Basic Fuel Mixture Procedures (911SC)

[fireant911]

I am attempting to adjust my basic fuel mixture using the test connection. According to the procedure, as I understand it, with the engine at normal operating temperature and the Lambda sensor connected I am to connect a digital meter's positive lead to the green/white wire pin and the ground to the brown wire pin. My multimeter meter does measure duty cycle and what I am looking for is to adjust the 3mm screw until I acheive a 50% duty cycle - this same procedure is listed in How To Tune & Modify Bosch Fuel Injection by Ben Watson. The problem that I am running into is that the measured reading is always 50% regardless of any adjustment to the 3mm adjusting screw. I pulled the 15° C switch last weekend to ensure that it is functioning properly (the hot water test as recommended in the Bentley repair manual) and it was operating properly to allow my car to go into closed loop.

The gas tester procedure is currently out of the question since I do not own this equipment and I do not wish to continue to pay my mechanic for this procedure.

My question is - has other 1980-1983 911SC owner's had luck setting the basic mixture using this procedure? Why does my duty cycle continuously stay at 50% regardless of the setting of the adjustment screw?

[Paulporsche]

Are you sure the screw is actually turning? Do you get any changes to the sound or rpm of the engine when you turn the tool?

[Superman]

I have used this procedure, and the duty cycle did adjust as the screw was turned. I'm not sure why yours would not.

Of course, like many others, my best suggestion would be to backdate both your exhaust and heat plumbing systems to pre-'74 designs, and that it is not important to incorporate an O2 sensor bung since our cars frankly run better when the computer is not leaning the mixture out. And besides, I think the computer mostly works at idle, whereas nothing is really getting electronically metered while the car is cruising. Certainly not at wide-open throttle anyway.

[chuckr]

All I can say is that I had my mechanic unplug my O2 sensor
and use a five gas analizer to set my motor. It ran much better
and I got good gas milage. This would pay for itself in performance
and milage.