Idle problems at 911SC

[IF80ACE]

Hi,

After installing/retrofitting an AC system on my 911SC (taken from an 85 Carrera 3.2) , I see that the idle is not staying in the same 950 rpm spot like before, but lower.
I am thinking that this might be because of the new "load" given by the belt from the AC compressor.
In order to fix the idle now - shall I work with the main idle or the mixture one?

Thanks!

[Schulisco]

Do I get it right that you've taken the AC components from the 85 Carrera, the car is a stock SC?
Which model year is the SC?
US or RoW model?
is it a lambda controlled CIS?
Do you cheched the engine for vacuum leaks?
Pressure (system and control pressure) are in factory spec?
Idle mixture / CO is adjusted to factory spec?

If it's a lambda controlled engine this should manage idle with extra load from the AC. On others you probably can adjust idle speed on hot engine with AC. But this can lead to a high idle when engine is cold. I don't remember if there's an extra idle control on SCs with AC.
On other cars like Mercedes they manage the extra load from AC with 10° more ignition advance on idle only. Others might open an extra air valve to give more air when engine idles with AC.
This could also be an option to handle your problem.

Thomas

[AndrewCologne]

Originally Posted by IF80ACE

... I am thinking that this might be because of the new "load" given by the belt from the AC compressor.
In order to fix the idle now - shall I work with the main idle or the mixture one?
Thanks!

Yes, its very normal that with a running A/C the idle drops cause of the additional load of the belt/compressor.

Just adjust the idle using the main idle air/bypass screw on the left side of the throttle body. So you can just try and find out what's the "middle" setting so you end up with an nice idle rpm with compressor running and without.

[AndrewCologne]

Originally Posted by Schulisco

On other cars like Mercedes they manage the extra load from AC with 10° more ignition advance on idle only. Others might open an extra air valve to give more air when engine idles with AC.
This could also be an option to handle your problem.

Thomas

This is done by the original SC dizzy as well, if the rpm drops below a specific level, the ignition advance rises which lifts up the idle rpm. Not 10° but enough to keep the rpm at a regular level.

[Schulisco]

Originally Posted by AndrewCologne

This is done by the original SC dizzy as well, if the rpm drops below a specific level, the ignition advance rises which lifts up the idle rpm. Not 10° but enough to keep the rpm at a regular level.

Ahh, very good thanks!
Now it's clear to me why the advance curve on lower idle speeds increases again...I thought it was only to help starting the engine. But this makes sense as well in case of extra load from the AC!

Question: If idle speeds drops too way in case of the AC - could this mean that the dizzy is not properly adjusted?

Thomas

[AndrewCologne]

It depends what you call "drop", if the A/C makes the rpm only falling 100 rpm from 900 to 800 or even 800 to 700, then you won't recognise it.