Topic: Carb FAQ

[Harry]

How is carburetion affected by throttle opening, temperature, humidity etc?
Throttle Position:
Idle Set at 1700 RPM + or - 100 RPM
Idle -> 1/4 Air screw Turn in for Richer out for Leaner
1/4 -> 3/4 Needle Jet Clip up =Leaner Needle Clip Down = Richer
3/4 -> Full Main Jet (bigger the number richer the mixture)
Conditions:
Air Temperature  High = (less air so you have lean out the mixture)
Air Temperature  Low = (more air so you have to richen the mixture)

Altitude High = (lean the mixture)
Altitude Low = (Richen the mixture)

Humidity High = (Lean out the mixture)
Humidity Low = (Richen the mixture)
 
How can I temporarily richen or lean my mix without fiddling the carbs
Colder Glow plug to Lean it
Hotter Glow plug to Richen it

I need to sync my carbs but have no vaccum meter. How do I bench-sync?
When I rebuild a set of carbs, I always bench sync them before I re-intall them. Basically, I use a 1/8" drill bit as a gage. Here's how you do it: Adjust the idle screw (the main one) until the 1 carb outside of it is open enough to just let the 1/8" drill bit through. Then adjust the _sync_ screws on the other 3 carbs until the drill bit just fits them as well. You'll probably want to close the idle screw before you bolt them back on the bike, 1/8" openeing is pretty big. This isn't a perfect sync, as there are other things that will affect the airflow to a given cylinder, but it will get you pretty close.

Comment from Bob Wessner:
I was reading the FAQ's, on carbs to be specific, about bench syncing. It's probably trivial and only affects those with pre-K1 750's, but they really can't be bench sync'd off the bike because of the four independent cable linkage. I must have read through I don't know how many earlier posts on this trying to figure out how it would possibly help on mine until it dawned on me that as described, it only works for models that have the carbs all linked via the bar, so to speak. Jus thought may a note to the affect that the described procedure is for K1's and later might be useful.

How do I tune the carbs?
1.  Pick the main jet first.  You can take a plug reading at wide open throttle.  Accelerate over a long distance (preferably uphill) getting into 5th gear and keeping throttle fully open.  When fully tapped out, grab the clutch and hit the kill switch.  The motor should stop immediately, then look at the plug.  Should be tan, but not too dark.
2.  The pilot jet affects cranking, idle, and particularly acceleration from idle.  If too lean, the motor will hesitate when accelerating (in neutral) off idle.  If too rich, motor will smoke excessively, foul plugs, and be unresponsive off idle.  Driving along in first gear just barely above idle, there should (actually MUST) be some blubbering, but it should clear up immediately at about 1/16 throttle.  Air screw affects pilot jet somewhat.  It is useful in 1 to 2 1/2 turns out and within the extremes of this range, is about like changing pilot jet 1/2 step (if that were possible).
3.  Slide cutaway has a tremendous effect on throttle response.  Its range of operation is approx 1/16 to 1/4 throttle.  Too lean and the bike will be a bear to control.  Too rich and throttle response will be sluggish.  Slide cutaway and needle DIAMETER are very similar in their actual effect on jetting.  It's usually easier to leave the slide stock and try to get jetting optimum with the needle diameter. 
4.  The needle is perhaps the hardest part of jetting.  There are various aspects to selecting needles.  Hopefully, the manufacturer was pretty close so all you have to do is change the clip position.  This, however, only affects the range around 1/2 to 3/4 throttle. By selecting special needles, it is possible to tune any desired throttle position, but hopefully you won't have to do that.  It's not as easy as picking the right clip position.  If the clip is too lean, the bike will be VERY crisp but after warming up it will usually develop a hesitation when accelerating using mid throttle positions.  If too rich, the bike will be slow and unresponsive and you may even get the blubbers
typical of extremely rich jetting. 
5.  Note we have been talking about throttle POSITIONS.  It is useful to mark your throttle grip so you can SEE what position your hand is in when jetting.  Remember, it's the throttle POSITION, not the engine rpm that you're using to jet the motor.
6.  In general, lean conditions will overheat a motor.  It might take a few seconds (main jet or needle clip too lean) or 30 minutes of woods riding (needle diameter too lean), but an overheated motor will become harder to ride, lose power, and hesitate on acceleration.  If you're lucky and smart, you'll recognize this before you seize it.  A motor jetted too rich (in any one area) will suffer loss of acceleration and responsiveness, may smoke, and may foul plugs.  It is possible to have one jet too rich and one too lean.  The bike could display both lean and rich characteristics depending upon where you hold the throttle.
7.  To develop your skill at jetting, you need to experience too rich and too lean with EVERY jet (pilot, needle diameter, needle clip, and main jet).  Only after you FEEL what too rich and too lean is will you be able to jet your bike perfectly.  Start with the main and go way too rich and feel it blubber at full throttle, high rpm with low load.  As you come down, you'll feel the power and responsiveness build considerably and the bike will begin to rev out cleanly..  About the time that the plug starts saying "too lean" you should begin to notice hesitation on full throttle acceleration and a reluctance to rev very high.  Don't press your luck.  One main jet size or one needle clip position is often the difference between a bike that's unpredictable and miserable to ride and one that always has great power whenever you twist the grip.

What happens when I turn the airscrew out - richer or leaner?
If your airscrew is on airbox side of carb, turning out will lean mixture; opposite if airscrew is on engine side of carb.

[Harry]

What are the symptoms of rich/lean running:

>>>Typical lean conditions:

• Poor acceleration - feels flat
  Engine doesn't respond when throttle is snapped open - picks up speed as throttle is closed
  Engine runs hot, knocks, pings and overheats (end result- hole in piston)
  Engine surges or "hunts" when cruising at part throttle
  Popping/ spitting through carb when throttle is opened, or popping and spitting through pipe on acceleration with a closed throttle (classic lean pilot circuit symptoms)
  Engine runs better in warm weather, worse in cool
  Performance gets worse when the air filter is removed

>>>Typical Rich Conditions

• Acceleration is flat, uneven
  Engine will "8 stroke" as it loads up and skips combustion cycles
  Throttle needs to be opened continuously to maintain acceleration
  Engine works better when cold
  Black smoke from the tail pipe
  Poor fuel economy
  Engine performance improves when air cleaner is removed
  If the pilot screw is overly rich, idle is rough and the engine won't return to idle without blipping the throttle
  Black sooty plugs, sooty exhaust pipe

How do I diagnose faults "on the fly"
When you are limited to street riding, take note on how easily and quickly the engine reaches the proper RPM for shifting gears. The engine should accelerate smoothly and quickly through all gears. When the main jetting is to rich, the engine will feel sluggish and acceleration will 'feel' slow. You can check the main jet by quickly closing the throttle from wide open to 7/8 position when the engine's RPM is greater than 4500. If the engine accelerates slightly, the main jet is to lean. A larger main jet is needed. If the engine hesitates or misses slightly, the main jet is to rich. A smaller main jet is needed. If the engine just slows a slight amount, the jetting is very close to correct.
Another test is to accelerate through the gears at full throttle. If the engine backfires through the carburetor, misses, cuts out or quits running, the main jet is lean. Increase the size of the main jet. If the engine acceleration seems sluggish, does not react to the throttle or sounds flat, the main jet is rich. Decrease the size of the main jet. The main jetting can be set quickly at a 1/4 mile drag strip. For this test, you can ignore the ET (elapsed time). Making sure the engine is warmed up, make your run down the strip making note of your final MPH. Keep increasing the size of the main jet until MPH begins to drop. Then drop back one jet size. Your jetting should now be correct.

To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture)), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich. If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich. If you rev the bike and the revs hang up after closing the throttle this = Lean; if the revs dip down then come back up = rich.


How do I adjust the carb?
The backfiring indicates air/fuel mixture or pilot jet leaness. I'd start, as you correctly stated with the idling mixture as it's the easiest and cheapest. Remember there's another carb under your tank which you'll also have to do. Manufacturers specs are normally one and a half turns out from the slightly seated position - don't force the screw it's got a taper on the screw you're turning as well as in the carb. Start the engine and get it to working temp. Light a cigarette or whatever you smoke as you'll be busy from here on. You'll be letting the engine run for a while so do it outside where there's air circulation or put a fan on the engine and put the beers within reach. Adjust the idling speed to the spec or to where you can hear it's running at a constant speed (the idling speed is adjusted by hand by turning the brass screw with the spring on it sticking out of the back of the rear carb). Now turn the pilot screw out one quarter from the manufacturers spec. If the engine speed drops, return the screw to where it was and turn it in one quarter. If the engine speed drops, you're OK (and I'm OK). Drink the beer.
Don't go for a test ride after the beer. If the engine speed increases as you turn the screw out, keep going until you reach the point of max RPM. If it takes more than 3 turns for the engine speed to level off, the pilot jet is too large. Install a leaner one. This will probably not be your scenario. If the engine speed increases as you turn the screw in, keep going until it levels off. If the pilot screw ends up less than half a turn open, the pilot jet is too small.Trick : When you've found the peak idle speed, turn the mixture screw in one quarter turn. If it doesn't change the idle speed leave it there. If it does drop, return it to the last setting.

Any "shade-tree mechanic tips?
Sure - if you dont want to go the trouble of rejetting, a colder plug will often compensate for a slightly lean mix!

[SteveD CB500F]

We might do an in-house movie!, until then...

http://www.salocal.com/sohc/tech/carb/carb.htm

Additional information (see link below posting) regarding carb overhauls.

Provided by http://waynes-cycles.com/.

[SteveD CB500F]

For an idiots guide to carbs, take a look at http://204.71.0.3/motoprof/moto/mcycle/carb101/carb101.html

[Harry]

Carb tuning is the last thing you want to do.  Perform all other tune up items first, cam chain adjust, tappets, new correct heat range plugs, spark timing, new filter, etc.  Then carb sync and dial in the mixture. (Thanks Lloyd)

[Harry]

Warlock has sent us this 1975 Honda Technical Manual on Motorcycle Carburetion, which is hosted at http://www.sohc4.net/?q=node/65

The manual fills 11MB, but is well worth downloading for the excellent insight it gives in all matters regarding carburetion.

[Harry]

How do I clean bowls / measure float height?

There should be a brass screw (plug, actually) at the bottom of each float bowl. After you've turned off the petcock, you can drain gas from the float bowl by unscrewing each these.

Once you've drained the gas from the float bowls, you can unscrew the bottom of the float bowls to reveal the jets and float itself. You may want to order a set of float bowl gaskets before you attempt this, as you'll likely have to replace.

You're looking for anything that might prevent the float from seating correctly or anything that might be clogging either of the jets. Buy a can of compressed air and blow everything clean while you're in there. Carb cleaner spray helps as well.

If you start taking things apart, pay careful attention to the order and orientation in which you removed them. The parts are small and the diagram confusing.

If you want to get serious about working on your carbs, you should build a stand to suspend your (or another) gas tank away from the bike.  Then attach longer fuel lines to this tank.  This allows you to work on the carbs (especially synching them) without the tank in the way.  Just don't forget and launch into 1st gear for a test ride

The bowls for the carbs come off easy.  Stubby screwdriver is all you need.  You don't really need to drain them, as long as you're careful in removing them.  Keep them level so they don't spill.  Keep some rags handy for errant fuel.

Check the bowls for gunk.  If there's gunk (there will be) clean it out.

Raise and lower the floats with your finger.  Free movement?  They should be LOOSE.  No resistance at all.

To measure the height, you need to raise the float to just the point where it touches the little pin that stops the flow of fuel into the bowl.  It's just like a toilet.  When the float 'floats' in the fuel to a certain height, it shuts off the flow of fuel into the bowl.  So, as your bike uses fuel at varying rates, the float will be at different heights in the bowl.  At idle, the floats will rise to the top and shut off fuel to the carbs until the fuel in the bowls is depleted enough that the float lowers and open the valve.

So, when the float is at that point of just touching the little pin, you measure the 'height' as shown in the manual.  Just get a ruler and eyeball it for the moment.

Putting (bowls) back on, you'll note some resistance.  They push up against a 'leaf-spring' type deal that is sitting hanging from a groove in your main jet.  This keeps the main jet pressed up into the carb body.  They aren't threaded in.  Just shoved in with an o-ring.  If this leaf-spring deal is missing, replace it.  You don't want your main jet vibrating out of it's little hole.

[SteveD CB500F]

This info originally from TwoTired, resurrected when I re-found it recently.

Anyone able to fill the gaps and/or provide similar for CB350/400, 650 and 750?

[SteveD CB500F]

Go here for carb specs for all(?) US models 4 cylinder and 6 cylinder engines...

http://www.motorcycleproject.com/motorcycle/text/specs.html

[Harry]

Heres some useful info from Remi (Flood):

Carb main jet, float valve seat and bowl drain screw O-ring Sizes for CB350F, CB400F and CB550.

"Carb O-rings"

When rebuilding carbs on the old Honda fours with press-in main jets,
 you will often find that all the metal parts are fine after a good cleaning,
 and all that you really need to buy are new O-rings. Honda doesn't sell the individual O-rings separately....
instead you have to buy a complete carb kit at $10 to $15 per carb. I found an industrial O-ring supplier
 who was willing to do small-volume counter sales, and was able to get new O-rings at about 50 cents per carb. Sizes are:

Main Jet: 1.2mm cross-section x 3.5 mm ID

Float Seat: 1.5 mm cross-section x 5 mm ID

Float Bowl Drain Screw: AS568-009 (7/32" ID x 11/32" OD)

------------------------------------------------------------
CB550:

Shifter lever seal = inside diameter - 14mm, outside diameter - 24mm, depth - 5mm

And the one behind the sprocket = I.D - 33mm, O.D. - 57mm, depth - 7mm

[SteveD CB500F]

More from TwoTired:

http://hondachopper.com/carb.specs.html

[SteveD CB500F]

by Craig Faith

    The stuff is called Yamaha Carb Cleaner. The Part Number is ACC-CARBC-LE-NR
    For lightly gummed up carbs, use per the bottle instructions.

    For really gummed up carbs, where they are going to have to come apart,
    mix the carb cleaner 50/50 with water, put the carbs-or parts-in a can,
    cover them with the mixture, and heat it to an "almost" boil - surface
    should be barely rolling. Let them cook for approx. 15 minutes, then
    rinse the parts in fresh running water, and blow dry with compressed
    air. Check the bodies and bowls to make sure all the small passages are
    clear, and soak them a bit longer if they aren't.


    Specific comments and cautions:
    1. Do this outdoors, or in a VERY well ventilated area - the fumes are
    pretty rough..(cough, hack..)

    2. The cleaner will not harm rubber parts-unless they've already started
    deteriorating. It WILL quite cheerfully remove paint from painted parts,
    like black GPz Kaw carb bodies. Brass parts left in too long-or at too
    high a temp-will discolor.

    3. The stuff is reusable and pretty long-lasting. After it cools down,
    pour it into a jug and cap it - it will evaporate very quickly if left
    in an open container. Every once in a while strain it thru a paint
    filter to get the grunge out.

    4. I've got a collection of cans and containers of various sizes to use,
    all the way from a tuna tin which is handy for small parts, up to a pan
    that looks like a large mud pan for drywall work, which will hold an
    entire rack of carbs for an inline four. Heat comes from a double-burner
    hot plate, picked up in a yard sale for a few bucks.

    5. Wear rubber gloves. This stuff will pull the oils out of one's skin in no time.
    Hope this helps!

Original is here:  www.vjmc.org/techtips/carbclean.htm

[Bob Wessner]

Pods And Carb Adjustments/Jetting.

Submitted by: TwoTired

Begin with knowing what you currently have.   For example, a 76 CB550 could be an F model or a K model.  The carbs could be set up as 022A, 087A, or 069A.  These numbers are stamped on the carb bodies.  The set up determines what internal parts they have and how they were adjusted by the factory.

ALL the set ups sized the fuel metering devices inside to flow properly with the barometric pressure drop in the carb throats provided by the stock air filter arrangement.  There is no Pod filter offering that duplicates this pressure drop.  And, there isn't even any consistency between brands of pod filters.   Therefore, tuning to changes are done on an individual case by case basis.

Elsewhere in this FAQ you will find a chart of the known adjustment and internal parts changes among the different carbs available to the CB500/550.
This is a starting point.  The next step is to determine the effects of the changes you've made to the air induction and exhaust restriction has had on the combustion needs of the motor.  This is done either with a Dynamometer and instrumentation.  Or, with a test track of some sort and frequent "readings" of the combustion deposits on the spark plugs.  The deposits tell how the engine is running with the last changes made to the carb adjustments.  It is a reiterative process, test run, read, adjust, and repeat until the plugs yield the correct color with performance, and throttle response either meeting the driver's goals or he tires of the process and accepts the way it currently runs as good enough.

There are three aspects of carb fuel metering to be addressed in response to engine "breathing" changes.  They are interactive to some extent. But, dominate at certain throttle position settings.
1) The idle, slow or pilot circuit.  Active from idle to 1/8 throttle.  This is a separate parallel circuit from the main and throttle valve fuel delivery circuit (discussed later). It is comprised of the idle, slow or pilot jet orifice size and the pilot or Idle air bleed adjustment screw.  (alternately an Idle Mixture Screw in some carbs, the 77-78 models for example.)
Depending on the individual carb, the adjustment range may be limited, requiring changes to the idle, slow or pilot jet orifice size to achieve proper behavior.  The extra tricky bit with the 550  and earlier carbs in other models, is that they have no enrichment pump to compensate for the rapid loss of vacuum when the slides are opened quickly.  Therefore, these carbs are adjusted sufficiently rich to achieve adequate throttle response with moderate changes in throttle position.  Some CB750 carbs do have an accelerator pump to enrich the mixture on sudden throttle openings.  The Idle Mixture Screws (IMS) are adjusted for maximum lean conditions or fastest idle speed in this case.

2)  The throttle valve controls mixture from about 1/8 to 3/4 throttle positions.  It is comprised of the slide needle, and the slide needle jet orifice.  The taper rate, needle width, and depth position in the jet orifice determine the fuel delivery at the various throttle positions.

3)  The Main jet determines the maximum required fuel draw required at 3/4 to Wide Open Throttle (WOT).  The slides throttle valve is withdrawn from the throttle valve orifice to present less restriction to fuel flow than the main orifice.

Now it gets more complicated:
Float bowl fuel level also effects mixtures throughout the throttle range as the fuel has to be drawn up into the carb throat bore.  The longer the lift, the more pressure is required to make the lift.  Given the same pressure, less fuel is delivered for a longer lift.

Emulsion tubes:
Both the idle, slow, or pilot circuit and the Throttle valve/Main circuits have these emulsion tubes.  These tubes premix air into the fuel creating a froth that is more easily drawn into the carb throats and more thoroughly atomized after insertion into the carb throat.  These emulsion tubes have cross drilled holes for air insertion. The number and size of these holes determine how much air is frothed into the fuel delivery mix and this also effects the ultimate fuel mixture delivered to the engine.  The factory used different emulsion tubes in setting up the carbs for different machines.

There are also air jets that supply air for the emulsion tubes.  These are usually drilled metering orifices in the inlet throats of the carb bodies, and is part of the set up configuration stamped on the carb body. 

One final note about why many of the above metering orifices have to be readjusted.
The partial vacuum in the carb throats is a barometric pressure change in the carb throats relative to the surrounding atmosphere.  The depth of the pressure change, determines the rate of flow though metering orifices, as the surrounding atmosphere pushes the fuel into the carb throats as it tries to equalize the pressure difference.  Nature abhors a vacuum and will rush to fill it, using everything in it's path to do so.  In this case, fuel is part of what is in the path.
The intake stroke of the engine creates this partial vacuum.  Resistance to the inward flow deepens the partial vacuum.  The intake runner, carburetor venturi, slide, and air filter media all present resistance to the air flow, and thus, the deepening of the vacuum in the carb throat.  In this way a filter that presents less restriction to airflow does not increase demand for fuel from the engine.  However, it makes the existing fuel metering orifices deliver less fuel.  Increasing the orifice size is a compensatory measure.

Honda engineers determined all the settings in the carb to work properly with the engine's breathing characteristics, exhaust, and intake restriction.  Change these and you must find new settings for the changes you've made.  Changing air filters is not like changing the color of the bike.  If you don't enjoy the engineering set and adjust process, get the stock air box arrangement, set the carbs to book values, and enjoy the ride.  Otherwise, be prepared to tinker for a good while before focusing on ride enjoyment.

Cheers,

[SteveD CB500F]

Submitted by Aaron J Williams:

Superior and when he got back he told me that 1&4 carbs were dripping fuel. Since I had replaced sunk floats on 1&4 I suspected that the used floats I put in were sunk also or the float level was a little too high. I checked the floats and level but both were spot-on. I lowered the levels to see if I could stop the problem but the dripping continued. This morning I decided to put 2 different used floats in and see what would happen but when I removed the float bowls they were almost empty. A closer inspection revealed this:



There were vertical cracks up the side of both float bowl overflow tubes! I took some bowls off of parts carbs and one of them was cracked too. After replacing the bowls with good ones the bike no longer drips. So if you can't stop your bike from dripping no matter what you do, look for cracked tubes.

Some suggestions as to fixing this without replacing the bowl:

• Solder
• JB Weld

Anyone actually tried this with positive results to share?


Aaron came up with this:

Ok, here's what I came up with for a solution. I took a piece of shrink tubing for 18 to 20ga wire and cut it to 29/32nds and put it over the cracked brass tube. After shrinking it on I put it in carb acid for 1 day to see how the tubing would hold up. The acid swelled the tube enough to slide it off the brass but after a couple of hours it shrank back down and was tight on the tube again. I then put the bowl back on the bike and after sitting in gas for 2 weeks it shows no sign of degradation. I think this is a viable alternative to JB weld or soldering and it looks a lot cleaner too!

[Bob Wessner]

Submitted by Mystic (and others)

Not trying to sell anything, just thought I would regurgitate an old trick that went around the dealership I worked at in the 70's. Since we're about to have another gas crisis it just seems appropriate again. This is all from memory and was confirmed by myself. Don't know if it works with other bikes but it should.

I don't have parts in front of me now but it went something like this... Remove main jet,... Look into throttle bore and see a small brass thing sticking up,.. push down on it with a screw driver and it should fall out. Crawl around on knees till you find it and you will be holding the emulsifier tube. It has a series of holes drilled on opposite sides. Drill more of the same size holes between the existing ones, essentially doubling the holes. Do one at a time and put it back in the same way it came out. Compare to other carb to get the orientation.

This tube mixes air with the fuel coming up around the needle and makes it sorta foamy (or emulsified) Apparently it atomizes better when it enters the airstream as a foam? The main jet and needle/needlejet do the fuel metering. somewhere there's an air jet feeding the emulsifier area. None of these things have changed and no re-jetting is required,.. It just emulsifies better.

I did this on my 75 CB550 right before a 750 mile trip. Prior to that it got around 50-52 mpg. After the mod with no other changes it got 60mpg. Now I'm old and forgetful but I remember this because gas was 60 cents a gallon so it cost me exactly a penny a mile or $7.50 for the whole trip!!! I also remember my buddy had a CB360 and we got the exact same mileage so he was impressed!

Does anyone else remember this? The grapevine I heard it through was comprised of reputable mechanics and I heard that there was a Honda bulletin describing it but I never saw it.

Possibly it just corrected a rich condition at part throttle and will only help if your bike needs it,... so I'll go with the standard disclaimer. "Your mileage may vary"


More


I was curious about this myself so I did some googling and turned up the following:

From The Secret Life Of Carburetors:

The effect of the emulsion tube will depend on the hole pattern. Here is how to read it: First, hold the emulsion tube upside down and inspect the hole pattern. Holes at the top of the emulsion tube will affect the top-end of the rev range. Holes in the middle will trim the mid-rpm range, and holes at the bottom, the low-rpm range. Where there are no holes, the mixture will be rich. Where there are holes, the mixture will be leaned out. Just how much the mixture is leaned out by the presence of holes depends on how many, and how big. The more holes present, the more the mixture is leaned out at that point. Because it is fed with air from the air bleeds, the emulsion tube's overall function is influenced by the air bleed size. A larger air bleed leans out the mixture, but at low rpm and small throttle openings, the air bleed has little influence over the mixture. As the engine's demand for air increases due to an increase in throttle opening and rpm, so the air bleed's influence increases. At high rpm, just a few thousandths change in the air bleed diameter can have a significant effect on mixture.

One other aspect of the emulsion tube and well is that they act not only as a means of calibration but also as a control element for fuel atomization. By emulsifying the fuel prior to it reaching the booster, the fuel is easier to shear into fine droplets at the point of discharge. Generally, the more it is emulsified with air in the emulsion tube, the easier it is to atomize at the venturi.

From Rotary Engineering:

Emulsion tubes control the metered fuel and air introduced into the Carburetor.  When air enters the emulsion tube through the air jet and fuel enters through the the main fuel jet this condition emulsifies the fuel delivered into the carburetor.  The emulsion tube has a series of small holes from top to bottom which regulate the fuel mixture.  These holes allow air and fuel to enter the main circuit and emulsify fuel.  Low speed engine conditions or engines at idle do not require use of the emulsion tube or the main circuit.  As engine speed increases the fuel level in the float bowl drops uncovering these holes and allowing air from the air jets to enter the main circuit resulting in a lean mixture.  As the engine speed increases the fuel level in the float bowl continues to drop. This uncovers even more of the holes in the emulsion tube, which makes the air jet have a greater effect on the low to high rpm fuel delivery mixture.

From Pre-emulsion bleed formulas:

From memory, as the air and fuel are flowing at low speed, the air only enters the emulsion tube through the holes high up, but as speed increases, the air travels further down the well.

If the top holes are to big, to much air enters the tubes at the top at high speed and does not mix well thus giving poor fuel delivery quality, if they are to small, not enough enters at low speed, giving poor fuel delivery quality.

Also, the bigger the air leak into the emulsion tube via the air correction jet and the emulsion tube holes, the leaner the mixture as the pressure drop across the main jet is reduced and there is more air introduced to the metered fuel.

Interesting stuff.

mystic_1


More (HondaMan)

I would make a minor correction to the author from Rotary Engineering, as regards our carbs, though:

It's not the float bowl level that drops, in our carbs, but the level inside the tube surrounding the main jet holder (aka emulsifier tube). This is the result of the limited flow from the size of the main jet itself. The float bowl level does drop in real life: at engine speeds over 6500 RPM, the level drops about 2-3mm.

However, this 2-3mm would have little effect inside the emulsifier section, as the holes are spread out over about a 10mm length. But, the main jet itself limits how fast the tube will "fill back up", and it is this pumping action that defines these carbs as "pulse carbs" instead of "flow carbs", like found on cars. Each engine intake stroke sucks a little fuel out of the emulsion chamber (above 1200 RPM, anyway), and that level starts dropping as the RPM rises, exposing more holes, "bubbling" more the remaining fuel for atomization.

The impasse comes at wide-open throttle (WOT). These carbs run out of mixing ability above 7/8 throttle: that's the nature of the beast. This is why all bikes like these don't seem to "have any more" in the last 3/4 turn of the handle: the fuel level has reached the bottom of the emulsifier chamber, and the fuel is rising straight from the bowl to the needle jet, and not enough makes the trip: it runs lean. Raising the float level of the bowl helps a little, at the risk of leaking around the edges when running at more normal speeds. Our "standard" change for road race applications was to raise the float bowl 2mm in the 750 (24mm) and then seal the float bowls with new gaskets about twice a year. 

[SteveD CB500F]

TwoTired has written this up a few times; I guess it's time to put it in here 

Removal:
Remove completely the band clamps for the rear rubber couplers on the carbs. 
Loosen the band clamps for the front rubber couplers only on the carb end. 
The air filter box frame mounting bolts must be removed so the filter and air box can move rearward about 1/2 inch or so.
Remove throttle cables, disconnect bottom hoses and fuel line.
Disconnect engine breather hose and move it out of the way.
Work the rear rubber couplers off the carbs at the rear. 
Fold the bottom of them in on itself so the rear of the carbs may move down.
Twist the carb bank down in the rear and pull them back out of the front rubber couplers simultaneously.  The rear couplers will crush some with the band clamps gone.
Once the carbs are out of the front couplers work the bank sideways out the left side of the bike mashing the rear couplers out of the way as needed. The rear rubber couplers are thin wall, more compliant, and flop around way more than the front ones.  With them yielding, and the 1/2 to 3/4 inch gain from moving the air and filterbox rearward, you get just enough room the get the carb bank out.

Fitting:
Going back in is pretty much the reverse.  Using a wedge or tying the air plenum and filter box rearward eases installation.  Leave those rear band clamps completely off the bike.  Push the carb bank in from the left side and work them across,  almost ignoring the rear rubber couplers until the carbs are aligned with the front couplers.  Then wiggle them home into the front rubber couplers.
I use a dull pointed "L" shaped scriber to guide the rear rubber couplers onto the rear carb throat nipples in the same fashion tires are mounted onto rims.
Push the air/filter box forward and reinstall all the band clamps on front and rear of carbs.
Reinstall filterbox fame mount bolts.
Reinstall hoses.
Reinstall throttle cables.
Bandage knuckles. 

[SteveD CB500F]

Ever wondered if you can fit 750 carbs on a 500?  650 carbs on a 550?  ZX600 carbs on a ??

Thanks to Soos who has been out measuring:


1977 or 78 CB550 and PD series carbs (Soos)
depth (front to back)               -100mm
OD (engine side)                     -32.9mm
OD (airbox side)                      -38.1mm
spacing between #1&2 and 3&4 -75.8mm  (c to c)
spacing between 2&3               -81.1mm

Venturi: 22mm (unverified...)



CB500 and 627B series (Soos)
depth (front to back)             - 101mm
OD (engine side)                     - 33mm
OD (airbox side)                      - 38.8mm
spacing between 1&2 and 3&4   - 69.6mm  (c to c)
spacing bewteen 2&3               - 69.7mm


Venturi: 22mm (unverified...)



PD50A stock '79cb650 carbs: (Soos)
Type of carb                  - Mechanical linkage
depth (front to back)       -99.8mm
OD (motor side)              -35.4mm
OD (airbox side)              -45.8mm
#1&2 and 3&4 (c to c)     -76mm
between2&3  (c to c)      -80.9mm


Venturi: 26mm


PD50A stock '80cb650C carbs: (from manjisann)

OD Airbox Side: 35.46mm
OD Engine Side: 45.95mm
Front to Back: 100.13mm
Top to Bottom: 149.58mm (this doesn't include the overflow tubes or the cable control linkage)
Approx Center: 1-2 77.04mm
Approx Center: 2-3 81mm


"7A" '73(?) stock cb750 carbs: (Soos)
Type of carb                  - Mechanical linkage
depth (front to back)       -88.26mm
OD (motor side)              -36.95mm
OD (airbox side)              -40.08mm
#1&2 and 3&4 (c to c)     -76.3mm
between2&3  (c to c)      -81.5mm

Venturi: 28mm

VB44a stock '81 cb650 carbs: (Soos)
Type of carb                  - CV
depth (front to back)       -122.1mm
OD (motor side)              -39.8mm
OD (airbox side)              -53.8mm
#1&2 and 3&4 (c to c)     -66.3mm
between2&3  (c to c)      -92.7mm


Venturi: TBC


'85 kawasaki ninja (zx600 - A1 motor AKA GPZ600) carbs: (Soos)
Type of carb                  -  CV
depth (front to back)       -94.1mm
OD (motor side)              -38mm
OD (airbox side)              -50mm
#1&2 and 3&4 (c to c)     -67.5mm
between2&3  (c to c)      -91.4mm


Venturi: 32mm

If you have a set that is already listed, but yours measure differently post here .
Perhaps there are minor changes through the years/models that will show with information from enough people.

[SteveD CB500F]

The following is Copyright © 2002 Aircooledtech.com.  All rights reserved.
More on www.aircooledtech.com/tools-on-the-cheap/
Thanks to Toycollector10 for the lead.


Cleaning and rebuilding your carbs can be difficult these days.  The really good cleaners of yester-year are no longer available now that they have been deemed "bad for the environment" and outlawed.  The cleaners we could always count on are now effectively rendered almost useless.  Awe yes, I remember being able to buy a gallon can of carb-dip at the local parts store and it would strip decades of grime away over night with one 12 hour soaking.  Now the same brand barely loosens varnish let alone cleans it away with days of soak time. . .  You still need to get carbs clean, but chemicals today just can't do it alone and you don't want to spend an afternoon scrubbing all the nooks and crannies of your carb housings.  What's a guy (or gal) to do?? 

Blast them!!  "But wait", you say, "Blasting my carbs will fill them with grit that I'll never get out and my expensive carbs will be ruined."  The solution is to use baking soda as the media.  Yes, common, household baking soda!!  "But don't I have to own a blasting cabinet or pressure blaster in order to blast my carbs??"  The answer is NO.  Below is a list of the items you need in addition to an air source like an air compressor (and you don't need a large compressor either):

1 - air gun attachment

1 - two foot length of 7/16" clear vinyl hose

1 - one foot piece of wood dowel, metal rod or thick wire

1 - box of baking soda

1 - small roll of tape (masking or duct)

1 - sharp knife (or razor blade)

 

    You start by cutting your length of vinyl hose approximately 2-2.5 feet long.  Vinyl hose is available at home stores like Home Depot, Lowes, etc. for around $2.00 for a 10 foot roll.  Then measure from one end in about 2" and mark it with a Sharpie. 



Take your knife or razor blade and make a small cut across the hose ONLY through one side.  The cut needs to be just large enough so that the tip of the air wand attachment will stick into the hose.  Stick the tip of the air wand into the hose and let it stick in about 1/4" - 1/2". 



Positioning your wand tip like this will create a ventricle effect in the hose causing the soda to be pulled up from the box and mixed with the high pressure air from the wand.  Use one length of tape to wrap around the end of the air wand and hose.  This will keep the hose in the proper position on the wand tip during blasting. 



I cut the other end of the vinyl hose at a 45 degree angle to allow better flow of the soda into the hose.  Then I tape the piece of dowel, metal rod or wire to the other end of the hose.  This will help keep it from curling up in the box of soda and help you keep good flow into the hose. 



Now you have your ultra low-tech & cheap soda blaster ready for blasting!!



WARNING:    Blasting should ONLY be done outside in a well ventilated area such as a patio or concrete driveway.  Of course proper eye protection and a face mask should always be worn.  Baking soda is not poisonous but it feels like your breathing a soft drink if you inhale it and it's irritating, so protect those lungs!!

Baking soda will get all over you and the surrounding area, but don't worry, it won't harm anything.  Just wear some cloths you don't need to wear for a hot date later because they will be coated white when you're done.  Adjust your air pressure to about 80 - 90 psi on your compressor.  You only need to hold the blaster tip about 6" or so from your carb parts and begin the blasting. 

Soda is a soft media and will not harm the factory finish of your carbs.  It removes all organic matter from the carb bodies as well as heavy corrosion scale in aluminum.  It will not remove rust or corrosion from steel hardware; it only removes the scale build-up. You can freely blast into passages and the carb bowls since soda dissolves with water!! 

Once you are finished blasting, simply place all your parts into a pot of warm tap water.  The soda will dissolve into the water and leave NO RESIDUE behind.  Then blow out the passages with air just as you normally would and your carbs are ready for re-assembly.

The mess on the  concrete cleans up just as easy.  Simply spray it down with the garden hose to dissolve the soda.  NO caustic chemicals to seep into the ground water for future generations to deal with; this is a real "green" solution to carb cleaning and it's inexpensive too. 

Below are a few before and after photos of some rare Porsche 356 carbs that I cleaned with this exact low-tech blaster.  The results speak for themselves.  Total cost for this blaster is about $5 including the box of soda!!  So the next time you need your carbs cleaned, give this AircooledTech; Tools-on-the-cheap soda blaster a try and see what you think.  I'll bet you never go back to chemical carb cleaners again!!





This from mcuosso:

If you have a compressor, blasting cabinets are under $100 at a lot of discount tool places like Harbor Freight.  You can blast with soda for your delicates and than switch over to coal slag or aluminum oxide for rougher stuff.

If you are going to paint or powder coat any blasted surface here's the order to do things

1. Degrease
2. blast
3. rinse with Acetone or similar
4. Heat (to temp that part would normally see) if possible
5. Repeat steps 1-4 if necessary till surface is clean
6. Paint or powder coat

I don't know that paint on carbs would hold up.  And I would think you would need to totally disassemble the carbs to isolate the carb bodies for powder coating, since they would need to be heated to at least 375 degrees.

Check out the blasted pics in my gallery (I haven't done my carbs yet)...http://gallery.sohc4.net/main.php?g2_itemId=306724

and from Grumpol:

Vinegar is better to neutralise the soda if your going to paint, if there's any soda left embedded in the alloy you will never get any type of paint to stick.

[TwoTired]

....Or, why didn't they use POD filters like all the really cool racers have on them.

Here is a report/ analysis thread on air induction for the SOHC4.  Maybe someone can learn a bit from the thread.  But, I warn you, it's turn out to be long and has taken several days to assemble.  If you can, wait till the end to post in it, OK?

Ilust. 001
Item 1 depicts slow air moving in laminar fashion around a sphere.

Item 2 depicts faster air moving in turbulent fashion in the shadow of the sphere, and the air returning to laminar beyond the shadow position.

Item 3 depicts even faster air moving in increasing turbulent fashion downstream from a sphere , with the eddies rotating in counter rotation.  These eddies can differ in size and number as the speed increases.

When these eddies are present before entering or extend within a tube, they present a pressure difference applied to the tubing wall.  This pressure effects the amount of draw and fuel quantity through the jet effected.

[TwoTired]

A snapshot showing the turbulence in a fluid.
Two more snapshots showing the turbulence in a fluid.  Again what we are concerned about is this turbulence reaching into the carb throat.

[TwoTired]

Ilust. 004
A smoke trail showing laminar flow over a vehicle.

Ilust. 005
A depiction of air moving in laminar fashion within a tube.  Note there is little/no air movement at/near the walls.  However, pressure differences WILL be present in a turbulent flow.

Ilust. 006
Another representation of  laminar and turbulent flow within a tube.

[TwoTired]

Ilust. 007
When ducts change size (cross sectional area), the air tries to fill the space available.  When going from a small to larger size the air slows in velocity.  Abrupt changes (steps) create turbulence and drag.  When the diameter of the pipe is decreased abruptly, turbulence occurs both before and after the transition.

Ilust. 008
A slightly better representation of turbulence occurring after a stepped transition in an air duct.  Here they used the turbulence and pressure difference to atomize a fluid.  To make (I think) a smoke trail generator at high altitude.

[TwoTired]

Ilust. 009
Here is a CB550 Carb to intake runner coupler.  Note there is a stepped inner diameter change.  Did Honda make a mistake?  Won’t this cause turbulence?

Ilust. 010
Nope, when the coupler is installed on the manifold, the stepped portion matches the inner diameter of the carb throat.  The stepped portion also matches inner diameter of the manifold intake runner inner diameter (not shown).  This sure looks like Honda was determined to minimize turbulence and velocity loss within the duct.  As a bonus, the rubber step also doubles as a spacer to keep the carb metal from rubbing on the intake runner metal.  I really admire when a part can serve more than one purpose, don’t you?

[TwoTired]

Ilust. 011
An Inlet design to maximize air flow.

Ilust. 012
Another depiction of Inlet design to maximize air flow.

[TwoTired]

Ilust. 013
Honda inlet coupler.  Notice the ramped inlet?  Whatever velocity at the wide bell diameter, the speed will be increased as it transitions to the smaller diameter.  This velocity change helps convert turbulent flow back into laminar flow.

Ilust. 014
Honda inlet coupler with a stepped wall.  Looks bad until you see it mated to the carb in a later picture.

[TwoTired]

Ilust. 015
Here is the inlet coupler mounted in the air plenum and onto the carb inlet.   Notice how well the inner diameters match without any stepped edges in the wall connection?


Ilust. 016
Here is a POD filter.  While I think it is an old K&N, I can find no markings on it to verify.
Note two important design errors.
1 – There is no molded step to match the inner diameter to carb throat inner diameter.
This is certain to cause turbulence, with eddies reaching well into the carb throat.

2 – The straight wall section ends abrubtly with a sharp corner and irregular features.
This is also certain to cause turbulence, with eddies reaching well into the carb throat.

IMO it may look cool on the outside, but is certainly inferior on the inside.

A third point about this filter is that the filter media certainly does nothing to keep the air flow laminar.  In fact, it is a near certainty to cause turbulence right at the entrance to the carb and reach well into the throat.

[TwoTired]

Ilust. 017
I would much rather see something more akin to this.

Air filters by nature present an obstruction to laminar flow, be it paper or foam or whatever.  If air has to move around fibers or through tiny tunnels, the obstructive and redirecting properties need, at least change direction of air movement and induce turbulence.


Ilust. 018
From K&N website.

Ilust. 019
Also from K&N website.

[TwoTired]

What did Honda do about this filter turbulence?

 
Ilust. 020
The 550 Induction diagram.

Ilust. 021
The 750 Induction diagram

[TwoTired]

Ilust. 022
This is the induction system of a CB550.  It has the same elements found in other SOHC4s, but in a slightly different arrangement.  (I didn’t have any dismantled CB750 inductions to photograph.)
 
Ilust. 023
The inlet air horn located under the seat.  Note the ramped entry.
 

[TwoTired]

Ilust. 024
Here you can see the air horn feed the filter chamber on the left.  Air flows through the filter, and is certainly turbulent behind and near the big oval section of the box.  The clean air quiets a bit as it enter the oval of the air plenum on the right, where negative pressure directs the air to the next demanding inlet tube where the flow is straightened out even more.
 
 
Ilust. 025
There’s that nice molded inlet funnel again.

[TwoTired]

Ilust. 026
Here are the bits of the carb that you do NOT want to have turbulent eddies of higher or lower pressure to reach.
The slide needle jet exit in the center of the venturi.
The mains emulsion tube air inlet.
The Pilot system air jet.


In the 550’s, the distance from air filter and duct turbulence is about 7 to 8 inches.

No POD or filter placed at the carb inlet can offer that isolation or buffering from turbulent air flow.

I estimate the 750 path is a bit shorter , around 4-5 inches.  But it has a much larger filter, and still has those beauty air inlet bells to straighten out flow to the carb inlets.

[TwoTired]

Finally, a comment about K&N claims.

Ilust. 027
This is also from their web site.  An artists representation of what K&N wishes you to believe.  First 4 points I have no argument against.  The 5th, however says: “Bonded cotton/wire mesh straightens airflow reducing turbulence.”
Sorry, saying this doesn’t make it so, even if you pay an artist to represent a fictional desire.  Based on all the physics and science of air flow principles, anything you put in an airflow where each fiber or obstruction isn’t aerodynamically shaped to smooth airflow transition, you WILL get turbulence.  Round wire mesh and bonded cotton are by no means aerodynamic.
Brag 6 from the above advert states: “Straightened and filtered high-volume air enters the intake system.”
Huh?  The exits are all wavy or folded with mesh and fiber that induces turbulence.  Yet the air is somehow straightened?  How can people accept this baseless brag as fact?

I use K&N air filters on my 750’s and like them in the stock air box, where the turbulence they undoubtedly induce is quieted well before getting to the carb throat.  But, sticking any filter at the carb entrance for all around performance reasons, doesn’t have any science or engineering to support the practice.

Well, that's my report.  I hope you find it of some value.

Cheers,