Richard Stephens, Register Technical Advisor, reports

If you have had your 911 SC engine hunting up and down when started from cold or it has a rough idle, then here are few thoughts about the subject. I have a 1978 model 911 SC (built June 1977), which has had this problem on cold start. I purchased it in 1993 and, in winter when it was cold, the engine hunted around a lot at first start-up. People thought I was purposely blipping the throttle to make lots of noise! I did a fair amount of investigation into the issue, carried out quite a lot of practical adjustments to the engine and read fair amount of literature on the CIS system. I found I could dial the rough idle out a bit by adjusting the fuel mixture, but it would lean the mixture off when it warmed up, which was not a good solution.
 
My thoughts at the time were that the tolerance on the warm up curve of the fuel mixture unit was quite wide (diagram A). Units in the bottom of the shaded area will cause the fuel mixture to be richer than average, causing hunting of the engine (lower control pressure equals richer fuel mixture).


Diagram A

The warm up is just controlled by a bi-metal bar acting as a thermostat to control the fuel pressure with a spring (old technology). As the bar heats up it pushes the plunger up to increase the control pressure and lean off the fuel mixture. There is a modification where you drill an adjustment screw into the control unit for fine tuning (see Wheeler Dealer reference below).
 
On Wheeler Dealer they addressed this rough idle issue. Ant modified the control unit on a Porsche 924. The information comment on the summary for this episode was “Warm-up regulator cleaned and modified to address rough idling”. You can see the adjustable screw added on the left of diagram B.


Diagram B

The mixture control unit went through a few iterations from 1978 to 1983. In my 1978 car it had the most basic version as shown in diagram C.


Diagram C

If the electrical connection is broken, the bi-metal strip will not heat up and control the fuel pressure causing a misfire when driving. Later mixture control units had an extra chamber for sensing full throttle, enriching the fuel mixture under heavy load (see diagram D).


Diagram D

There are vacuum connections, on the later models, to these mixture control units, so you should make sure these are not leaking. Later units also have an extra vacuum for altitude adjustment as well as full throttle. 
 
There is also a little mesh on the fuel inlet, which traps any debris. If you see rust particles here this means your petrol tank is rusting and the fuel filter is starting to get non-effective. A while ago my mixture control units got blocked with rust debris. This caused the car to run very badly and blew the injection box with a large misfire. The tank and mixture control unit have now been replaced.
 
There have been a few comments about the cold start valve causing a rough idle. The cold start valve just sprays a load of fuel in at start-up for a short time on ignition start below certain temperatures. This is unlikely to cause this issue, as it would flood the engine if stuck on.
 
There have been some thoughts about the anti-stall causing a rough idle. I accidently knocked the anti-stall pipe off once. It did not cause any idle issues. When coming to a quick stop for a junction, from higher revs, the engine would stall. Re-attaching the pipe quickly cured this!
 
In summary, my practical cure for this rough idle on cold start was to put my foot lightly on the throttle, getting the engine to rev to about 1500rpm for 15 to 20 seconds. This usually was enough to get the car idle nice enough to drive off.


 Reference Spec for Mixture Control Unit