Hi, Don!
This is an everlasting problem, as long as one does not go to the bottom of the issue… flow rules.
It is not a thing depending on the linearity of one dispensing “organ”.
The task is to provide “equal opportunities” for the two gases to be dispensed in a desired proportion, independent of the mixed (resulting) flow volume.
It should be absolutely clear, that the two “arriving” gas streams in a traditional setup have COMPLETELY different “flowing opportunities” at cranking, idle and WOT settings of the common main throttle.
The “arriving” secondary air intake is just depending on the air-flap setting, but the “arriving” gas-line has never a constant gas resistace.
Putting together these two flows at cranking, idle and up to WOT is a constant play of cut and try, more or less.
Above the main throttle, the for the moment prevailing underpressure is always set by the momentary gas-line resistance and motor consumption.
So, the air-flow will increase with increased gas-resistance, and vica versa! The more fluctuancies in the gasline, the more variation in the the resulting gas-mix.
These pressure drops of “arriving” air and “arriving” gas above the main throttle flap are relatively smal in comparison to the vacuum under the main throttle, at idle and part load.
This is the picture of “traditional” T- suspension.
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BUT, if we use WHOLE the pressure drop down to the manifold for the gas and the air INDIVIDUALLY, then this (part load) vacuum in the manifold will make the “arriving” and fluctuating gas pressure less significant, compared to the “arriving” air pressure.
And the simple rule for equal (or proportionate) flow:
IF TWO GASFLOWS WITH EQUAL PRESSUREFALLS FLOW THROUGH IDENTICAL PASSAGES (HOLES), THEN THE FLOWS WILL BE OF EQUAL MAGNITUDE
This, in all simplicity means that we suspense gas and air through identical tubes (area and lenght) with identical flaps (ganged) to a swirrel mixer and then to the manifold.
Cranking a 4-cycle motor with closed flaps, with no “leaking” under the flaps from the crankcase or idle slotting, can generate a vacuum of 2 - 5 m H2O. This is a completely dominant vacuum in comparison to the gas system’s resistance-vacuum.
Already a half meter of H2O is totally dominant at cranking. This means that the ganged gas and air flaps can have only a minute opening to maintain the dominant gas and air resistance for proper suspense control.
This can be controlled automatically by a bellow or a cylinder and piston (~1,5" diam.) sucked inward by manifold vacuum and balanced by a countering spiral. The piston rod will try to pull up the flaps when the vacuum overcomes the spiral setting. The spiral setting rules the achieved vacuum.
If the motor starts, it will cause a runaway if there is no manually set stopper to set the wanted idle.
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At WOT the suspense tubes with the ganged flaps will no longer be the DOMINANT resistancies, and the gas system resistance can play it’s game again.
To overcom this, a second airflap is installed some distance ahead of the ganged airflap.
One can use this one manually from the instrument panel (dashboard), or then it can be automatically constantly operated by a resonably big membrane comparing the “arriving” gaspressure and the “arriving” air pressure just before the ganged flaps. Maintain symmetry in the placing of the sampling tubes!
Max