How to decrease idle when adjustment screw is fully screwed in???
 

After fixing my exhaust leaks, my idle has increased and now noticed the adjustment screw is fully screwed in but idle is still 1100rpm instead of 900rpm... Any suggestions how to decrease further?

High Idle
 

Disconnect the throttle linkage at the airbox lever and make certain that the throttle is closing completely. If not, there are two places on the throttle linkage where you can adjust the throttle rods (3 if you count the end by the throttle pedal). The rod gong up to the air box lever is the easiest one to access, but there is another shorter rod down on the throttle console mounted on the engine. If everything is adjusted properly the idle screw should be slightly open.

You can just disconnect the throttle and see if it matters. I have the same issue and the throttle is not significant. Probably need to move on to the 3mm screw adjustment, which is probably best done with an exhaust analyzer.

Make sure you don't have any vacuum leaks.

JR

After fixing my exhaust leaks, my idle has increased and now noticed the adjustment screw is fully screwed in but idle is still 1100rpm instead of 900rpm... Any suggestions how to decrease further?


Sounds like you have vacuum leaks. Check around the injectors and boots with an unlit propane torch. Can you wiggly the injectors and are the sleeve O rings original ?

some previous owner or tech may have adjusted idle speed at the stop screw instead of the air bleed. i've found many a crack in the brake vacuum hose down by the rear toesion tube. pinch it off in the engine area and see if the idle changes. too much ignition advance will up the idle. an overactive deceleration valve also.

Thanks for all the tips!

Disconnected throttle linkage but still same problem
Sprayed starter fluid on injectors -> No change in idle -> No vacuum leaks there
Rechecked all CIS hoses, several clamps were tightened up a bit but don't know if that could be the cause..
Will test further this evening as I didn't want to wake the neighbours yesterday ...

Take a look at this thread for a little info on how to adjust the throttle butterfly:

http://forums.pelicanparts.com/porsche-911-technical-forum/625887-startup-idle-rpm-all-time-1977-2-7-cis.html

Check around the back of the airbox. There are several hoses that connect there which can look tight but may not be because they are hard to reach. I found a major air leak back there.

Just for my understanding, how does anyone know exactly what car he has? I see a Pic but to small to make out and later I saw he mentioned CIS.

FYI, this concerns a 1978 SC

Vince - I looked at previous posts - he has a 78.

If your car does not die or nearly die when the air bypass is screwed in, you have extra "false" air in the system. Check the base of the intake runners with the spray or propane. Check around the airbox as mentioned. I use starting fluid - more dangerous but effective.

Do what JW and psalt say. If you are afraid to pinch the brake booster hose, disconnect and plug on the engine side - use duct tape if you have to - and see if change occurs.

You need to attack the leak(s) and "Terminate with extreme prejudice". It should be your absolute first order of business.

For JW; where is the stop screw?? Is there a way to check the deceleration valve?

The Bentley SC Repair Manual would be a great help to you re TP stop screw and Decel. Valve.
Lots of info with pictures.

Throttle plate has a basic setting but for that you'll need to take the big black bellows off. Before you start with the TP stop screw:

Have you checked for vac leaks like others have suggested? :confused:

I took it all apart and changed the runner gaskets and the rubber between the runner and the airbox etc and the o rings at the injectors.

Gunter - mspirito is not the OP. Url is. Url will get back into it tonight he says.

Auxiliary air valve sticking open, ignition timing like John said?

open the throttle by hand, back in the engine compartment, and close it. the lever stops on a screw with a lock nut. you can see it below the lever with a flashlight easily. it's adjusted from the factory so the throttle plate closes 99.9% of the way, but open just enough to keep the throttle plate from digging into the aluminum housing. the farther it's beem opened, the more you have to turn in the bypass screw to lower the idle. some people mess with them because they don't know about the air bypass screw. rare, but it happens.

I had the same problem with my 1979 SC and it was a problem with the idle control valve. It ended up being the earth for the idle control valve was disconnect off the back of the alternator.

http://forums.pelicanparts.com/uploa...1321920036.jpg


I also found the throttle valve not closing properly.

It's amazing what you find and what other mechanics have done to fix other problems that they have created.

http://forums.pelicanparts.com/uploa...1321919991.jpg

Don't mean to sabotage the thread, but I just adjusted my throttle stop screw and the idle is now much much better, thanks