What causes the mains to kick in on a set of Weber IDFs?

[71ghiavert]

The title says it: Just trying to learn what causes the main circuit to come in on a set of Weber’s? Is it how far the throttle plates are open? A signal from the amount or speed of air passing thru the carb? Something else? Does it vary, say, according to the cam, heads, valves, timing, venturis?

Yep, they’re a mystery to me. I think I’ve managed to tune them and get the jets OK, but I’m still working to understand what’s going on in there. Thanks.

[modok]

[67rustavenger]

[Slow 1200]

[Dale M.]

Want to know more about your Webers...

https://vwparts.aircooled.net/Weber-Tuning-Manual-By-WEBER-p/weber-tuning-manual.htm
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[sled]

well, to simplify things, and to actually answer your question specifically, it is Bernoulli's principle.

air speed is partly correctly, because air speed causes the PRESSURE DROP, which is actually what causes the mains to come in. As the air speed increases through the throat of the carb and speeds up through the main Venturi, it in turn, even further accelerates the air through the auxiliary Venturi, creating a negative pressure area where the main nozzle resides. Because the fuel in the float bowl is subject to ambient air pressure (14.696 PSI at sea level), the fuel is actually PUSHED through the main circuit into the auxiliary Venturi as there is a significantly lower pressure zone there.

same goes for idle circuit and progression circuit


and yes, how this acts in a carburetor is dependent on engine specifications because each engine will create necessary vacuum (pressure drop) at different strengths and at different rpms.

all else equal, a smaller main venturi (choke) will cause the mains to come in sooner than a larger, because for any given rpm the smaller venturi is increasing air speed sooner and thus creating a lower pressure zone at the bottom of the auxiliary venturi.

now WHEN and how STRONG the mains begin to tip in and control mixture, is another and considerably more complicated topic
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[nsracing]

It is all of the above -

The carburetor was invented long ago - but as simple as it looks, it anything but.

There are pressure-differentials above and below the throttle plates. That is how the air moves is by this pressure differential. If the above and below are equal in pressure, there is not any air movement.

There is a restriction or choke point -venturis - to create this pressure differential. Bernoulli guy's area.

As the air moves thru the carb throat, someone invented fuel circuitry to introduce misty fuel along the air stream. YOu have the idle area, the mid-range area, and top end area...to introduce fuel. I idle area will die out as the mains take over and the engine speed increases...dependent on the throttle plate position.

you can actually hear the mains take over - this pronounced swooshing as you crack the throttle plate open. Webers' mains come on as early smidge past 1000 RPM.

Just get the WEber book - and memorize it.

[71ghiavert]

OP again.

Bernoulli, huh? Hey, if he knows so much, how come he never comments on Samba? That said, I bet his Webers are real Italian.

Thanks for even more detailed info. It all DOES seem like an adventure. I switched exhausts today--from a VS Sport to a VS143 Supersport--and, if anything, the mains now seem to come in just a tad later. I shall have to sit in the full lotus position for a while and see if I can figure out why that would be.

But the sound is GREAT. Quieter, smother, and slightly higher pitch. Especially when it winds up a bit.

[modok]

The idle/transfer circuit is pretty simple, holes open as the throttle sweeps past, but once you are BEYOND those..... then the mains will ether work or not work or kinda work.
but IF they work right depends on the "carburetor signal"
Which has to do with a heck of a lot of things.
Compression ratio is a big one, intake manifold length and volume, and also what the exhaust system is doing.
yep airspeed at the venturi is the main thing but it's not just the average speed it's the PEAK speed that will bring the mains to life or not.