'79 911sc initial CO setting

[jerkan]

Anyone know the procedure for doing a preliminary setting on the mixture screw for a 79 911sc? I seem to have read one time that there is a way of dialing in a rough setting by turning in the screw until the injectors start to spray and then backing off a turn or so.
thanx

[Jim Williams]

That's only a starting point, and only if you've replaced a fuel distributor or and air flow sensor. If the engine runs at all, this is too basic an adjustment, and you will lose a lot of ground. The factory manual calls out a number of turns or part turns for this procedure and I can't recall if from memory, but I can look it up.

[Gunter]

NO, no......jerkan:
Be careful with turning the mix adjust screw any which way.
The mixture is adjusted after the engine runs.
The injectors with K-Jetronic spray continuously that's why it is called CIS (Cont. Injection System)
Get a book on CIS and also get the Bentley SC Repair Manual.
If you dismantled the metering body, refer to the initial setting of the airflow sensor plate .

[jerkan]

I have dismantled the mixture control unit. I have all the manuals, including the bosch fuel injection guide, however, nothing tells me how to set the initial setting before i attempt to start the car. I just need the rough setting to get me in the ballpark so i do not blow up my airbox on the initial start.

[john walker's workshop]

if it's the same fuel distributor that was on there originally, nothing would have changed. if it ran before, it will run again. you may need an exhaust analyzer to get it right on though.

[Paulporsche]

Do you have an airbox popoff valve?

[jerkan]

popoff valve installed.
I changed alot of settings when I rebuilt the car including the mixture screw. I just want the thing to be close when I start it up.

[Paulporsche]

Can you try to put the screw back to about where you think it was prior to your work?

At least the popoff valve should prevent airbox explosions if too lean. If you are way off , the engine probably won't even start.

Are your AAR, AAV and WUR all within spec?

I was fooling w/ mixtures & WURs this summer. At times both were way out of spec, but I was still eventually able to get started, and any leanness was handled by the popoff valve popping. Richness was quickly apparent via smoke, smell and idle hunting.

As Gunter said, make sure the sensor plate is @ the right height for starters.

[Gunter]

Originally Posted by jerkan

popoff valve installed.
I changed alot of settings when I rebuilt the car including the mixture screw. I just want the thing to be close when I start it up.

Check out page 240-4 and 5 in the Bentley: Basic settings.
It gives you the sensor plate height in the rest position.

[T77911S]

he is wanting a preset for the mixture like you would preset the mixture after rebuiling carbs.
if he took the AFM apart and removed or turned the mixture screw, he wants a basic initial setting to help get it started without have to crank it for hours.