my 78 idels high rpm unknown for the tac is out . the idel screw is at lowest idel . with engine at temp i pinched off aux air hose thinking an air breach in the valve but no dice. i then inspected the idel stabalizer it passes air at -15 lbs vacume and increses passage rate with increased vacume the the vac meausred from throttle body was 15 lbs so i remove the supply line from the valve to find the engine settle to a idel rpm, shouldnt the valve increse bypass flow when recivig ruduced vacume caused by retardig rpm. i though i unerstod the stabalizing sytem but after my experiments im left in a state of ignorance. also has anyone heard of leaky idel bypass screw thanks ror listening

 

It has been too long since looking at a CIS system to be able to help you; however, the guys who are current on CIS cars are reasonably represented in the posts within this "CIS high idle" search.
I thought that the aux air regulator was to be closed when hot, open when cold .... have you tested that? Also, it would be unlikely that the idle screw in the throttle body would bypass that much air - if it did, then removing the supply line to the aux valve would have had no effect on the idle as you had observed.

 

What idle screw? Maybe you have been messing with the mixture screw? That's very sensitive, and very, very hard to get adjusted right without a CO sniffer in the tailpipe. '78s don't exactly have an idle stabilizer. I suggest you get a good book on CIS, a set of test gauge/fittings, and the 928 manuals and go by the book. With those you can troubleshoot 100%. Or get the test kit and send jim morehouse a PM and he can help you with the tech docs.

I'd like to help but need better info. Find out the correct names for parts you are testing, or post pics. The TB has both port vacuum and manifold vacuum ports. Find out exactly which port you are taking your readings from -- again, maybe post pics.

 

I think he means the vacuum limiter, which limits the vacuum level on overrun, so the engine doesn't pull gas out of the mechanical injectors.

On L-Jet, I would say remove it, but I can't say for sure for K-Jet.

 

Dave, c'mon Dude .... you've had an idle screw or 2 .... you do live in CA, don'tcha?

 

My K-jet has that limiter as well. Of course, I know that there isn't much that you wouldn't recommend removing, Ken...

 

yes you are correct it is the vacume limmiter at least acording to pet cd. what is this item for

 

Alright, alright... busted...

 

It's to prevent too much vacuum when the throttle plate slams closed. Prevents excess fuel from being sucked out of the injectors(which never close). Also to help preven stalling on sudden stops.

 

Dave, me thinks he was referring to the 1/2" slot head thingey on the throttle body ... for us L-jet types, that is an idle adjust screw .... . I suspect that WH928 has it screwed in tight without any effect on idle - so was questioning whether it was sealing when closed. Plausible?

 

Duh.... thanks Garth. Big brain fart there. Thanks for the reality check!

 

Here are some air leak issues I had on my '79 cab.
1) There is a rubber boot between the throttle body and intake plenum (center of the spider). Mine was hard as glass and had a 1/4" crack up the middle
2) There is a plastic fitting through which the hose from the aux air valve plumbs into the air plenum. These are easily cracked. Mine was.
3) The Vacume Limiter (called a deceleration valve on L Jet cars) was leaking. In my case there was an adjustment on it and when I tightened that up it helped.
4) After I solved all the air leak issues I still had an idle that was way too high when the aux air valve was open. I could adjust it for a good idle when cold or when hot but not both. I put a plug in the hose to the aux air valve and drilled it with larger and larger bits until I got a nice idle at both cold and hot.
Hope this helps.

 

I'll suggest a sticking vacuum limiter. Right side (US passenger) of throttle body with a regular (small) vacuum hose attached to the outside. Other suggestions for leaks are also good possibilities.

The idle speed adjuster is on the front of the throttle body. A long flat-blade screwdriver can get to it just behind the oil fill. Look for a screw head about 3/4" across canted at an angle.

 

After carefully reading through this thread again, I wonder if you are talking about the Auxiliary Air Regulator... looks sort of like the attached schematic and is shown in the center of the attached pic. It should not pass air after it's had 12V applied for a while. Verify 12V presence, verify it closes after a couple minutes. I've also attached a functional schematic.