Quote:
Originally Posted by 99boxster559
Can you explain what the air sensor plug does other than keep the FP off when the ignition is on and the engine isnt running? I think it has to with something. As when I turn the key the FP will run. I lean out mixture screw, turn key and no FP or FV running. Then go back to plate and move up ever so slightly it fires on again.
If it is that close to the trigger point is the mixture extremely rich?
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No, the mixture screw is nothing else like fine tuning for the sensor pkate height. When you look closer on the lever from the sensor plate this not only a single lever arm but a lever system of two levers with a connection rod outside the pivot point. This results in an overproportional transmission of the travel from the sensor pkate to the plunger of the fuel distributor. That's this - I call it working point or injection begin - is so sensible because of this non proportional translation the system is able to enrich the mixture on higher speeds.
That particular point where the sensor pkate is being lifted due to the idling or starting engine defines the injection begin = where the fuel pump starts to pump gas and the engine starts or keep running.
When your car won't start with this basic setting then there's something else is wrong. You have to adjust both sensor plate height as well as the injection begin meticously as the documents describe. The basic adjustment procedure is taken from the 911 1972-1983 Porsche workshop manual, the sensor plate procedure is described in the Bosch K-Jetronic Workshop Manual. Both are linked here in the forum or available on the web.
So: With non running engine and ignition on fuel pump mist not run (security switch plugged in). When engine cranks the sensor pkate gets lifted so less but that enough to start the fuel pump and supplying the lambda control incl. the fequency valve if present.
That working point defines the working window of the whole CIS as this defines the (math.) function for the mixture.
Here's a video showing how it sounds when only the fuel runs when (security switch unplugged for demo) and then when the injection valves barely opens up and injecting fuel. You must follow all the steps described in the procedure precisely.
https://youtu.be/4iWZdvOow0g
Explanation of the lever system:
https://forums.pelicanparts.com/porsche-911-technical-forum/1158127-air-sensor-plate-height-why.html#post12203484
When you move that working window either too low or too high you enrichen or lean the mixture in wrong (working) areas (of the CIS) resulting in bad performance, poor mileage and bad up to impossible starts. Worn injection valves, bad / old fuel and other issues may worse the situation.
There is an excellent video series from Kurt from KlassikATS about the 2,7 911 with CIS incl. Troubleshooting and adjusting it:
https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj
Thomas