Topic: 60 mpg with a CB550!

[kayaker43]

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, esentially 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"

[HondaMan]

Check my FAQ: that same procedure is outlined there (minus the best part, the "part hunt"  ). And, yes, it does improve MPG, as well as lowering octane requirements, and reduces the "suddenness" of the throttle on the 750 at small throttle openings. 

[Buber]

Ok, this sounds really good, and logical. So, what can be the negative conswqueces of e.g. drilling too many holes? Or why factory didn't do it? Or why there aren't "aftermarket" solutions (mayb ethey are, but i didn't found them)?

Just it sounds a little too good to be truth. Such a simple operation, and such a gain. I will gladly do it myself, but just want to ask for "advocatus diaboli" opinion.

Thanks!

[mystic_1]

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

[HondaMan]

Excellent find, Mystic!

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 mainjet 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 mainjet 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 roadrace 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. 

Moderators: can we request to have Mystic's find added to the FAQ - carbs section? It's excellent text.

[Bob Wessner]

Moderators: can we request to have Mystic's find added to the FAQ - carbs section? It's excellent text.

Done.

[neil young]

i learn more on this site in an hour than any other site after days of searches.
nice job Mystic 1
i will do a search on Hondamans info.
but i was wondering how you would secure the emultion tube to drill it.i imagine they are pretty fragile.and obviously you have  to apply a bit of pressure when drilling them.and i dont think holding it with 2 fingers is a good idea 

[mark]

I'm thinking that a block of wood with a notch in it would do the trick...



edit: It did. The stock 550 has 5 rows of holes about .035"(#65 drill fit easily - #64 was too big). I added another set (roughly) perpendicular to the first.



.....now if the weather would just warm up enough to try them out properly!

[andy750]

Can we have a photo of an emulsion tube with extra holes added? This would be very helpful to go with the text.


**EDIT - found it in the Hondaman FAQ - always a great read (after several times I still pick up new things!) ****

Photo is here....

http://forums.sohc4.net/index.php?topic=7401.15



Great info!
Thanks
Andy

[patpollin]

just when I think I've got my carbs assembled and running for good for the season, I come across this- can't wait to try it, thanks for the tip

[neil young]

I'm thinking that a block of wood with a notch in it would do the trick...



edit: It did. The stock 550 has 5 rows of holes about .035"(#65 drill fit easily - #64 was too big). I added another set (roughly) perpendicular to the first.



.....now if the weather would just warm up enough to try them out properly!

right on Mark.
you dont piss around eh.

[mark]

ha ha. I cheated.

The carbs are already apart. Have been for months.

I'll toss this one in as it seems relevant.....

My emulsion-tube-pusher-outer tool. A scrap of acrylic after a touch of the grinder. The rounded end pushes the tube down flush with the bore - it will usually drop out from there.


Happy trails.

[Buber]

Thanks guys! Good info!
Actually I did it last saturday - took me maybe 2 hours, and most of it was getting the carbs in and out. Anyway I had it easy to hold - in CB650 th emulsifier tube has a hexagonal head (no7 spanner) , so i just had it in the... whachamacall in eynglissh? Vice, there you go. So, in the vice it went, and being soft yellow metal, drill went through without any problem. The we went for a ride (a beautiful one, as spring has sprung!), and at the end my feeling is that bike is maybe more responsive, although it may be superficial - it ran good in the first place anyway. At the end I looked at the plugs - no plug-chop, mind you, just look at them at the end of the ride. And? They are darker.... They were really white (I think condidtion was leanish), and now its proper milky chocolate, light tan colour. BTW, having carbs out, i took out and cleaned pilot jets, and moved them around some for mobility sake - it's a bummer to regulate them when in the engine  , and i left them leaner - 1,5 turns instead of 2 turns of factory, as i was thinking that with more holes it will draw more mixture, but, mixture is regulated by the main jet anyway? I left them at 1,5 turns, as I'm trying to get best mileage. So, all in all, nothing bad happened, I feel better about whole thing, and the most important thing - mileage.... will be known when i go through full tank of fuel  as this only will be a good indicator. I will report, promise.

And for the order sake - I have a european cb650z 50hp, with CV carbs. I had 2 rows of 4 holes (2 sides, vertically) and on the top of the tube,  4 holes on the same level. I added holes, so i have now 4 rows of 4 holes. I hope it makes sense... 

P.S. Today I'm going to buy some hex, or anyhting-else-than-philips screws to replace all of those on the carbs... the fight you can have with some of them.. ehhhh, you know the drill.. 

[HairyPotter]

I can't find the small drill bits to save my life. One of the drawbacks to living in this one horse desert town. Any suggestions on where I might find these small bits to drill out my emulsifier tubes?

HP

[super pasty white guy]

Available in many places, but this comes to mind first.  Search for pin vise.

http://www.micromark.com/

[HairyPotter]

Thanks...
Now can anyone tell me where I can find a replacement emulsifier tube for the same bike. I bent mine over tightening it. I've gone to some of the honda parts places, but the parts fiche are pretty broken up and I'm not sure what they call that part. What should I order?

[cb650]

Last tank i got 60 mpg anyway.   A few weeks ago wandering on dirt roads it went to 66.  80 650 and used to do the same on my 81.

[HondaMan]

Thanks...
Now can anyone tell me where I can find a replacement emulsifier tube for the same bike. I bent mine over tightening it. I've gone to some of the honda parts places, but the parts fiche are pretty broken up and I'm not sure what they call that part. What should I order?

I've seen it called "mainjet holder", "needle jet", and a host of bad words. Look at the fische here to find your part number for your dealer: www.hondaparts-direct.com . They show the Honda parts lists with old and new numbers, makes life a lot easier.

[cleveland]

I get 35 mpg on th HWY.  So any increase would be great!  Thanks for th info.

[HairyPotter]

[/quote]

I've seen it called "mainjet holder", "needle jet", and a host of bad words. Look at the fische here to find your part number for your dealer: www.hondaparts-direct.com . They show the Honda parts lists with old and new numbers, makes life a lot easier.
[/quote]

I looked there. The diagram for the carburetor is a little hard for me to discern. I can't tell what the part is exactly that I need to replace. That's why I was hoping to find out what Honda calls it.

[super pasty white guy]

looks to me like it's the "needle jet set" item #16012-323-004

http://www.cmsnl.com/img/needle-set-jet_medium44411.jpg

http://www.cmsnl.com/honda-cb500-500-four-k2-us_model461/partslist/E++22.html

[HairyPotter]

looks to me like it's the "needle jet set" item #16012-323-004

http://www.cmsnl.com/img/needle-set-jet_medium44411.jpg

http://www.cmsnl.com/honda-cb500-500-four-k2-us_model461/partslist/E++22.html

That's a very cool site. I like how the you can pull up a picture of the actual part from the schematic.

I have to make a correction here, and It appears I'm on the wrong topic, but I'm running a '76 CB750. The Emulsion Tube Mod Discussion has sort of been talking about both the 550 and th 750. My emulsion tube doesn't appear to be included in "the needle jet set". This is very perplexing.

[super pasty white guy]

Ah.  That'd be #22 the "Needle Jet Holder" then.   Apparently not available from honda, at least for a '76 F.  You'll have to shop around.  You have a f or a k?

http://www.cmsnl.com/honda-cb750f-750-super-sport-76-us_model490/partslist/E++18.html

[HairyPotter]

I have a K.

[mono]

I'd really like to try this for the upcoming riding season if anyone has shown any [recent] proven success with this method.  The articles seem to make sense about emulsion/air mix/etc, but it also sounds like hole placement can be important (wow, i just said that).

Any updates, guys?