Calling All CIS Specialists
 

I drive a 1973.5T, first with CIS which they should have ended that year!!!! I believe I have tinkered with the mixture setting and idle screw to a point where it still runs a bit rough and I could be on blend when I should be on puree!

I do not possess a CO meter so I have to start from scratch with the help of you wonderful folks. I was told once that the mixture setting is in the sweet spot when the idle needle on the tachometer hits bottom and bounces back to the correct tack setting (8-10 rpm). Its that bounce back!

A bit of background: I picked my car up recently from a shop that rebuilt my top end (valve job, new nikilsil cylinders/piston set), clutch, suspension bushings and a few more small things. Compression went from 7 to 8 with the nikkies and horsepower increased. I did not instruct them to rip the whole CIS system apart; however, they did a fuel pressure test and it was on the numbers, cleaned and lubed the distributor cap and new plugs.

Driving home after I picked up the car I was noticing a slight bump or hesitation at mid range, however, the car felt more powerful and idled well. No backfire, just a slight hesitation at speed. When I got home I started to tinker with the mixture setting. I took the car out for an errand yesterday and some of that was on the interstate. The car ran great, with no hesitation at speed, however, once I hit the exit it went back to the hesitation in the midrange. No backfire, but was rougher. I even stopped to fill up with 93 octane thinking that would help.

So this morning, I inserted the 3mm adjustment tool and went CRAZY trying to get this settled. This goes for the idle screw too.

Without trying to troubleshoot anything else (I was suspicious of the fuel filter, but they said they changed that out), I gave it a try and even at idle I get the hesitation now.

What is the best way to reset the mixture from scratch assuming I turn the wrench to the left (left -lean, right-rich) until it stops and work toward rich. Quarter turn at a time to rich?

How do you manually set a CIS mixture without a meter? and idle screw? Some of you CIS old timers know the secrets. Or am I dealing with another issue?


Thanks

Bob
1973.5T

Have you tried this thread's adjustment method?

Edit - Linked wrong thread Let me look some more.



Ok Here are the correct links.

Base adjustment from Base setting of mixture for CIS 1980SC? post 3


Now for the adjustment without a meter.

CIS Idle Speed and Mixture Setting Without an Analyzer

A bit of background: I picked my car up recently from a shop that rebuilt my top end (valve job, new nikilsil cylinders/piston set), clutch, suspension bushings and a few more small things. Compression went from 7 to 8 with the nikkies and horsepower increased. I did not instruct them to rip the whole CIS system apart; however, they did a fuel pressure test and it was on the numbers, cleaned and lubed the distributor cap and new plugs.


Take it back to them.

CIS Primer for the 911
 

I hope you are using Jim's Basement CIS Primer for the 911 troubleshooting guide. This is the best reference I've found for our CIS cars.

K-Jetronic Troubleshooting Guide

I believe we can thank Brian Bodart, RarlyL8, for maintaining these pages now.

I recommend getting a book on CIS not specific to Porsche. I bought the yellow Bosch paperback for $13.00

I use the voltage/duty cycle of my O2 sensor to adjust my mixture but just realize 1973 doesn't have one.

You sound lean to me and might suspect a vacuum leak. There is a mixture adjustment screw on the top of the CIS that often gets left open. I closed mine with a plug from our host and things worked much better.

ST

I would bring it back to the shop that did the work, tell them want happened and what you tried. Hopefully they will make it right.

If you want to do it yourself. You need to be sure that:
1-Valves are properly adjusted
2-No vacuum leaks in the intake
3-Points gap is within spec.
4-Timing is dead on at 5000 rpm
5-Idle is on spec and not changing when fully warmed.

Then and only then do you mess with the mixture.

I have had my 1973.5 CIS car since 2001. The only time the mixture has been adjusted was when I first got it and it was surging, when my valve guides were replaced and when I had a full top end done.

I got it with 116k miles and it has 162k now. While CIs has it limitations, it makes for a wonderful touring car.

If you are going to own CIS system and work on the vehicle BUY A FUEL
PRESSURE TESTER. Sorry I had to yell at you. Don't just fiddle with something or replace this or that. There are easy tests to do once you BUY A FUEL PRESSURE TESTER. Cold, hot, system pressure, full throttle, part throttle hot bleed down, etc. An example of its use to your stated issue would be a test at part throttle. What is your pressure. Well?

I had a vacuum leak at the backfire valve that drove me batty. That repaired, car runs amazingly well.

A pigment of your imagination.......
 

ST,

That hole is open to atmospheric condition. You could put JB Weld over that hole and your CIS will not change a bit. It was just a pigment of your imagination. Plugging it will prevent tiny bit of dust particles getting into the surface area of the air box. The other side of the hole is still the outer section of the air box. Go check it.

Tony

well you are close.
adjust it leaner until it JUST stop doing that and let it idle, it should not surge.
if it does either, adjust leaner until it stops. that will get you very close. .

After following the above steps, If you want to see/hear what the rough idle mixture setting process looks/sounds like, watch this starting at 2:30 https://youtu.be/8TcAzqKxWgY?t=150

Dont listen to his explanation before (he kind of jumbles up some terms), just watch the process while the engine is running.

And now you are normal?

Just did this last weekend to my 74'. Fixed a couple small vacuum leaks. Wanted to check the mixture as I believe the previous owner had been messing with it. Had the 3mm allen key conveniently located in the glove box.

911 CIS Primer - Testing: Mixtures

CIS Troubleshooting.........
 

Bob,

The biggest mistake you did on your CIS was tinkering the fuel mixture setting without verifying the absence for source/s of unmetered air. SOOK’s procedure for adjusting the fuel mixture is good except he failed to mention to verify the absence of unmetered air. I called his attention about this matter and he could not edit the posts due to the time frame. After a certain time, the posts become locked.

Anyway, no amount of adjustments will correct your problem if you have sources of unmetered air. The best you could achieve is to make it more RICHER to compensate. At this point, we are not even sure you have unmetered air going into the system. But we don’t know that as fact.

You are hoping it is free from any significant sources of unmetered air. You could be lucky and all is good. But you can not rely on LUCK for diagnostic testing. Maybe you could not find the leak but that does not mean you don’t have an air leak. Test and verify.

Tony