[alfa] L-Jetronic Idle Mixture

["Tom O'Neil" <tomoneil41@xxxxxxxxxxx>]

Ciao a tutti,

I have a 1984 L-Jetronic Spider. The car runs fine, but it has a rich idle, 
judging from the smell and from the carbon soot on the floor of my garage. 
What's driving me crazy is that the idle mixture adjustment screw doesn't 
seem to do anything.

As I don't have access to a gas analyzer, I'm using the oxygen sensor 
voltage method of setting the idle mixture. At idle, and with the O2 sensor 
nice and hot, you disconnect the O2 sensor and check voltage from the sensor 
to ground, using a high-impedance digital multimeter. The correct idle 
mixture should show about 500 mV. (To be more specific, you can check the 
high and low range of the sensor and then set the idle mixture at the 
halfway point, which should be somewhere in the neighborhood of 500 mV.)

I can turn the mixture screw from "full in" to "full out" and the voltage 
hardly budges from about 900 mV. I'm convinced that the O2 sensor is OK, 
because if I introduce some unmetered air into the stream (e.g.: by cracking 
open a hose), the voltage drops quickly and predictably. This led me to 
think that there might be an obstruction in the airflow meter. I removed the 
AFM from the air box and was able to pass a thin wire from one end of the 
idle air passage to the other, so there's no obvious obstruction there.

This is pretty straightforward stuff, and shouldn't be so freakin' 
confusing. Can anybody tell me what I'm overlooking??

Grazie mille.

Tom O'Neil
'84 Spider Veloce

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