Topic: CB550 air fuel mixture

[iancb550]

Hi all, I have found a number of questions asked about adjusting the air / fuel mixture but I couldn't find any clear definition of which screw actually does the adjustment.



I have seen videos where the screw on top are being adjusted when balancing the carburetors, but there are also two screws on the side which I have highlighted in this image. I believe "1" is the air / fuel screw for the slow jet and "2" is the drain screw?

Could someone let me know if firstly whether I am right about the purpose of the screws and secondly, if "1" is the air / fuel screw, do I turn it clockwise, inwards to enrich the mixture and out to lean? And finally, what is the purpose of the "2" screw (my guess would be to drain the bowl through the overflow pipe but no idea really)? Also do they affect the 'balance' of the carbs.

My reason for asking is, that I have had my carbs balanced at a garage, prior to this I fixed a big vacuum leak which was affecting one carb, I was hoping that when they balanced it they would get it to idle correctly from cold.

When I start it cold, I have apply the choke fully, then turn the throttle a little, remove choke after a few seconds and hold throttle for about a minute or two until it's warm, leaving choke applied kills it, it won't start with just choke and no throttle and won't start with no choke.

Prior to fixing the vacuum leak I could start the bike with a touch of throttle but leave it idling with the choke applied, as the revs increased I could back off the choke. (But of course as soon as I wanted to go over 40mph it would splutter like crazy).

My suspicion is that the previous owner adjusted the air / fuel screw particularly on the carb with the vacuum leak to make it run correctly in order to sell it. I would like to adjust it correctly now there is no leak.

[iancb550]

Thank you in advance for checking out this post I was going to change the oil, oil filter, air filter and spark plugs tomorrow but because the current running condition means the spark plugs are getting blackened I didn't want to ruin the new plugs.

[phil71]

You're right about what the screws do, but you may not be out of the woods yet when it comes to vacuum leaks. Did you put new o-rings in the manifolds, where they meet the cylinder head?
it's easy to overlook, and they are almost always shot.
Also, i'm assuming you did a thorough cleaning of the carbs. Are you running pods or stock airbox?

[TwoTired]

Screw labeled "1" is an air bleed mixture adjustment for the pilot (idle) circuit.  Inward or clockwise is enrichment, the reverse is leaning the idle mix.  The pilot circuit has a minor contribution at throttle settings above 1/4 position.  The air filter matters for this adjustment and must be attached during this adjustment.  In my experience, this adjustment is more about throttle response than ideal idle mixture setting.  The goal being smooth power advance in any gear from idle speed under load with up to one half total remaining throttle twist.  You can then add more throttle as the RPMs go up.  Snapping the throttle full open at idle RPM will just make the engine wheeze and produce NO power (may even die until you return the throttle to a low position.

Screw Labeled "2" is a fuel bowl drain which releases gas AT the screw.

Cold engines require different mixtures than warm engines, due to efficiency changes and these carbutetors are optimized for normal operation.  This is why you need choke for cold starting.  You also need a throttle position bump to raise the idle speed a bit during the warm up period.  The computer that controls this is located approximately 2 feet above the seat and directly operates the twist grip. 
N.O.T.B.

Cheers,

[iancb550]

Cheers guys, regarding the vacuum leak, it was a pretty huge split in the rubber which connects to the airbox side. I didn't change the other rubbers as they looked to be ok but could be wrong.

After replacing that rubber it made a huge improvement, before that change it literally would not get past 40mph, now it will ride at 70 around 6k rpm. Unfortunately it won't do a much more than that (not that I need it to but would have expected 80 to be not too much trouble for it), and is fairly hesitant once fully warmed in the mid range.

I thought before I looked for anymore vacuum leaks as I guess if they're present they must be pretty small compared to the situation before I would try and get it to start like it used to with the choke working normally. Do you think adjusting the air bleed mixture screws will help with that?

If the idle mixture *is* rich at the moment, could it be making the plugs black or is that more likely to be running rich when riding too, and if so how do you lean the running mixture?

[iancb550]

Also, I am using no pods just the normal air box and filter, my jets are the standard size, I cleaned out the bowls (they only had a little bit of dirt at the groove at the very bottom and jets were clean already). The exhaust is not the original although not a performance type exhaust and has been on there since I got it, and I suspect quite a long time as the company that makes it is no longer in business so I thought he would have re-jetted for it by now but then again based on the vacuum leak sneakiness I don't know what other corners he's cut.

this is the bike, its done 20k miles and apart from the exhaust I believe it's just all original

[TwoTired]

How old is the air filter?  They don't last forever no matter how they look.
Prolonged idling and/or choke operation will soot the spark plugs.

Have you checked for head pipe heat?  All 4 hot?

Do all the plugs show the same deposit patterns?

With the exhaust change, you may need to raise the slide needles from the stock position and up the mains to #100 from #98.  Replacement exhausts were often made for the K model without regard for F model set up (mixture-wise).  So, a K carb set up would offer better carburetion.

Cheers,

[iancb550]

The air filter is 8 months old now, I have a new one ready to fit, so the oil filter, plugs, air filter, oil etc are all changed at once.

Since I first got it, I have fixed a few electrical issues such as it not charging and replaced the points system (as it was arcing) with an electronic ignition and a new battery, these fixed their own issues but never affected the issues mentioned earlier.

Yes all pipes very hot pretty quickly. The plugs are all black and I cleaned them recently with a little copper brush. There is also a little black sometimes around the exit of the exhaust.

[LesterPiglet]

That's pretty bike. Mine once looked like that.

[phil71]

In my experience, poor top end performance on these engines is often caused by the rock hard o-rings (part #11 pictured), and a failure to clean the emulsion tubes. Sooty plugs at idle don't tell you too much. A plug chop, under load, at high rpm will give you a better idea what's happening. When it comes to air screws, I usually sync carbs, then adjust air screws till I get the highest possible idle , then drop the idle with the idle stop screw.
  You say the jets were cleaned, but were they cleaned by you? Don't take the PO at his word, or overestimate their ability.
  Bike is super pretty!

[iancb550]

Cheers Phil, I mean when I took the jets out I looked through them and they looked totally clear. And a little carb cleaner and a pin just in case.

And cheers for the comments on the bike guys :-)

[phil71]

you don't mention the emulsion tubes... when they're dirty, you'll have some really annoying running issues.
The emulsion tube is what the needle moves through to regulate fuel flow. they need to be pushed from the top to pop out.
They are often overlooked because you don't know it's there, above the jet. On older honda carbs it was called a 'jet holder' because the jet and emulsion tube were unscrewed from the bottom as a unit. your carbs don't work that way.
this drawing is pretty crude, but # 1 and & 3 depict the area I'm talking about.
Don't feel bad, most people miss that, as it's not strictly a jet, but it's HUGE .

[iancb550]

Yep I didn't even know about those. I think I will try the air screw adjustment this morning to get maximum idle rpm and back it off with the idle screw as you say to see if I can resume normal starting. Then troubleshoot the top end performance by taking the carbs apart properly and changing the o-rings while they're off.

Also change the oil and filters and plugs as they've not been changed for 12 months now apart from the air filter.

[TwoTired]

I think I will try the air screw adjustment this morning to get maximum idle rpm

This is exactly the WRONG way to adjust these screws for these carburetors.

To get max RPM at idle, you'll turn them outward about 5 turns.  While it will idle well.  It will be a bucking beast to drive as any application of throttle under load will wheeze out the engine (no power) until you back off the throttle to near idle position.

You DON'T have an accelerator pump for the throttle and you have mechanical slides.  Raising slides will make the mixture so lean, the engine can't pick up.
These carbs must be set for a slightly rich mixture, so that raising the slides won't make the mix over lean.
Raising the slide drops the carb throat vacuum (the force that draws fuel through fuel jets) and allows more air to pass through carb throat.  Lean mixtures can become so lean they can't simply combust.

The bike wasn't made for prolonged idle and long durations of applied choke.  The plug will go slightly dark with a prolonged idle.  However, operation above 3000 RPM under load conditions and at operating temperature should allow the plug to self clean nicely.

If you never want the engine operate faster than idle, then by all means, max lean the idle mix.

I completely agree though, dirty emulsion tubes will make the mid and high range throttle mixture over rich.

Cheers,

[matt mattison]

+1 to inspection of the emulsion tubes or needle jet as some call it. Ask the shop if they did it. If they didn't, guaranteed your engine will benefit from a good cleaning. Also, cleaning plugs with a wire brush isn't the best way. Small pieces of wire get imbedded in the insulator, which ain't great for making a good spark. Try a blowtorch to burn off the carbon, or a spark plug cleaner tool.

[Scott S]

 +100 on emulsion tubes.

 Have you checked the advance mechanism/timing? I've seen the timing too advanced cause issues at speed/upper RPM's.

[Deltarider]

On older honda carbs it was called a 'jet holder' because the jet and emulsion tube were unscrewed from the bottom as a unit. your carbs don't work that way.

But they do.
Your carbs are similar to mine. So...
As for the needle jets or emulsion tubes, you don't have to remove much.
step 1 Remove float bowls
step 2 Remove main jets
step 3 Twist the throttle say halfway in order to raise the needles some and then insert from below a match or wooden toothpick into the needle jets. When it is snug in the needle jet, cant or turn the wooden toothpick a mm or so and you can wiggle the emulsion tube out easily.
Some WD-40 at forehand can help.

[iancb550]

Cheers all. I tried small adjustments to those air screws but it just made it run worse. May have to go for the clean of the emulsion tubes then. Seems odd it started better when it had the big vacuum leak.

Regarding timing it uses an electronic ignition rather than the points. When that was changed I saw improved consistency with acceleration matching how it ran just after points gaps were set.

[phil71]

Emulsion tubes wont effect your idle, but everything above that. It's not uncommon to have to clean the carbs a second time. Pay extra attention to the idle circuitry , not just the jets. Not to sound like a broken record, but those O-rings I mentioned earlier are ALWAYS overlooked, and can make a huge difference, even if they don't show an obvious vacuum leak.
IMHO, it's a rare weak-link in honda design to put rubber in that space, subjected to so much heat. I'm sure they deform within a few months, and then get brittle. You'll see what i mean when you pull the old one.

[dave500]

its only common to have to clean carbs a second or third and fourth time when it wasnt done properly the first time,it only takes once.

[phil71]

Dave, I think we've established that he doesn't know the extent to which they were cleaned the first time.

[dave500]

i was speaking in general,do you think its normal to do it twice?

[iancb550]

I cleaned the bowls and jets without removing the carbs as a first pass really. As the bowls and jets looked pretty clean I tackled electrical gremlins, vacuum leaks and serious brake issues and am now coming back to fettling to get its top end performance up to scratch and its starting to be what I would consider normal.

Where I want to get to regarding starting is, a bit of throttle and full choke, throttle released once its started, then let it warm up with the choke on until the revs climb then to back off the choke once warm.

Is that what you guys get with your bikes out of interest?

At the moment I have to keep the throttle applied for the whole warm up at the moment which isn't that bad it just seems to me to say something's not right with the idle circuit especially when cold.

However once warm it will purr away at 1000 rpm no probs it's never cut out on me so far touch wood.

It seems like a proper carb clean is in order then, just seems a shame when most of the time it works so well, also I use it pretty much daily for work so I just try and get these things done at the weekends.

[dave500]

if your going to pull the carbs just be sure to remove all the brass,use compressed air/fine wire pokers/carb spray cleaner every which way,you only need to do it once,,too many people do it over and again,use the factory jets and settings,dont try fiddling with the needle jet clips in the slots.

[TwoTired]

Where I want to get to regarding starting is, a bit of throttle and full choke, throttle released once its started, then let it warm up with the choke on until the revs climb then to back off the choke once warm.

I already explained this dream is never going to happen.  See reply #3.

Is that what you guys get with your bikes out of interest?

There is no fast idle cam coupled to choke on cb550 carbs except for the 77-78 K models with PD carbs.  They won't idle cold without twisting the grip or changing the idle knob position.
It's no big deal really, as they do not need a stationary warm up.  Start and go with some choke and throttle applied.  While driving, your hand is usually on the throttle anyway, so keeping the revs up to 1200-1500 is no big issue.  Reaching down periodically to nudge the choke off as it warms, leaves the bike to idle on its own and without choke in a few blocks.

At the moment I have to keep the throttle applied for the whole warm up at the moment which isn't that bad it just seems to me to say something's not right with the idle circuit especially when cold.

You're wrong. That's the way this machine operates.