Lean at cruise, rich at idle.

[Walter_Middie]

I have a stock 1975 911s with CIS. Like the subject says, my car is running rich at idle and lean at cruise or under power. How do I know? I recently switched to 1974 heat exchangers, and expected the mixture to be lean. But when testing the car, it acted rich. That is, when I let my foot off the gas, the idle would drop way low and almost stall. So I leaned out the CIS and the idle improved – but now at cruise or under slight load, the car would buck and hesitate, like its lean. Richening the mixture improves the slight load hesitation, but the idle changes back to dropping way low and even stalling. How do I richen the cruise and lean the idle? If I screw in the idle screw, that will lean the idle, but it will also drop the idle. I’m idling at 1000 RPM now, and don’t want to lower it any more.

I had a problem a couple of months ago with a hunting idle, and it was diagnosed as an underpowered fuel pump. A new (and very expensive) fuel pump cured it.

Any advice?

[Walter_Middie]

Another possible clue - I also fixed a few other problems when I had the car apart for heat exchangers - like a new starter, new fuel accumulator, and I had the chain housings off - so I now have new cam timing (it should be very close to what it was).

I'm going to try and lean out the idle adjustment and hopefully not drop the idle speed too much, and see if I can reach a happy medium between the idle and cruise mixture.

[dweymer]

Check the decel valve, look here for more info on CIS

http://members.rennlist.com/jimwms/CIS/CIShome.html

[Walter_Middie]

Donnie,

Thanks for the suggestion - I've got the decel valve adjusted so that it just holds the RPMs from dropping too quickly. When it's rich, the idle will come down slow, but still drop to almost zero - and when it gets really warmed up, it will stall. But when it's rich, the cruise or part throttle acceleration is nice and smooth.

[2.7RACER]

Rex,
Have you looked at the injectors? CIS injectors are easy to check by placing them into a glass jar one at a time and actually looking at the spray pattern. Chevron Techron added to your gas can clean up dirty injectors. It's known as a "tune up in a bottle" by the Pro's.

[Walter_Middie]

Doug,

Good suggestion - I'll try a new set of plugs and a bottle of techron and see if that doesn't smooth out my cruise.

[RoninLB]

Originally posted by 2.7RACER

Chevron Techron added to your gas can clean up dirty injectors. It's known as a "tune up in a bottle" by the Pro's.

I'm not sure if this is worth anything as I never had to deal with my old CIS, but

the CIS FD /fuel distributor is very dirt sensitive and thus I always used Techron before an oil change. And there is a way to easily clean out an area of the FD that accumulates dirt of which I don't remember what fuel line to remove for access?

[RoninLB]

Originally posted by Walter_Middie


Doug,
Good suggestion -

2.7 R routinely offers great info for sure.. But


JW Workshop in Seattle has posted CIS info of that he sees happening and its fix.

[T77911S]

i am having the same problem with mine after doing the same as you. PO replaced many parts trying to fix a pinging problem above 3500 rpm. new WUR,AAR, ACCUMULATOR,FUEL FILTER, NEW INJECTORS, REBUILT DIST.,NEW WIRES. (advanced cam timing was the problem). i am very rich at idle like you and still have a stumble when i accelerate from a stop. i havent had much time to mess with it nor have i located someone with a gas analyzer to set the mixture since resetting the cams. is there some sort of enrichment device for that year? also, how is the ign. timing?

[boxermania]

The CIS system is in essence a "mechanical system" since the delivery of fuel is controlled by the sliding piston, in the fuel distributor, actuated by the position of the flapper valve.

What it means is that there is only one adjustment and that is reflected across the rpm range.....the CIS injections are very fair in providing a flat A/F ratio across the rpm band.

The idle screw is nothing more than an air bypass, it doesn't regulate fuel delivery.

The addition of the SSI will lean the mixture a bit because there will be less restriction on the exhaust side....but that is easily corrected.

So if you are running well at idle and feel that you are leaning out at higher rpms the following can be happening:

1) The fuel pump can't keep up pressure at higher rpms, therefore reducing flow of fuel to the cylinders. Need to check pump pressure.

2) Fuel injectors are partially plugged. Not very common in this day and age of fuel with cleaning agents, however, all is possible. The cheapest thing to try is the Chevron Techron, I would go for the concentrated one....then move to checking the injectors for flow pattern and then move to physically removing the injectors and having them cleaned and flowed by one of the many services available. RC comes to mind.

However the absolute best bet, in my opinion, is to acquire and install an Innovate LM1, A/F ratio meter and install it on your engine, they come with a Bosch wide band oxygen sensor and everything needed. This will allow you to really monitor your A/F ratio in real time and see what is happening with the injection.

Keep us posted on what you find out.....

[Walter_Middie]

T77911S,

I'm going to have to start checking the basics again - ign timing, plugs, fuel pressure, injector performance....all good ideas. I have very recently replaced my fuel pump, filter, accumulator, and had my injectors cleaned by Witch Hunter.http://www.witchhunter.com/ . Many of my problems were cleared up with the new fuel pump - I'd check yours first.

Bxoxermania - thanks for the response. By saying I'm richening the idle mixture, I'm just screwing in the idle screw. Yes, this just restricts the amount of air slipping past, but since the flapper valve doesn't move, the amount of fuel input does not change. Less air = a richer idle mixture. Once you come off the idle, the small amount of air from this idle passage will not affect the running mixture.

RoninLB - Having JW here can be a blessing - except that he is kept so busy, it is sometimes hard to just pop in and get advice. Plus I like the challenge of fixing things myself - I'm always learning.

[Paulporsche]

Something I'm not clear on--are you turning the fuel mixture screw, the idle speed screw or both? Also do you have points? Have you checked them?

[T77911S]

the idle screw, big one on the backside, is after the AFM, so changing the idle up causes the plate in the AFM to rise also with the increased air flow. my WUR is vacuum controlled to richen the mixture for sudden pressing of the gas pedal, like an accelerator pump on a carb. does your car have a device to do this?
mine runs fine when cold, no stumble, until the WUR warms up.
all pressures are good.
ive checked the fuel pressure but not volume.
i am going to play with the timing and mixture a little more.
i have never put one on a analyzer, but i understand a small change with the allen=a big change in CO%.

[Walter_Middie]

are you turning the fuel mixture screw, the idle speed screw or both? Also do you have points?

So far, I have only changed the fuel mixture screw - the idle speed has been OK at about 900 RPM. No changes to the idle speed screw yet. A long post from SAUK tried to explain several things about CIS - one of which was how changing the idle screw also changed the idle mixture. Maybe I misunderstood what he was trying to say.

I don't have points anymore - Pertronix Ignitor.

Car was running perfectly before the fuel accumulator, heat exchangers, and cam timing.

My plans are for new plugs, some techron, and ign timing check.

[dweymer]

Rex:

The big screw on the throttle body is the idle speed screw.

Mixture screw is a 3mm allen down in(for lack of better term) fuel distributor.


P.S it is: souk

[Walter_Middie]

dweymer,

I have no problem knowing which is which - but thanks for checking. You never know the level of experience of someone on the internet. I once worked with a guy that literally did not know which end of a screwdriver to use - I had to explain it to him.

[Paulporsche]

Have you checked your cold and warm control pressures? Sometimes the WURs can go out of adjustment not only in the usual way, which seems to create a too lean startup (too high ccp) but also in the relationship between the cold and warm cp. Others have said this can be corrected via the adjusting screw accessible from the bottom of the WUR, behind the brass plug.

Or maybe your WUR is off just enough so your CO setting is right just for one temp range.

Or possibly the bimetal strip or some other internal part has packed up, or there is no power to it.

Check operation of the distributor, and for air leaks.

[Walter_Middie]

I’m back on the road. I pulled the plugs and they didn’t look very good. The base was dark and the insulator was too white (lean). I like them to look a little brown, like weak chocolate milk. It makes sense that it was running lean, because I had just changed to more efficient heat exchangers and I was getting some hesitation under part throttle. Richening the mixture cured the hesitation, but created an idle that would drop way low and even stall.

I replaced the plugs with NGK BP7ES. I noticed right away that the car was running smoother. After warming up the car, I richened the mixture (using the 3 mm long allen wrench). This smoothed out the part throttle cruise like before, and the dropping idle was still there – just not as bad. I then unscrewed the idle adjustment a little – making the idle leaner. I know from experience that the idle screw does not change the idle speed right away – you have to turn it a few times before anything will happen. I think this just changes the idle mixture a little – eventually the idle speed will change. The result on my car is that the idle speed is now up a little – 1050 to 1100 rpm, and I no longer get the idle dropping way low. The car runs great – I probably have some more small tweaks to make, but that’s half the fun of owning a car like this.

[T77911S]

i dont see how the idle screw will make it leaner.
do you get any spark knock at high rpm's? if so, then you are lean and/or timing too far advanced. i am wondering about air leaks, i have all new seals on the intake and i have a popoff, but i am going to check my airbox and the intake boot, it is a good source for leaks. what type of gas are u useing?

here are some good sites on reading plugs for mixture and timing. i have more if u want them.
http://www.dragstuff.com/techarticles/reading-spark-plugs.html

http://www.4secondsflat.com/Spark_plug_reading.html

http://www.4secondsflat.com/Spark_plug_reading.html

http://www.carcraft.com/techarticles/64379/

http://www.strappe.com/plugs.html

http://www.dragstuff.com/techarticles/how-to-read-plugs.html

http://corvettefever.com/techarticles/153_0503w_plug/index1.html

[fred cook]

After reading all this, I could not tell if you are using an exhaust analyzer to set the fuel mixture. When I put SSI exhausts on my SC, I richened the idle CO mixture to about 3%. At that setting it pulls strong all the way to redline and seems to idle pretty well with maybe just a hint of richness. If there is a major leaning out at high rpm, you are probably experiencing a fuel delivery problem. Maybe as simple a a partially clogged fuel filter. Did you check the CIS plate to make certain that it is lifting easily? If it is binding, that will prevent the system from increasing the fuel mixture as the rpms increase.

Good luck!