple times and it's fine. A sticky float. I'm going to drop the bowls and see if I see anything out of the ordinary.

Now a question regarding slide orientation. There is nothing preventing me from putting the slides in 180° from the correct orientation. But I've been aiming the raised part of the bottom of the slide towards the airbox. I noticed if I have the slides out and look down them that there are three holes. Two for screwing the arm plate to and one unused. That's a hole that goes directly into the carb throat where all the magic happens. If I have the covers off for a sync do those holes not screw results? It looks like a hole so the slides can move up and down without making pressure in the top chamber. Anything abnormal going on here? I'm using the old gaskets for the top caps but they seem in perfect condition.


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1978 Honda CB550K

[Maurice]

Top caps don't matter when sync'ing, as long as you take them all off.

Have you run the bike without any airbox/plenum at all, and looked at slides operation? Wear eye protection. They have to close almost all the way, no snag, smooth operation. If it's a lean condition it would also improve worsen it a bit.

IME a bench synch' is good enough to run almost normal, most it would do is slow the return to idle, i.e. when you blip the throttle revs go high, stay there a bit then slowly come down. But YMMV here.

In your case it sounds like either air leak (between carb and head, after carb won't likely account for this symptom), exhaust leak (have you checked that?) or slides stuck open.

Good luck.

[Maurice]

Oh and I believe the slides raised part is towards engine. When putting them in, IIRC the way I check is that looking forward the third hole is on the left side.

Last thing is to ensure bowl vents are not clogged/obstructed, this would make bowls overflow and the bike would run weird as proper gas level in the bowl would never be achieved. There is a single thin tube near the gas inlet, and all carbs share that via more thin tubes across, I believe the rubber tubes as opposed to hard ones, those are for gas.

[DaveBarbier]

Have you run the bike without any airbox/plenum at all, and looked at slides operation?

In your case it sounds like either air leak (between carb and head, after carb won't likely account for this symptom), exhaust leak (have you checked that?) or slides stuck open.

I haven't run the bike without airbox or plenum. I have only looked in the top caps while it's running and watch the slides from the top. They all seem to close all the way...that is until the idle stop hits the idle screw. I could check on that this weekend. It's raining here and I have to run it outside.

I haven't checked for an exhaust leak but I don't hear anything abnormal. There's no rusted out holes on the exhaust and there's new copper crush gaskets installed.

Wait, you said, "between carb and head, after carb won't likely account for this symptom." To me that's the same thing. Between the carb and head IS after the carb. Do you mean leak on the airbox side or engine side?


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1978 Honda CB550K

[DaveBarbier]

Oh and I believe the slides raised part is towards engine. When putting them in, IIRC the way I check is that looking forward the third hole is on the left side.

Last thing is to ensure bowl vents are not clogged/obstructed, this would make bowls overflow and the bike would run weird as proper gas level in the bowl would never be achieved. There is a single thin tube near the gas inlet, and all carbs share that via more thin tubes across, I believe the rubber tubes as opposed to hard ones, those are for gas.

I'm going to check but I think if the hole is on the left side (clutch lever side) then the raised portion is facing the airbox.

And ok, I'm going to see about the tubes you're talking about.

Thanks for the help!


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1978 Honda CB550K

[Maurice]

Have you run the bike without any airbox/plenum at all, and looked at slides operation?

In your case it sounds like either air leak (between carb and head, after carb won't likely account for this symptom), exhaust leak (have you checked that?) or slides stuck open.

I haven't run the bike without airbox or plenum. I have only looked in the top caps while it's running and watch the slides from the top. They all seem to close all the way...that is until the idle stop hits the idle screw. I could check on that this weekend. It's raining here and I have to run it outside.

I haven't checked for an exhaust leak but I don't hear anything abnormal. There's no rusted out holes on the exhaust and there's new copper crush gaskets installed.

Wait, you said, "between carb and head, after carb won't likely account for this symptom." To me that's the same thing. Between the carb and head IS after the carb. Do you mean leak on the airbox side or engine side?


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1978 Honda CB550K

By "after carb" I mean on the intake side of the carb. Same as running without anything attached to it. It'll run ok, albeit LEAN, actually, unlike what I said above, I'll fix that. In other words an air leak somewhere on the air cleaning apparatus is not going to have much of an effect, except it'll let dust and crap in.

[Dave Voss]

Hi Dave, have you checked the condition of the four O-rings that seal the intake manifold to the cylinder head?  If they are original, one or more may be allowing an air leak between the carb and the engine, which will cause a leaner mixture and a higher idle for the cylinders that are affected by the leak.  My '78 CB550K used to be sluggish to return to idle, and was difficult to achieve and maintain a carb synch until I replaced those O-rings, but afterward both symptoms went away.

By the way, your dynamic compression readings seem a bit low (150-160 psi is closer to 'normal'), but not so low or uneven enough to be causing your idle issue, the engine will just be down a bit on power.  If this engine has been sitting for any length of time, the pistons rings may be dirty or sticking slightly, in which case additional running time on the engine will likely show an increase in compression on future checks.

[DaveBarbier]

Hi Dave, have you checked the condition of the four O-rings that seal the intake manifold to the cylinder head?  If they are original, one or more may be allowing an air leak between the carb and the engine, which will cause a leaner mixture and a higher idle for the cylinders that are affected by the leak.  My '78 CB550K used to be sluggish to return to idle, and was difficult to achieve and maintain a carb synch until I replaced those O-rings, but afterward both symptoms went away.

By the way, your dynamic compression readings seem a bit low (150-160 psi is closer to 'normal'), but not so low or uneven enough to be causing your idle issue, the engine will just be down a bit on power.  If this engine has been sitting for any length of time, the pistons rings may be dirty or sticking slightly, in which case additional running time on the engine will likely show an increase in compression on future checks.

Hey Dave,
I haven't checked the condition if the o-rings but I just sprayed carb cleaner and propane gas around it to check for air leaks...nothing that I found. I DID order the o-rings, though, in anticipation that that WAS the problem but haven't put them in since I didn't find anything. I might just put the new one's in anyway. Think it's needed?

And yeah, the bike was sitting for a while so hopefully as the season goes on the compression will improve slightly.


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1978 Honda CB550K

[DaveBarbier]

Oh and I believe the slides raised part is towards engine. When putting them in, IIRC the way I check is that looking forward the third hole is on the left side.

Just to be crystal clear. This is my slide. I just took a video. The hole is on the left and the raised part is facing the airbox.

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1978 Honda CB550K

[DaveBarbier]

Last thing is to ensure bowl vents are not clogged/obstructed... There is a single thin tube near the gas inlet, and all carbs share that via more thin tubes across, I believe the rubber tubes as opposed to hard ones, those are for gas.

This is what's you're talking about? The little rubber pillar with the metal thing in it? I see that it connects all carbs. Can I shoot carb cleaner or compressed air through it with the bowls off to clean it out? Or do I need to take the carbs off and disassemble the rack?




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1978 Honda CB550K

[DaveBarbier]

Ok, I removed that little air vent thing but I thought it vented out to the air. But the metal thing inside is a plug. Doesn't let air in or out. Is this venting just to keep the bowls equal in pressure and not to actually vent to the outside air?


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1978 Honda CB550K

[Maurice]

Ok, I removed that little air vent thing but I thought it vented out to the air. But the metal thing inside is a plug. Doesn't let air in or out. Is this venting just to keep the bowls equal in pressure and not to actually vent to the outside air?


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1978 Honda CB550K

Should run better now... It MUST be open to the air, or bowl level will be funky to say the least.

BTW you are correct WRT slide orientation: raised part toward back, third hole to left. One way to be absolutely sure is that the plate that bolts into the slide and actuated by the arm must not obscure that hole.

At any rate, just leave the vent hole open and report back... Eventually just run a rubber hole there, long enough to loop around and down so things don't fall into it. Will take a picture when I get a chance.

[DaveBarbier]

Should run better now... It MUST be open to the air, or bowl level will be funky to say the least.

Whoa, really!? WTH is this PO doing to me? I'll have to try to start it tomorrow or Saturday and report back.

In any case, I removed it and attached a longer piece of hose and gently blew into it. I could hear air coming out the bottom of all the carbs with the bowls off. So it seems it's all free and clear. I'll do as you say and run a small hose. But a picture would be great.


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1978 Honda CB550K