2.7L High Idle CIS Question

[Arminho]

So we finally got the 2.7L back in the 911 after having the case split and refinished at Ollie's. Pretty much a complete rebuild. Car fired up right away and idles at 1500-1700rpm. CIS pressures were checked and appropriate. No vacuum leaks noted. Fuel delivery system checked and appropriate. When throttle is applied, idle surges and stays surged/high until the air flow sensor plate is touched, at which point idle returns down to 1500-1700. I can tell there is air getting past the sensor plate after idling. I have a feeling the car idles high because the air flow sensor plate isn't adjusted appropriately.

Here is an attached video after messing with mixture and idle adjustment
At about 24 seconds, you can hear the throttle surge and stay high (approx 4k rpm). It returns to normal after messing wit the air sensor plate at 28seconds.
https://www.youtube.com/watch?v=3pq8JO8Oo_I

Another video: https://www.youtube.com/watch?v=pVNDEzwBBTQ

Can anyone comment if they had a similar problem or if there is an easy way to adjust the air flow sensor plate? All drawings/material I have found in the technical handbooks have pictures which are too grainy to learn anything from.

[Dave Kost]

Start here-

https://www.youtube.com/watch?v=8TcAzqKxWgY

[Arminho]

Originally Posted by Dave Kost

Start here-

https://www.youtube.com/watch?v=8TcAzqKxWgY

I've watched this video, informative for sure! The fuel distributor was checked and plunger is not stuck. In fact, both fuel distributor and WUR are newly rebuilt on this.

[fanaudical]

Which year 2.7? What CIS components installed?

[Arminho]

Originally Posted by fanaudical

Which year 2.7? What CIS components installed?

1974 2.7L. All original components.

[Dave Kost]

You condition could be a lot of things- Cam timing. valve adjustment, air leaks, ignition system- are the ignition wire correct, Its easy to make a simple mistake.

The way it backfires thru the intake you will want to do a smoke test to check for unmetered air entering the intake. You could have a cracked air box. The fuel distributor needs to be calibrated to the air flow meter(below). It looked like the AFM arm was stuck in your video. Make sure it moves freely and smoothly.

Did you replace all of the rubber parts on the CIS system while the engine was rebuilt?

Make sure you do or confirm the following Bosch procedures.

Bosch K-Jet Air Flow Sensor

posted on August 31, 2009 by Larry Fletcher | Comments Off on Bosch K-Jet Air Flow Sensor
It is very important to have the air flow meter set up properly. If it is out of adjustment you will never have the proper mixture throughout the rpm range, you will have lean and/or rich spots throughout the operating range. Inside the air flow meter there is a leaf spring that serves as a stop for the sensor plate. This spring is adjustable by moving a steel pin up or down. This pin is usually flush with the top surface of the air flow meter located directly under the fuel delivery line fitting on the fuel distributor. To move the senor plate down, drive the pin down with a small drift. To move the sensor plate up disassemble the airflow meter and drive the pin up from the bottom. The result you are trying to get is the edge of the sensor plate should rest at the narrowest part of the cone shaped opening at the point farthest from the fuel distributor both with and without fuel pressure.


Bosch K-Jet Mixture Setting

posted on August 31, 2009 by Larry Fletcher | Comments Off on Bosch K-Jet Mixture Setting
After assembly of the fuel system, run the fuel pump and depress the air flow sensor plate for a couple of seconds to bleed air out of the fuel distributor. Remove one injector line from the top of the fuel distributor, and while looking into the injector port in the top of the fuel distributor turn the mixture adjustment screw clockwise until the port just starts to fill with fuel then turn the mixture screw 1/2 turn counter clockwise. Reinstall the injector line, start the engine and adjust mixture when it reaches operating temperature. This is the Bosch recommended procedure for initial mixture setting and adjustment.

[boyt911sc]

What are your fuel pressures (CCP, WCP, & SP)? Use numbers in psi. or bar instead of word “appropriate” for describing your fuel pressures. Do you have a CIS pressure gauge kit? Keep us posted.

Tony

[fanaudical]

Couple of questions/suggestions after watching the videos:

- Have you verified throttle plate is returning full closed after you blip the throttle? '74 has the hand throttle still? Verify the throttle linkage isn't getting hung on the hand throttle and holding it open. (Happened on my '75.)

- Verify the fuel piston slides freely in the fuel distributor and isn't "sticky".

- And what Tony said - Verify fuel pressures are correct and report values along with the part numbers on the WUR and fuel distributor.

[Dave Kost]

Please confirm if you have a throttle regulator? I think 1973.5 and 1974 911's had these. You should and if you don't, you may have to confirm the CIS part numbers as asked by Fanaudical.

If so there are other fuel tests that need to be preformed of the throttle regulator, some call it a throttle valve. I don't recall the test, off hand, but some of the experts here will know. It change the control pressure as the throttle body opens.

I believe there are 3 fuel pressure tests- one at idle, part throttle and wide open throttle.

Here's what it looks like.

https://www.************.com/Porsche-911-1974_p-3331-cis-throttle-position-valve-1973-74-models.aspx?gclid=Cj0KCQjwjer4BRCZARIsABK4QeXFFWyc 4iyNAdfKJ5lq0nqvKSBITRM2gX24lx9Hk1RXlifAxjHomtAaAo gkEALw_wcB

or
https://www.yumpu.com/en/document/read/25703914/72-83-porsche-911-series-cd-repair-manual-renntech
page 18-19

If you PM me and can send you more Cis info if you like.

[Dave Kost]

Something more to think about - the best way to set the idle.

When you go to set the idle the spec is 900 rpm + or - 50 rpm and 2.5% CO. The best way is to find someone that can measure CO. In Pennsylvania we have state inspection station done yearly that include emissions testing. They can measure CO at the exhaust. You will go back and forth making adjustments to the big screw on the drivers side of the engine bay- it by passes air in to the intake at idle, and the 3 mm adjustment set screw in the mixture control unit.

OH and don't forget to check that the ignition distributor in advancing properly too. Some time the advance weights stick.

[Arminho]

Update: Thanks for all the help folks. It's been addressed!

Air flow sensor plate was taken apart and readjusted. Mixture was then set by measuring exhaust CO. All is well in the world!

[Dave Kost]

Originally Posted by Arminho

Update: Thanks for all the help folks. It's been addressed!

Air flow sensor plate was taken apart and readjusted. Mixture was then set by measuring exhaust CO. All is well in the world!

Arminho,

Glad you solved the idle issue and up dating us with the solution.