idle speed dropping way too fast car stalls

[LCOX]

I'm trying to work through what appears to be multiple problems on a 77 cis 911.

When changing gears while I very lightly lift off the gas pedal or when I take my foot off the pedal the idle speed drops very fast ot 0 rmps and the car stalls.

I've got the air bypass screw a full 5 turns out which only helps alittle and doesn't change the idle rpm speed that much. I know this isn't the fix as well but the car is driveable as long as I feather the pedal alot to keep the idle speed from dropping fast.

Any ideas on where to start?

Thanx everyone. L

[john walker's workshop]

try adjusting the deceleration valve. it's on the throttle housing forward and above the throttle lever. loosen jamnut and turn the threaded tube. or shift faster.

[timmy2]

How does it idle and drive other than when lifting off accelerator?
Is it idling fine when you start, have lots and lots of power?

Running really rich will cause your symptoms, (as well as the decel valve needing adjustment).

Not knowing what all the issues you meention are:
I'd recommend you test your fuel pressures and check your exhaust CO level to make sure they are in spec, then adjust the decel valve if it is all correct.(and all vacuum lines are good and run to the right locations)

[wasserkuhl]

What John said. and if that doesn't work, what Dennis said.

However it is possible that the Decel valve is buggered. remove the top vacuum hose and see if there is any difference, if there isn't then the valve is buggered.

Alot of people block the valve off anyway............

Get some nice exhaust crackles happening!

[T77911S]

you have other issues besides the decel valve(DV). even with the DV removed, the RPM's should drop and the car idle fine.

you could have air leaks. those need to be fixed first. there are lots of places for leaks. the hoses that run from the oil tank to the engine. the oil cap. the brake booster line. tested mine with a hand vacuum pump. pulled a vacuum and it held. that checks the line and the booster.
the intake runner sleeves. Orings, injector sleeves. even the aluminum pipe behind the CIS can rub and get a leak.
a smoke test is a good way to find the leaks.

next i would do a tune up. plugs, cap, rotor and wires if needed.
then i would check fuel pressures. system, hot control (HCP) and cold control(CCP).
check timing, make sure the advance works
then check the mixture

you can also check and adjust the DV with the hand vacuum pump. i would also check the vacuum lines from the WUR, to th thermo time valve (TTV) (if you still have one) then to the DV and then to the throttle body(TB).

you can also check the TTV with the hand vac pump, or just check for vac to the WUR when the engine is running. if you remove the vac line to the WUR with it running, then engine should run worse.

make sure you have power to the WUR and TTV and aux air regulator(AAR).

you have gone the the most of the CIS, you might as well check the AAR. make sure the idle increases when cold, then drops down after a few minutes.
you could pull the AAR and make sure it closes when power is applied.

[LCOX]

I've looked in to the decel valve and it's not the problem.

I'm going to start with some of the other suggestions.

Going to hook up gauges and run fuel/control pressure tests.

What are the fuel pressure and control pressure readings for my 77 911. Also my wur has 2 vacuum lines connnected. Do these stay connnected for the control pressure readings?

[timmy2]

1976 911S – from garage find to on the road
If you have the same WUR this should work for ccp I don't know your specs for the wcp. Do a search.
Now that I re-read your post you may have more issues.
Best to describe how it idles, how it drives, Is this a change or are you just getting it running? what you've done etc.

[T77911S]

one is a vent, the other is a vacuum enrichment.

the DV holds the RPM's up then drops them down slowly. more so than just closing the throttle. if the RPM's dont hold then drop some what slowly, this IS the problem.

now, the RPMs can drop quickly and the engine should keep running. so you have more than one problem.

CCP is based on outside air temp.
remove the power to the WUR and check this with the engine off. you will get one reading.
start the engine and if you have the TTV, in about 20 seconds, you should get another reading. should be around .8 bar higher.

connect power to the WUR. after several minutes you will get the WCP reading. should be around 3.6bar.
if you remove the (?)top vacuum line, you will get about a .8 bar lower reading. so 2.8bar.

but, if you are adjusting the big idle screw and not getting much of a change ,i would really start looking for air leaks.