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This seems to be a very common problem. The fact that it fires up right away, suggests that the thermotime switch and the cold start valve are working properly. The next thing that should happens is that the engine should idle at about 1800 rpm, and then slowly reduce to 950 rpm in about 4 or 5 minutes. When your engine finally idles without stalling, what is the idle speed? If it's not increased over normal, that suggests that the aux. air valve is not opening when cold.
Also during cold starting, the warm up regulator enrichens the mixture by decreasing the control pressure to the fuel distributor. The warm up regulator may be the trouble. The CIS system has lots of little components all over the engine connected by fuel lines and vacuum hoses. There are good books on Bosch fuel injection that do a good job of describing the different CIS systems, and have procedures for troubleshooting each component.
I'm still working on a very similar cold start problem on my 79SC (as are many other people).
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Bill Krause
'79 911SC Euro
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