IDF Transition and you...
Sometimes you can use different main and air jet combinations to get your transition straight, and to help with issues related to WFO mixture problems. For instance.. suppose you had some problems with weak transition, and you were running 11mm floats, and you felt pretty sure that the floats were dead on, and that fuel pressure and delivery were behaving. What can you do to make the mains come in sooner to meet the idle at the right time?
Look down the throat of the
carb at the auxiliary venturi. There is a tube with a nozzle in it, and there is
a port that feeds
the nozzle. The nose end of that port is
further out at the top, and two things are happening.. one is that the
difference in pressure of the air flowing down through there (the inherent drop
due to air acceleration) PLUS the fact that the nose end is further out at the
top, makes the pressure at the port opening lower as you increase air velocity
through the carb throat, right? Right. They're designed so that when the
relative amount of low pressure area around the idle circuit starts to taper
off, the port is developing enough vacuum under the nose end to start to suck
emulsified fuel up the well and draw it into the throat of the carb. When
sufficient vacuum work happens, the emulsified fuel mixture will start to flow
out of the port and into the throat.
Float height is important, period. It is important because it sets the height of the fuel in the float bowl, and hence in the jet wells. This level is critical for the things that happen with the emulsion tubes. They must happen at the right time, and there is a narrow window of float heights in which that works best.
Float height is important as a first thing to look at in getting good transition. Don't be fooled into thinking that raising the floats will solve all your problems. It might, depending on where your floats are currently set. If they are too low, it will be weak in transition and there will not be a lot you can do to fix it except set the floats right. The correct height for IDFs is 10-11mm and the correct height for DRLAs is 5-6mm, in both cases as measured from the cover to the furthest edge of the float assembly.
Too high is a bad thing too.. you start to defeat the purposes of the emulsion tubes, may even delay transition, or cause a lot of extra fuel spillover. I would never recommend to someone that they raise the float level to less than about 9mm.
How does transition actually start? Being familiar with the anatomy of IDF or DRLA carbs helps with understanding this. You have an idle circuit that is comprised of a mixture screw port, and a set of progression ports along the wall of the barrel. They are all exposed by the time you have pressed the gas pedal a small fraction of it's total travel. This gives you a smooth low speed operation and happens while the throttle is open enough to rush air past the progression ports, causing an amount of fuel to be delivered to the engine. The idle jet size determines this mixture strength once you crack the throttle, until the point that you have exposed all the progression ports.
By the time you have exposed all the ports, you have reached a throttle position that is open enough that two things are starting to occur:
1. When the throttle plates are just opening.. the majority of the air flowing past them is passing by the walls of the cylinder. The first progression port is exposed in a situation where a lot of air is flowing right by the progression hole, and the resulting turbulence pulls fuel from the ports. The more you open the throttle, the more ports are exposed, BUT, at these angles, more and more of the air is passing far away from the port holes. This means that more air is now doing less work to the progression ports to pull fuel out. Somewhere in this effect, you will no longer be able to support the needed air-fuel mixture on the idle circuits, so you need to get more fuel some other way.
2. As the plates open more and more, you increase the airflow through the carburetor, and the increased rate of airflow eventually reaches a point that it does substantial work on the venturi. The venturi is basically a mild restrictor, and the result is that as air is pulled down with a specific amount of force, the air will be accelerated through the venturi. As this happens, the pressure of the air develops a gradient along the length of the venturi.. such that there is the relative high pressure region (above the venturi) and a relative low pressure region (typically somewhere right under (downstream of) the minimum diameter region of the venturi. Note the height position of the auxiliary venturi tube. The top of it is above any region that would have a venturi effect, and the bottom edge of it is close the the minimum diameter region of the venturi. The auxiliary venturi experiences a net flow of air through it, as a result of the inside of the tube being isolated from the area where the pressure gradient is changing the most. The pressure difference in turn pulls a suction on the port in the auxiliary venturi, sucking fuel out.
What we hope for is that the main circuit is there to cover the area where the idle circuits are starting to taper off in effectiveness.
If you pull out a jet stack and shine a light down the hole (i.e. the fuel wells), you will see gas in there. If the float bowl is full, that fuel should be about 4mm below the port in the jet well that leads out to the auxiliary venturi. That gas is at the same height as the gas that is sitting in the float bowl. It takes a certain amount of work to draw the fuel from it's resting level in the well to the height of the aux vent port. No matter what main jets you have in there within reason, the air jets, vent size, engine speed and the throttle position largely control when the main circuit wants to come on, for a given float height.
The air jets, main jets, and emulsion tubes form a system that introduces air into the fuel in the jet well. Liquids have a surface tension property, and one of the effects that can be observed is one that causes the liquid to attach and pull itself up along the walls of a container. If you've ever put water in a test tube and read the quantity of the liquid by way of the meniscus, you know that the liquid does not sit level in the tube. This is also true in the jet wells. But the introduction of an inner tube inside the jet well (emulsion tube) can make the fuel leech up between the emulsion tube and the jet well by about 2mm or so. The fuel that is resting in this area is pretty resistant to going anywhere due to the same surface tension that has drawn it up into the area. But it's closer to the aux vent port when the tube is in there, and so half the work is done. The other half of the work has be be done ON the fuel, by the auxiliary venturi's inlet port.
Since the fuel is resistant to being pulled up any higher than the surface tension has affected, we have to do something that will "mobilize" the fuel. The early action of the auxiliary venturi's inlet port isn't enough to physically do the work on the fuel that is necessary to pull it up the well and into the port.
AHA! Well, news flash...the fuel is mobilized by way of the air jet and holes in the emulsion tube. The air drawn down through the jet stacks, into the emulsion tubes is used as a control to mobilize and to put on the brakes for fuel consumption through the full range of the main circuit. The AIR jet size affects how easily and how much air is fed to the emulsion tube.
When the fuel mixture STARTS flowing at low velocity (when the main circuit is
just kicking in) the amount of flow through the main jet does not really depend
a whole lot on the size of the jet.. but as there is a higher flow demand, it
does matter. As the flow rate goes up, relieving pressure difference (fighting for
equilibrium) in the port becomes a contribution between the air jet and the fuel
jet. Their sizes RELATIVE to each other will start to matter more as the flow
increases. But when the mains JUST start to kick in, the amount of vacuum seen
at the port is the same, no matter what jets you have in there, until flow
increases and the dynamics of that duo of jets really starts working.
In order to bring the mains in sooner you need a LARGER air jet. Seems sort of backwards doesn't it? Well I learned my lesson on this the hard way. Why does it work this way instead of sucking more fuel when you reduce the air jet? Because the fuel that is reaching the inlet to the secondary venturi is fuel that has been introduced into the two phase flow (emulsified). The larger air jet pulls air down into the fuel well easier and so it takes less to emulsify the fuel, break the surface tension, and pull it into the inlet. Prove it to yourself. Put smaller air jets in there and watch your transition get weak. Put larger ones in and watch it smooth out.
What you want is the smallest air jet that will allow you to get transition with no dead spot. It's more of a struggle with larger venturis due to net airflow through the 4.5mm aux vent compared to the amount of air that goes through the whole main venturi. With larger vents and especially with larger butterflies, you will start to move enough air, far enough away from the progression ports, that the idle circuit will die out before a lot of velocity and vacuum reaches the aux vent horn, so you need to make it more sensitive. With larger air jets, you can do that.
There is a point at which an emulsion tube swap starts to make sense too. With a 40 vent on a 48 IDF, it's a good time to put in an F2 emulsion tube in place of an F11. With the F2, your mains and airs will be a little smaller to get similar effects.
Differences in emulsion tubes will be discussed more as I have time to write about them. They don't seem to be widely discussed in books, and the diagrams require some understanding about what's happening to interpret. Even a lot of Weber gurus don't bother trying to understand the dynamics.. just knowing when to change to a different one depending on what's going on seems sufficient for most of us.
The other thing you can do to help is raise the float levels but only do it a millimeter at a time, or even less. You want the floats about as close to 11mm area as you can get them and still have good transition. As the float levels increase, the distance from the fuel to the aux vent port will decrease, and less work will be required to get it up and out the port. This means transition can come earlier. But there is a limit and that limit is dependent on tolerances and carburetor application. A lot of buggy guys keep the floats lower to keep slosh down, and just run fat ass idle jets and main jets. Some just deal with lean transition as a result, and some deal with some richness under certain conditions.
So you have a few things at your disposal to do your tuning.. the problem here is that now that you have the idea you need a way to read your results. Save the pennies and get a wideband air-fuel meter. It's worth it. Once you have it there is no looking back. You'll have your carb tops off so many times your hardware will all shine like new. You'll have your jet stacks out so many times you can do a jet change in 2 minutes or less. It's annoying but at the same time, fun.
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