Topic: cb550 sucks air when you open throttle

[jakec]

Setup:

77 550K engine + carbs stock jetting

77 55F headers no muffler (muffler on the way)

no airbox (airbox on the way)

fresh battery

new magna coils, leads, caps, plugs (D7ea)

Symptoms:

Only starts with choke

Idles OK (no choke)

Cannot open throttle from idle - makes loud air sucking noise and dies

You can rev it if you keep the choke on but the RPMs tend to hang up


I know these aren't ideal conditions with no muffler or airbox. But having stock carb settings I should be able to have it run at least. I got my 750 running and tuned with no airbox and aftermarket straight - ish pipes.

[American Locomotive]

When I bought my 750F, the PO tried putting pod filters on it, and the thing would basically only idle, and that was about it. Anything about 1/6 throttle and it'd basically die. You could turn the choke on, but it didn't like that either. PO also included the stock air box and it was a night/day difference. You wouldn't think it'd make that much of a difference, but these carbs are extremely sensitive on the intake side of things for whatever reason.

[jakec]

Something I read on here is that any intake you use that doesn't have the v stack shape and perfect restriction like the original airbox will be bad for throttle "blip" when it comes to piston style carbs. I.e. pods. Totally make sense and I'm not coming in here to defend pods, but I have run my 750 with no airbox whatsoever and I even did my vaccum sync, so I had it idling and revving up and down for about 30 min that way. This bike (550) I can't even get to accept a little throttle.

I'm going to run downstairs in a bit and take a look at it, I think first thing I will do is adjsut mixture screws in until I can start / idle it without choke (it likes 25% choke to idle).

[Quattrocilindri]

Setup:

77 550K engine + carbs stock jetting

77 55F headers no muffler (muffler on the way)

no airbox (airbox on the way)

fresh battery

new magna coils, leads, caps, plugs (D7ea)

Symptoms:

Only starts with choke

Idles OK (no choke)

Cannot open throttle from idle - makes loud air sucking noise and dies

You can rev it if you keep the choke on but the RPMs tend to hang up


I know these aren't ideal conditions with no muffler or airbox. But having stock carb settings I should be able to have it run at least. I got my 750 running and tuned with no airbox and aftermarket straight - ish pipes.

I am no engine guru, but I would say that your motor is close to being perfectly set up as it is; with no back pressure provided by a muffler and no air restriction from the missing airbox, the air/fuel ratio would be too lean for the carbs to compensate. Once you put on the missing muffler(s) and the air box that are on the way, the symptoms you describe should disappear completely.

[TwoTired]

For 77, Honda was attempting to appease the EPA by doing what they demanded for other US models lowering hydrocarbon emissions.  And, that meant going to a lean burn engine.  Honda went this path with PD carbs, a slightly more restrictive air filter box inlet, and a 4 into 4 higher pressure exhaust system.  Then they jetted the carbs to get the proper mix ratios for running on the street and get close to 1978 emissions requirements with a finely tuned idle circuit adjustment.  Therefore, when changing the intake and not carb jetting, the engine runs far leaner.  Further, less back pressure increases scavenging and removes unburnt hydrocarbons from the last firing cycle.  So, they aren't there anymore to effectively enrich the firing mixture and reduce hydrocarbon content in the released gasses.  EPA smiles, anyone wanting motorcycle performance frowns.

The 550 never had and accelerator pump.  On other models with this feature (such as the 77-78 750), when the throttle is twisted and the slides raised, a shot of gas is forced into the carb throat.  As carbs with mechanical slides can suddenly open with the throttle, the vacuum in the intake runner and carb throat suddenly drops (or raises to equalize closer to outside atmospheric.  It is the difference in carb throat pressure relative to outside atmospheric pressure that drives fuel through fuel jets.  (Ever wonder why the carbs don't still feed gas when the engine isn't running?  That is why, as there is no differential pressure between the carb throat and the outside pressure.)  So when you whack the throttle on a 550 it will just wheeze and die unless... The idle mixture, and just the idle mixture, is set over rich in order to keep the mixture in combustion range when it gets a sudden inrush of air with no no corresponding extra gasoline (actually less).

So, after you put on proper intake bits to restore the carb throat vacuum as it should have, and an exhaust you plan to live with a long time, and if they are not the stock components, you will have to re-jet the carbs (Main, Needle, and possibly idle jet) in order get the mix ratios proper for your engine.   Until then, expect awful throttle response and an engine that runs way too hot and cooks the oil beyond functional effective use.  As it is, your current complaint relates to an engine operating way leaner than Honda ever anticipated.

Cheers,

[jakec]

Thanks for the reply. I am using the 4into1 headers from the same year and we are shopping for a replacement muffler now as well. It won't be the same as stock but I know that most people who own a 550F end up running some kind of aftermarket muffler. For the airbox we are using a period breadbox with UNI foam and about 80% of the free area of the breadbox blocked off (this is how most people run then when on a 750).

Once I have these parts installed should I try running with idle mixture changed and needle clip position changed? Or do I need to immediately rejet in your opinion?

[TwoTired]

The F muffler is also of a higher pressure type.  The F carbs are jetted leaner than the other carbs of that style to accommodate. I can't imagine anything aftermarket will compare.  Don't know how you are going to avoid a jetting change to preserve the engine.  Even the F carbs need a jet change along with a muffler change.

Imagine a long tube with atmospheric pressure at one end and a constant vacuum source at the other.  Lets say the pressure differential is constant with a linear fall throughout the tube.  So midway between ends could be 1/2 the pressure differential.  Lets put the carb jet exits there and this 1/2 pressure point is what is used to drive the fuel though the metering jets.  The jets flow a given volume of fuel relative the pressure differential applied across them.  Now lets cut off half the tube length to the carb on the side that's open to the atmosphere.  So instead of a foot between carb and atmospheric pressure source, it is only 6 inches.  The carb is no longer half way between source an destination, so now it "sees" 1/4 of the vacuum it used to experience, causing the jet to flow half the volume of fuel as they did with the longer tube they once had because there is half the pressure differential applied to them across their openings.  The engine runs way leaner, right?  Same volume of air passes the tube.  But, only half the fuel as before delivered from the carb.  The stock system has an air plenum with turbulence straightening V stacks at their entrance, attached to a filter box, and in later models on top of the filter box is a horn inlet where atmospheric pressure is presented along with air to the engine.  So, there exists a physical distance between carb venturi where engine suction is presented to the fuel jet exit, a distance from the vacuum source (falling piston), and a distance to the air inlet atmospheric source.  The stock jet size orifice was selected with these conditions.  Now you want to chop all that plenum  filter box and inlet horn off and present the atmospheric pressure source a couple inches away from the carb.  Don't you think the pressure at the jet orifice exit is going to diminish unless you change the orifice size to flow the same amount given less pressure differential it now experiences?  Yes, restricting the air flow with block off devices will raise the vacuum back up, but with such restriction comes an increase in restriction with increased air volume due to turbulence introduced.  The CB550 only makes good power when the engine is revved up and they aren't kidding about using 9200 RPM where air velocities are are getting up there.  Blocking off the "breadbox" is not recommended, unless its for getting you home to fix it right.  As, that's not an engine tune, its a temporary work around, imo.

  My advice, find size 100 main jets for your carbs, and raise the needle a notch or two.  Idle jets should probably be 38. And this should allow you to tweak the idle mixture to get a decent throttle twist response while driving.  But, this is just to save the motor from harm while you find the right jets to accommodate your intake and exhaust mods.  Gonna need a dyno or a test track for dialing in the changes your making.  Lastly, that plenum and filter box you hate is pretty well engineered by Honda.  Big bucks were paid to get that.  It's a pity to throw it away and seriously hurt the performance of the machine you'll hopefully drive a lot on the street.

Cheers,

[jakec]

Here is the explanation of how the airbox is "blocked off." I don't hate the stock airbox but we are building a chopper so this is how it must go...

http://www.lowridersbysummers.com/twt/13/index.html

What would you suggest that I do if I am dead set on using this breadbox filter?

I am not sure what muffler we will end up with, but being aftermarket I'm sure it will be less restrictive than the original.

Ok so first step will be to up jets and needle position. Thank you

[TwoTired]

Here is the explanation of how the airbox is "blocked off." I don't hate the stock airbox but we are building a chopper so this is how it must go...

http://www.lowridersbysummers.com/twt/13/index.html

Yes, I've seen that before.  Never found a follow up on how long the engine lasted, or what tuning procedure they used to get the mixture correct.  It mainly addresses cleaning the air fed to the motor, which is a good thing.  However, all the early carbs were set up for a much richer mixture, even from the factory.  They used "sloppy" carbs, even on the 750's that didn't have accelerator pumps.  And, I'm not sure the 750 required as much air velocity as the 550 near red line.   550 has smaller venturis and ducts and a higher red line.  But, I'm just guessing about that. The 750 also has a much different and shorter induction arrangement than the 550, too.  Not as drastic a change for the bread box alteration. 

What would you suggest that I do if I am dead set on using this breadbox filter?

Consider getting the earlier style carbs and manifolds.  As these carbs were also of the "sloppy" era.  Also, there are more options for jets and needles.  627B and 022a would be good ones to find.

Do you know how to read plug deposits to get a picture of how safe the mixture is?  Too rich will foul spark plugs and and wash oil off the cylinder walls.  Too lean will overheat and risk cooked valve guide seal and valve guides, as well as damaged oil.

Anyway, if the bike is just for show, and not ridden much, you'll probably be more worried about poor throttle twist response.  And that will definitely need a much richer mix for idle.  But, if you go too rich then the plugs foul out readily.  This motor is not a torque motor.  Power is rather anemic below 3500 RPM.  For driving escape maneuvers, you need this RPM, at least, and be ready to down shift a couple gears to make the bike scoot.  Of course, loud pipes and high RPM also attract attention from the boys in blue as well as the ire of those disturbed by it.  Don't know if you care about any of that.  But, know this: a properly tuned 550 will never respond with power from a snap open throttle twist at low RPM.  No accelerator pump. Normally from low RPM you can snap to 1/2 throttle and get smooth acceleration in any gear and as the RPM goes up you can add more throttle up to WOT.  That's how I tune the idle mix, after finding correct Main and slide needle positions and mimics the behavior of the stock set p and configuration.

Lastly, if you do shop for carbs, see if you can get some PD carbs from a 650, as they have accelerator pumps.  Not certain about manifold fit, though.  Measure before committing.  But, I know some on this forum have grafted them on in prior posts as I recall.

Cheers,

[jakec]

Consider getting the earlier style carbs and manifolds.  As these carbs were also of the "sloppy" era.  Also, there are more options for jets and needles.  627B and 022a would be good ones to find.

Unfortunately I had a set of cb500 carbs but I let them go as when I began the project I was instructed to stay with the carbs that came with the engine (matching 1977 carbs and engine from the same bike). Now I am wishing I still had the 1973 carbs. Not just due to the jetting issue but also the cable operated choke on these later carbs is not convenient.

I have the same breadbox setup on my 750 with only some more minor problems (puff back when cold, see my other post). So I assumed it would be as easy to get a 550 running with the same set up.

If you have a set of earlier carbs you want to sell or if you have some spare jets you would like to sell, let me know. Unfortunately this project is over budget at this point and the last thing I really need to figure out is these carbs.

[seanbarney41]

Jake, I have been running a bread box on my cb750f for a few years/about the last 15,000 miles.  I find it to work very well after just a main jet increase...A couple things...The article you link has good information as to how to fix up an old bread box but I have never found any reason to block off any part of the screen.  Also many of the breadboxes mpunting boots are velocity stacks very similar to the stock ones, and some will even accept the stock velocity stacks.  If they do, use em!  I think TT is probably right about the sensitivity of the 500/550s carburetion, but I don't really know for sure.  All the 550's I have messed with were very stock.

[jakec]

I tried to put the V stacks from my stock 750 airbox in the bread box that I have. But the holes in the metal of the box were much too large. They only seem to fit the boots that came with the breadbox (no stacks). What mains are you running? I am using round tops with stock jets and I now have the needle up one clip.

[seanbarney41]

I tried to put the V stacks from my stock 750 airbox in the bread box that I have. But the holes in the metal of the box were much too large. They only seem to fit the boots that came with the breadbox (no stacks). What mains are you running? I am using round tops with stock jets and I now have the needle up one clip.

yeah Jake, the breadbox's came from many different sources, so you are never quite sure exactly what you are getting, just as stated in the article.  As far as my jetting goes, I would have to check my logbook, but I know the mains are somewhere between 120-130 inclusive.  The needles and pilots and airscrew's are at stock cb750f0 which is 40 pilot, needle clip in middle slot, screws 3/4-7/8 out.  I run a Kerker exhaust with the most restrictive/quietest baffle.  This combo runs smooth as silk at any rpm/throttle position.  You have maybe noticed the sloppy, flappy intake noise you get with pods or a breadbox, well mine, at about 85 mph, you can start to hear the flabby sound hit a resonance, and by 90 everything gets super smooth and powerful and the sound is like nothing you have ever heard with the stock airbox.

I have a hunch that the jet needle's that came with the 750f0/f1 bikes have a slightly different profile, or maybe something else, that somehow makes for easier tuning to compensate for intake/exhaust mods.  Or maybe I just always get lucky with the F's...not sure yet.

[jakec]

I bought a set of 100 and 110 jets. I got the 110s on the suggestion of someone I know running straight pipes on a 1977 550 so assuming same carbs. We ran / rode the bike for the first time the other day with 110s in and it was obviously very rich as it was puffing white smoke from the muffler. After warming up a bit I was able to actually open the throttle like never before. However there were two flat spots, one right off idle and another at the top end.

Now I am planning to switch to the 100 jets as originally advised here. I'm thinking that also raising the needle one slot would help with the flat spot coming off of the slow jet (which is 38 or 40, that's what was in the bike when we got it). Of course there is more tuning to do after that. But we will be waiting for a new air filter to come in, we are running it open intake right now.

[bryanj]

The pd carbs you have are a nightmare to get correct on anything except a standard setup. As said the whole thing was built to pass the california rules and getting different pilot jets is just about impossible as they are pressed in.
I now refuse any jobs with pd carbs!!

[jakec]

Yeah they are definitely harder to get going than the last 550 I had which was a K0. I would happily take some earlier carbs if anyone wants to donate them

[TwoTired]

Certainly California was the initial impetus for the emission rules.  But, the USA Federal EPA made import restrictions and emission rules effective 1978.  Honda was trying to get ahead of the game in 77 trying to meet 78 rules.

White smoke from the muffler is usually water vapor.  If it's actually gas vapor, then a cylinder isn't firing.  So the exhaust smoke observation doesn't tell the true story.  However, spark plug deposits will.  If too rich, then soft black carbon will be present.

Be aware, there are three different fuel metering paths in the carbs; pilot, slide needle, and main, corresponding to idle, 1/4 to 3/4 throttle and W.O.T.
The driving force for all of them is the differential pressure between carb throat and outside atmospheric.  They don't flow when the engine is stopped because the differential pressure is zero without the fall of the piston in the cylinder bore providing a lower pressure.  The tube from the inlet valve to the input of the filter (inlet duct) is always trying to equalize while the engine turns and the carburetor is part of the tube path.  The closer the carb is to the outside air inlet the less differential is placed on the carb's metering jets, resulting in less fluid flow.  This is why pods or similar require fuel path enrichment over the stock filter arrangement.  It effect ALL the metering jet.  Usually, you can change the pilot screw to compensate at idle.  But, the PD carbs will need both a slide needle change and a main to keep mixtures from become too lean and overheating the engine and damaging the oil.

This could likely be calculated if we new the stock exhaust and air induction parameters and then compared them with parameters provided with replacement components.  I've no seen any such parameters for stock or aftermarket.  So wne making induction and exhaust changes, it's either a dyno and mixture map from the exhaust, or a test track to run the motor under load at specific throttle settings, then reading the spark plug under those condition to infer the mixtures for that setting.  Adjust and test again.

The third method of rejetting carbs is to try  a bunch of different parts and settings until you find a combo that is "good enough".  (Often to sell the bike).

Be aware that mechanical slide carbs, even with a perfect tune, will always wheeze and stumble when the throttle is whacked from idle.  This is because the sudden slide opening drastically drops the pressure differential in the carb throat that drives the gas through the jets. They should make a reliable power pickup from, up to 1/2 of throttle twist, and then take more as the engine revs.  This is why some mechanical slide carbs have accelerator pumps to shoot extra gas into the carb throat as the throttle is twisted.  But, the 550 doesn't have those.

Cheers!

[jakec]

Interesting point about the white smoke. I forgot that I had actually used WD40 in one of the float bowls to check for a broken drain tube (confirmed) then let it air dry before installing. So I bet the white smoke is largely caused by the WD40. We only ran the bike for about 5-10 minutes. There was no backfiring or at least such a small amount I didn't take note. So maybe it is not as overly rich as I thought. The muffler is mildly baffled inside. It has about 6-8" of punched holes similar to the slip in baffles you can buy. I would describe the performance now like this,

pilot / idle: good
transition from pilot to main: air sucking sound with small flat spot in power
slide needle? / 1/4-3/4 throttle: ok power but could be better
main? / WOT: air sucking sound, low power

[jakec]

I need to pull the plugs after work to check. But based on the crude analysis I did above It feels like perhaps needle position is ok, or maybe move up 1 notch, but WOT still felt lean.

pilot / idle: good Good
transition from pilot to main: air sucking sound with small flat spot in power Normal according to TwoTired
slide needle? / 1/4-3/4 throttle: ok power but could be better needle position ok?... hmmm
main? / WOT: air sucking sound, low power Needle fully raised no longer has any effect, too much or too little fuel flow through unobstructed jet (110 currently)

[jakec]

Interesting plugs. 1 & 2 look good and  3 & 4 lean

[TwoTired]

pilot / idle: good Good

you can’t know this without reading plug deposits, or metering the exhaust contents, or noting the engine response  from idle to a one half throttle twist in any gear.  It should respond reliably, if not particularly brisk with no stumble


transition from pilot to main: air sucking sound with small flat spot in power Normal according to TwoTired

NOT what I said.  And it doesn’t transition from pilot to main, it transitions from pilot to slide needle.

slide needle? / 1/4-3/4 throttle: ok power but could be better needle position ok?... hmmm
main? / WOT: air sucking sound, low power Needle fully raised no longer has any effect, too much or too little fuel flow through unobstructed jet (110 currently)

engine power is identical when rich of peak as it is from lean of peak.  Your seat of the pants tuning technique is prone to failure.  Learn to read spark plug deposits to determine actual combustion conditions or pay the man at the dyno shop.

[jakec]

Sorry didn't mean to mischaracterize what you said, I thought you meant the puff off pilot jet was normal under these conditions (non stock intake and exhaust). I know I need to do plug chops but the problem is that I'm right in the middle of the city, so not a good place to do it. I would need to trailer the bike somewhere to be able to do a true test. Otherwise there is a straight road around the corner from me where I could do do pulls under load up to about 3rd gear, but I would run out of room.

[TwoTired]

You can't do proper engineering or mechanic-ing without proper tools.

The Dyno and the test track are needed tools for the engineering changes made.  Of course, there is also endless guessing...  ...With commensurate frustration.

Best of luck!