Decel valve isolation........
 

Ernie,

Is your '81 SC a USA or Row model? The USA late SC models have a different decel valve than the ('76-'79) 'red decel valve' posted. Unless you have a RoW car, the decel valve would be similar. Removing and plugging the vacuum hose from the top of the decel valve would isolate it (decel valve) and do nothing as you suggested would work only if:
a). The by-pass valve inside the DV (decel valve) is at closed (sealed) position.
b). The diaphragm is good and not leaking air/vac.

Otherwise, you just plugged a leak source by plugging the vacuum hose with a tee and allowing a vacuum leak source (decel valve).

Tony

Tony I have had my car for thirteen years and have not been curious about row vs US up until a few days ago when I seen the "zzz" for vin. I will check ins papers tomorrow car is put up and I just had ankle operation. Anyway my valve is just like the red one not a pancake type. I had the same issues for years and sealed the hose this past summer. No more high or low idle at stop. I am open to any info you can provide. Also is the "zzz " the one sure way to tell if it is a row or not. I do not have the fender side lights
Thanks Ernie

Lambda/ECU........
 

Ernie,

Do you have an OXS sensor or ECU (lambda)? Euro or RoW SC cars don't use an ECU or OXS. Could you PM me the serial #? I could look at my reference booklet. Thanks.

Tony

That kind of stalling can also be an indicator of an over rich air fuel ration...when was the last time your co was checked/set?
BTW, I have a 930-10 euro motor in my US spec and vin car....I swapped out the US 930-16 for more HP and a fresh engine.
I have the decal valve like the pic shows not the US type.
What are your engine numbers, easy to tell.

Thanks everyone. I'll try plugging the vacuum line to the decel valve. I have a US-Calif 911SC. My decel valve is like the photo that ManniB posted NOT the pic with the red Euro one I posted. I was cornfused, but clarified with the help of Pelicans.

If I have a bad Decel valve, I will probably replace it, as I have to smog my car every two years.

I recently had my car's CIS system gone over before this started, and they did adjust my mixture using a CO meter.

If this issue started after that adjustment I'd have it verified elsewhere....if the machine hadn't been calibrated lately they can be way off....I went to two well respected indy shops on the same day for a check (both excellent actually) and the dif between the older machine and the new computer deal was 1.2 CO...that is a huge difference.
My car was doing the same after the first setting when cold...stop and die, when warm it would almost die...it was just too rich.
Since then I put an onboard AFR guage so I can now set it myself.

My motor looks like this, where is this small diameter hose I should plug? Near the front or back? I tried to locate it really quick before work but couldn't find it so easily. Wondering if previous owner got rid of it already...

Tested removing the oil cap during the warm up and it seemed to have affected nothing

I have had this in the past. I would call it dangerous in that it helped me get into a nice rear end collision. Coming to stop, engine started to stall/warning lights on, glanced down for a split second and boom - whacked the Jeep Cherokee in front me that stopped abruptly...Really my fault but I believe this helped me do a number on my front end by distracting me at the worst time.

You said you just had your CIS gone over and they set the mix.....I have had it come back from shops like this with mix freshly set. Some shops just dont care too much about driveability. I believe in these cases the mix was too rich as slighlty leaner settings later on alleviated it. Also bumping up the idle myself from 800 or so to 950 (leaner idle) helped the rpms not dip so severely upon coming off the throttle. I have heard timing can also play into it as well.

I would check mix, idle speed and timing probably in that order. I think a well functioning Decel valve could also help - and on paper is probably the quick answer - but may not be the only answer here depending on what settings you are running.

As a note, I have, at times, run my car with the decel valve disconnected, but with the mix/idle set at spec., had no decel stalling issues even with AC and lights turned on.

I have a feeling you are a tad rich on the mix as a result of whoever just set it.

Took my car to a shop, and the mechanic popped off the airbox cover and removed the air filter and adjusted the mixture. Yes, it was a tad rich. It runs great now.