1979 911 SC altitude idle issue

Maxhr203 · Maxhr203's Garage

Hi everyone. My 1979 SC runs great below 3000 ft in altitude but above that, it will not hold idle. It just stalls out. The big issue with this is my house is at 4,100 so downshifting or stop and starting are major problems.

Thanks in advance for any help or thoughts.

darrin · 04-21-2021, 07:16 AM

check out this thread, suggesting this stems from your mixture at sea level being a tad rich (and getting even richer in the thinner high altitude air) and proposing that you slightly lean out the mixture to compensate Altitude effect?

Glenfield · 04-21-2021, 07:22 AM

Definitely a function of altitude Maxhr203. CIS is pretty rudimentary and can't adjust for the changes in air density. My bet is you're probably running too rich because there's less 02 floating around. May have that the wrong way around but that was certainly my experience taking a car that was already running rich though CO last year.

stlrj · 04-21-2021, 07:37 AM

The question you might ask is "how do you lean it out"?

darrin · 04-21-2021, 09:31 AM

Quote:

Originally Posted by stlrj

The question you might ask is "how do you lean it out"?

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm provides good explanation on cis adjustment points and the 5th post in the thread I referenced above suggests adjusting the mixture 1/4 turn (or less) toward lean.

Maxhr203 · Maxhr203's Garage

Thanks too each of you for your insight and help! I'll attempt this......

stlrj · 04-23-2021, 07:49 AM

https://www.youtube.com/watch?v=XLUlnhLIMb4

turbo ride · 04-23-2021, 08:52 AM

I always wonder how this worked when the cars were brand new. So I buy a brand new 1979 Porsche 911 in Denver, CO at 5200 ft, then decide to road trip to see family in California. I get there and the car runs like crap. Did they expect me to know how to adjust the CIS? No forums, no Youtube videos etc. Or take the car to a dealer to have them adjust? then re-adjust when I get back home to Denver?
At least the naturally aspirated cars went to Motronic in 1984 - us Turbo guys were stuck with CIS until 1995. 1995!! Crazy. At least starting in 1991 the CIS had an altitude compensator...