No cold FAST idle, AAR is good

[Formula911]

I've got a bone stock 77 S with 2.7 CIS.

Ignition timing is set to dynamic 32* with vac line to the dizzy plugged and the retard port capped (IT falls around 6*BTDC @ idle). Idle speed is set to 950ish warm.

System pressure is 68psi
Warm control pressure is 52psi
Cold control pressure is 40psi (85*F ambient, summertime) and appears to fall into the graph path for spec.

AAR has been "opened" to 3/4 opening "COLD" (again, 85*F). It was open only a "crescent" 1/4 of the port, but since my problem, I thought, was associated with the AAR not allowing enough additional air, I adjusted the small plug to get the larger opening. AAR gets just under 12v with FP relay energized. It has 27 ohms resistance and will still FULLY close up in around 5 mins.

I get steady 18.5psi engine vacuum (pulled from vac line to top of the pear (decel valve) via a "T") with a warm engine at idle. Doesn't indicate a vacuum leak...?

The problem I'm having is there is no fast idle associated with a cold engine start. It starts on first ignition (assumes a good CSV and the AAV is assisting with air augmentation for initial start), however, the idle speed remains low, say around 550-650rpm, and will, after 3-4 mins, rise to around 900-950 (warm curb idle speed). There is no popping, backfiring, etc associated with the cold start. Neither is there any idle hunting during this warmup.

Idle mixture is set WITHOUT a gas analyzer, but rather using the turns to rich-lean-middle with 1/16 towards rich.

Warm performance/idle is really nice and strong.

Has new fuel pump w/ internal ck valve. New accumulator and supply and return fuel lines. Tank is flushed and clean with new 93 octance fuel. New plugs, valve adjustment, dizzy and fuel distributor.

WUR has adjustability and have tried lowering the CCP and it will NOT run if I go lower than 34 psi.

Sorry for the lengthy essay, but I wanted to provide as much pertinent info as possible. If you can add any direction it would be greatly appreciated.

JP

[Gemni911]

I'm having a similiar problem and am troubleshooting my 76 911S. Starts right away but will not idle when cold. Idles fine warm. Will die without keeping the throttle open to 2 or 3 K for at least 5+ minutes. Fresh rebuild with a rebuilt fuel distributor, new thermo switch, rebuilt tank, filters, etc... Not sure what fully open AAR looks like. (shows a cresent with the notch almost centered - seems to allow plenty of airflow); closes in less than 4 minutes. Fuel pressure readings were correct at all three stages. New engine is not running smooth although timing is correct. I have not spent too much time looking at this stuff yet but am just starting to and so will go over the basics including checking for vacum leaks.

[psalt]

JP

Unfortunately CIS cannot compensate for the drop in compression and manifold vacuum that comes with wear and if it is below spec, you are left with finding a compromise. A low cold idle usually means a lean mixture, vacuum leaks or not enough timing. My advice would be to do a compression test if you haven't already. Then I would check for vacuum leaks at the injectors, boots and throttle shaft with an unlit propane torch. There are too many variable involved to diagnose vacuum leaks with a gauge reading. If you can wiggle the injectors, the sleeve Orings are usually leaking. Any drop in vacuum, from leaks or a tired engine, results in less air flow meter deflection and improper fueling, especially at cold idle. After you fix the leaks, my advice would be to try a little more initial timing for several cold starts, then a little richer setting. My experience is the Bosch normal spark plugs give the best cold start performance. Unfortunately you only get one attempt at a cold start a day and finding the right compromise takes effort.

[T77911S]

very good explanation of your problem with all the right info. you seem to have a good handle on CIS already so that is good.

as paul has suggested, air leaks are always a good place to start. the injector sleeves are often overlooked.

timing? im not so sure about, fuel mixture, definately can be a cause of what is going on, which is tied into air leaks. since it sounds like you have already played with the mixture, i would go ahead and try it a little richer. try richening it up, warm, to the point it wants to either surge or when you blip the throttle, the rpm's dip when they come back down. then, lean it out to where it just stops doing both.

i did not see where you mentioned brand of plugs, but copper tops as paul said are the best.

is your vacuum circuit still connected to the WUR? and is the thermo time valve (TTV) still installed?

just curious. how is the ignition system? bosch CD?

new distributor and fuel distributor?

[Formula911]

I was reviewing posts that I had made. This was from a LONG time ago, but I wanted to finish it off just in case it may help someone else with a similar problem. Turns out there was work done to the CIS by the PO and the WRONG CSV flange (part # 911.110.264.00, at the Cold Start Valve) was installed. A $10 item that comes in two versions (one open and the other closed). The closed one was installed and prevented throttle-circumvented air from the AAR to enter the lower intake manifold. At first, I overlooked it thinking that the initial "puff" of air from the AAV was needed ABSOLUTELY so the cold engine would start. But that turned out not to be true. There's enough air gap around the throttle plate to allow cold combustion and so the engine wasn't depending upon AAV stored air to assist startup. Once the new correct flange was installed, cold fast idle was good (around 1800rpm @ 35*F, maybe a tad high, but I did open the AAR up further in my earlier diagnostic path). Thanks for the help and contributions from everyone that ironed out my thinking and pointed me in the right direction! JP