Bosch Motronic AFM => Point - Counterpoint

[Brad Anesi]

At the risk of continuing this discussion into the new millennium, I present
this response from Pat Braden, author of the book "The Alfa Romeo
Owners' Bible", in response to our recent discussion.

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"2-3 notches on the spring will change the Air Fuel ratio by about 2%"

Hate to disagree. Presuming that the spring has any effect on air/fuel
ratio, loosening it (that is, allowing more air to enter) would _lean_ the
mixture, not richen it. Conceivably, weakening the spring could allow the
flapper valve to swing further for a given air velocity, indicating a
higher volume of air, for which the control module would immediately
provide a richer mixture. However, the mixture would immediately be
corrected to the stoichiometric value of about 13.4:1 by the control
module, acting on information from the Oxygen sensor.
My understanding of the system is that the spring is there only to return
the flapper and act as a damper on its actions. Weakening the spring
reduces the damping effect, allowing a faster throttle response but not
changing the A/F ratio.

With any Motronic system, presuming it is in working order, nothing you
do to the mixture will change the actual air/fuel ratio because the Oxygen
sensor signal will simply force the control module to correct the mixture
strength to stoichiometery. With the Oxygen sensor disabled, the easiest
way to get a richer mixture is to fool the coolant temperature sensor  by
putting resistance in line.