Increasing your gas mileage.

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”
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.


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.