Nothing shows up when I search, so I figured I would start a discussion on the wizardry of emulation tube design. Or rather, see if anyone can answer a few questions I have about emulation tube theory.

1. Which holes affect which rpm range?

I have encountered conflicting information on whether the bottom-most ports affect high rpms or low rpms. My gut instinct says bottom holes are high rpms, because the fuel level in the emulation tube should be about equal to the level of fuel in the bowl. But I have also heard that high rpms increase depression across the main nozzle, and draw fuel up the emulation tube.

2. Is fuel normally moving up through the emulation tube, itself? Or around the emulation tube, through the same bore the emulation tube is in?

Once again, I have found conflicting information in this regard. I have even seen it said that fuel between the emulation tube and the inside of the bore is unmetered fuel, and a larger diameter emulation tube [that fits tighter within the bore] mitigates this aforementioned unmetered fuel.

I have more questions, but we will start with those.

 

Uhhh... don't you mean EMULSION TUBES??? They help the gas to better EMULSIFY into the air.
Emulation is the act of imitation of something....

 

Yes, that. My mistake.

I cannot, for the life of me, make any sense out of the Weber Emulsion Tube charts/diagrams. I am hoping having some really basic questions answered will help put a method to this madness. Some are bored all the way through, some have holes at angles [other than perpendicular]...

The F20, for example, is listed for both high rpms and big jets/alcohol. So is the F8. They are nothing alike in their respective section drawings. I have even heard of people drilling F11's all the way through... there has to be a reason Weber goes through the effort to port these tubes and contour their exteriors a certain way... and more people than just Weber's engineers have to understand what this all means. Even books on carburetors seem to barely touch on this subject, though.

 

Emulsion tubes are mainly used in Weber, and its copy, Dell'Orto carbs. I haven't seen them many other places.
It appears they sort of bubble air thru the gas before being sucked into the airstream. And the science on them is elusive at best.

 

I don't know $#!+ about carbs, and I generally regard it as stone age BS tech. But I like to tinker, and I like to learn, so carburetors have a certain intrigue. They offer opportunity to both tinker and learn.

My test mule for this is actually a little Honda GCV160 push mower, and its little carb happens to use an emulsion tube. The GCV190 carb has a different part number for its emulsion tube, but they are interchangeable. I have already ordered a larger main jet, and will order a larger pilot jet when I get around to it. Then I will just turn up the governor until it purrs like a kitten.

This pointless project of mine has sparked my curiosity on the broader subject of emulsion tubes, in general, and I was hoping to glean some understanding so I would "know" what I am looking for... like if lots of holes at the top or bottom is best for high rpms. Lawn mowers, in my particular case, run pretty much all out, all the time... so I don't much care about mid-range transitional behaviour... I just need to know basically how to identify if any random emulsion tube would be better suited for high rpms due to the position, number, and depth of its ports.

 

I'm not sure it is a very complicated concept. Especially in a lawnmower, something no right thinking engineer wants to over-complicate! lol
I think using an emulsion tube in a lawnmower carb is a basic attempt to encourage more rapid and thorough vaporization of gas to ensure FULL vaporization by the time it hits the combustion chamber, which in turn allows leaner jetting for cleaner operating. Honda, being their conscientious selves are just using as much simple science as possible to stay ahead of the small engine emissions game. Some of the larger V-twins are using an extremely simple form of EFI towards these ends.

 

I can see Honda using it for emissions/efficiency reasons. Any mixture has to be better than no mixture, and if it allows you to use smaller jets in a production piece, then I can see the draw in its use... so long as the part doesn't increase production costs.

This is one of the better explanations of emulsion tubes I have found. And it seems to reinforce my belief that the lower holes would affect high rpms, as they [the holes in lower positions] would be exposed as fuel demand increased and the fuel level in the bowl drops.

High Power Media wrote:
"... The essential points to remember are that the fuel passes around the outside of the emulsion tube at its base while air, coming from the top, comes down the inside of the tube and is extracted through a series of drilled holes of varying sizes and heights to mix with the fuel on the outside. When the engine is stationary, the fuel level will be the same as that in the float chamber, and will come to a level within the emulsion tube. As soon as fuel is demanded, the fuel level will drop in the chamber, uncovering more holes that will allow more air to mix, thus leaning the mixture. As well as altering the size and height of these holes, it is also possible to alter the diameter and thickness of the emulsion tube in its cavity within the body of the carburettor. This acts as a restriction to the flow of fuel which, when all fashioned together, can tailor the flow of fuel more or less precisely to that required by the engine throughout its operating map. Under wide-open throttle acceleration the emulsion tube plays little part since the overriding effect is that of the main and air correction jet. At part-throttle however, when the quality of the fuel atomisation arguably has to be significantly better, the emulsion tube can be considered more critical. Understanding this and being able to apply it in practice is therefore one of the dying ‘black arts’..."

 

Good stuff David! I agree, about as good an explanation as any I also have found. British, too by the two "t"'s in carburetor and "s" instead of "z" in atomization.
Thanks man!

 

I have learned a little bit more on this... emulsion tubes, that is.

There is a controllable restriction between outside diameter of the emulsion tube and the inside of the bore inwhich the emulsion tube resides. The bore's diameter does not change, but the outside diameter of the emulsion tube can change vertically along its length. Where there is greater tolerance between the emulsion tube and the wall of the bore, there is more fuel available to immediately be drawn into the venturi when the throttle opens more. This is metered fuel that is already past the main jet.

Where the tolerance is less, the restriction more, fuel does not build up and is not immediately available. The ability to control which level vertically along the emulsion tube will have more or less restriction [thus less or more fuel, respectively], allows you to control the rate fuel is flowing past the ports in the emulsion tube at those levels. So not only can you control the size, number, and location of the holes... but you can also control the amount of fuel, and rate that fuel is flowing past the holes in the emulsion tube.

The location of the holes does several things. Mainly, it leans the fuel at relatively specific RPMS, by adding air to the fuel flowing from the main jet before it reaches the venturi. The RPMS at which the air is added to the fuel is determined by the holes' position(s) vertically along the emulsion tube. Holes at the top of the emulsion tube, nearest the venturi, affect lower RPMs. The inverse being true of holes lower on the emulsion tube, nearest the main jet, affects higher RPMS.

Another important function of the holes' locations is the timing at which you switch from the pilot circuit to the main circuit... holes close, or equal, to the same level as the fuel in the bowl will make the switch the soonest. Some emulsion tubes purposefully do not put holes at this level specifically to delay the switch from the pilot circuit to the main circuit.

Hope I explained that correctly and the information is more helpful than confusing.

 

Looks like you explained it pretty clearly!
I can almost accuse you of "emulation" of a carburetor expert.... LOL

 

There is a book on performance tuning Webber carbs ,,
Mandatory if your doing Webbers most of what works on a Holley or Carter, doesn't even apply to a webber.

Been years since bothered with a carb but had a 69 911 years ago and also tuned friends cars..
Was funny how often a local mechanic had totally screwed them up..

I wasnt an expert by any means but I could get them to idle and run on teh highway correctly. LOL

Oh and the progressive 2 barrels, have little in common with DCOE/IDF etc..

 

I think Webers (only one "B") are the coolest carbs to be seen on any engine.
Closest thing to fuel injection without being there! One throat/cylinder, and, oh yeah!, they make full use of emulsion tubes.
Progressives ARE different, but still do a great job of carburetin'!

 

Individual carbs are beautiful. Synchronizing all those carbs would be... fun? I'm sure there is a word for it, but "fun" probably doesn't qualify. Tinkerers will tinker, though.

Anyways, my only realy experience with Webers is their two-barrel carbs, downdrafts, maybe? I don't know, they are found on old Toyota(?) trucks and need to be plugged and ported to go offroad... I think others might do similar modifications to these Webers for PCV or anti run on features, but we rednecks just wanted the carb not to flood when encountering extreme angles. Kinda of like extending the float bowl vent or adding anti sloshing baffles to a Holley. And I think there was something we did with Quadrajet floats, too.

But all that was back in high school metal shop modifying crappy trucks to play in the mud. Now... now I am much more interested in being able to produce a fine tuned machine that being able to drive upside down. I am most interested in how to tune the timing of the circuits... like how the size of air bleed a circuit has determines the strength of the signal, and can thus advance or delay that circuit. I'm even thinking about drilling and tapping all the air entry passages for removable air bleeds, just to gain control of that timing. Tinkerers will tinker.

The emulsion tube somehow can affect the timing of the circuits, but I am not sure exactly how this occurs. Like WHY do ports in the emulsion tube at the same level as the fuel in the bowl make the switch from the pilot circuit to the main circuit happen as soon as possible compared to other locations? This unanswered question is what sent me down the rabbit hole studying emulsion tubes. I was just studying the timing of a carburetor... everyone understands fuel jets... that's easy to get right. Or at least get right for that particular day and time of day when you finally got it running right... carbs are stone-aged junk like that. Lol.

But changing an air bleed to change WHEN the circuit engages or transitions... getting that timing right is what intrigues me. I appreciate a well-timed machine... had my 1911 timed by a gunsmith, and the slide returned, the barrel locked, and the sights leveled at the exact same... every shot. It was freaky. It was awesome. It was hard to believe it was actually the same machine as before. That is what I want to be able to do to a carburetor.

 

Trying to develop an all-position/attitude carb would, to my limited knowledge, be impossible, due to gravity being one of the restraints imposed upon proper carburetor operation. Even the downdraft Weber had to be predominantly upright for proper operation, and the side draft predominantly horizontal for the same reason. One MAJOR (but far from ONLY) reason for fuel injection's domination of the fuel disbursement bizz nowadays...