Ok, I kinda have to disagree just a bit. I understand what you are saying. But under that logic...... when you raise the slides by twisting the throttle you lower the vaccuum (larger opening), thus allowing the engine to suck LESS gas, resulting in a leaner mixture?
?
That is exactly right... until the air velocity increases to lower the pressure and draw more fuel through the jet orifice. Fuel is pushed through the orifice by the outside baro pressure only when the carb throat pressure falls. Watch your vacuum gauge when you crack the throttle open. The vacuum level rises closer to the outside baro pressure. This is why carbs are often fitted with and accelerator pumps to enrich the mixture for engine pickup. These old carbs are tuned at idle with an over rich mixture (and bonus fuel in the emulsion tube reservoirs) to provide reasonable pickup when cracking the throttle. It is also the reason they are no longer used on modern vehicles as they cannot achieve the low hydrocarbon emissions at idle mandated by today's emission standards. When these non-accelerator pump carbs are properly tuned, you cannot snap the throttle open from idle position and get engine pickup.
Carbs don't work by sucking gas out a jet by way of engine vaccuum directly.
Of course they do. That's why blocking off the intake with a choke butterfly enriches the mixture. More vacuum is created in the carb throat. And, the outside baro pressure pushes more fuel through the active jet orifice due to the larger pressure differential. The carb bowls are vented for a good reason. It is the pressure on the surface area of the fuel in the bowls that pushes the fuel through the jet orifices. But, only when the opposite end of the jet is experiencing lower relative pressure. The more the pressure differential, the more fuel flows through it.
Engine vaccuum draws a given volume of air through a tube (the carb body). The carb body is restricted in the center by shape, and by the slide. In order for this volume of air to squeeze through this opening it necessarily speeds up, which lowers its pressure (Bernouli's principle of aerodynamics). Its this drop in pressure over the jet nozzle which draws fuel up from the bowl. More fuel atomized into the airstream increases rpm, which increases the airflow, which in turn draws more fuel. The vacuum of the engine created on the intake stroke is what causes velocity but only when the carb slide is raised.
I was with you until that last sentence. Raising the slide also raises the baro pressure in the carb throat reducing fuel jet orifice flow until the engine picks up revolutions, then velocity through the venturi increases. You may be wrestling with transitional behavior vs static or stabilized speed behavior.
So more fuel is only drawn up by more airflow, maintaining the fuel/air ratio...... am i wrong in this??
No, not necessarily. However, carb bore pressures are not ONLY governed by air velocity. What we have here is four independent firing cylinders connected by a common crankshaft. Try to imagine the crank driven by external means with none of the cylinders actually firing. All the carburetors will still deliver an air fuel ratio. Engine vacuum is still being created in direct proportion to the crank RPM up to the limits of volumetric efficiency. When the carb slides are nearly closed, vacuum will be the highest in the intake runner and go higher as the crank speed increases. Air flow is very low in this regime, however the mixture ratio will go much more toward rich as the differential pressure across the jet orifice increases. When you suck on a straw in your beverage, don't you get more fluid when you suck harder? Bernoulli's principle doesn't apply to the straw case.
Open the slides and carb vacuum will diminish as the intake valve face is now more easily connected to the outside atmosphere with less of a barrier for equalized pressure. However, the velocity of the air will also increase the pressure differential seen by the jet orifice. The fuel RATIO equation is complicated by the slide needle taper which also allows more fuel to pass relative to throttle slide/needle position. However, at any given throttle position, the faster the engine is spun, the deeper the differential pressure is generated, and the higher volume of fuel will be delivered to the cylinders. It's combinatorial, engine vacuum and its effects on air velocity.
You may have noted that a lot of people need jetting changes when they simply change the intake filtration. Same carbs, same engine volumetric efficiency. The engine does not create any more velocity through the carbs at 1K or 2k rpm than it did before the filter change. However, the intake channel restriction DID change. As well as, the depth of the baro pressure in the carb throats. Therefore, jet orifice sizes need to be changed to compensate for the loss of vacuum now present in the carb throats. Generally speaking, the less restrictive the air intake, the larger the jet orifices for the same RPMs as before the filter change. The actual flow through the larger jets ought to be exactly the same as the original jets with the original air filter, at least for RPMs below WOT.
Cheers,
