Porsche 911 SC - Mass Air Flow meter - Mixture screw not accessible?

[PorscheCollecto]

Hi all,

I've got an Porsche 911 SC from 1978. Had some problems with CIS, so have tried to return to "initial mixture setup". Actually followed the guidance of the Joe Engineer youtube videos.
Now suddenly, when I started my car and wanted to do the mixture adjustments after it would get warm, I cannot find the mixture screw with my allen key (Unbrako), just spend 1 hour trying to get the key into the mixture adjustment screw, but no luck. I even tried to remove the air boot to see if it would help. But no success, it's like the mixture screw is gone....I know I've not "over turned" it or something like that.
But is it possible I some how unscrewed it completely? - meaning it's loss inside the Mas Air Flow meter unit?
I really hope not....anyone else tried the problem I'm facing? Perhaps has some good inputs?

Thx in advance.

BR
Chris

[Funracer]

So just to be clear you are using a 3mm Allen. Have you ever tried to adjust the mixture before? Use a flashlight. When you drop the Allen into the hole, tilt it very slightly forward (away from you) and wiggle until it drops into the 3mm opening in the mixture screw.

Took me forever to find it the first time. At least an hour.

[john walker's workshop]

Sometimes the adjuster hole is full of dirt. A stiff wire can stir it up and then blow it out with an air gun.

[PorscheCollecto]

Hi both,

God questions/comments. Thx alot. :-)

First - yes, I'm using a 3 mm allen key. I used the same key, when I about a week ago did the initial adjustment. Where I adjust the mixture to the injectors spraying lightly and then back half a turn CCW (leaner). Actually I hit the mixture screw there only after a few minutes. There's not alot of room for the allen key, I'll have to admit, since the insulation mat is still in the cars engine room. So I ended up removing the air boot, just to get room to ensure I could proof I could hit the mixture screw, but didn't help much.


Second - hmm, yeah dirt could be the problem, the insulation mat is in a shape where it's deteriorating so some pieces might have fell in. I did already try brake cleaner and air gun afterwards, but didn't help. But I didn't try the stiff wire, so I think that might be the next thing to try. I'll give some feedback when try it tomorrow when I get to the garage.


Would still be good to know if anyone knows if it's possible to turn the screw so much that it can get released. Since it's still my big fear, thought I didn't have the feeling I did that....

Happy to hear more help/comments while I try the "stiff wire" to remove any potential dirt.

[Funracer]

Originally Posted by PorscheCollecto

Hi both,
Would still be good to know if anyone knows if it's possible to turn the screw so much that it can get released.

I have one on my workbench. This is a pic of the actual fine threaded screw you are turning with the 3mm. I see at least 4-5 threads on this one. Of course this one is not yours but looks to me like you would have to turn it many times all the way around for the screw to come out. Seems unlikely.

[Funracer]

The screw might even be locked so it wont come out. But if it did come out it can only be three places.

1.On the arm and thus blocking the arm from full up travel. This would be very noticeable.

2. Fell into the airbox. You would see it laying there.

3. or here:

[scarceller]

I believe this is the mixture screw as seen from the bottom. I see no way for that screw to get messed up, unless it fell out?

[scarceller]

Is this the screw or just the stop? Is the screw on the movable arm? As I recall I think the screw was actually on the arm in my 75 2.7L?

[boyt911sc]

Use an endoscope camera to inspect the fuel mixture screw in your Air Flow Meter. While it is feasible to unscrew the fuel mixture screw, I find it unlikely someone would be able to dislodge it unknowingly because it would require more than 25 complete revolution or more to dislodge it from a running set point.

From a running set point, the fuel mixture screw is usually adjusted between plus or minus half a turn. So it is unlikely the OP, had dislodged the fuel mixture screw by accident.





Tony

[Schulisco]

For a quick view if the screw is in place a simple smartphone camera should also do the job ...

Thomas

[PorscheCollecto]

Hi all,

MAN YOU'RE ALL HEROS - all these pictures and info are perfect. Just what I need. So many thanks.
From them it looks like I should be able to see the bottom of the mixture screw from within the air box. At least that's my clear hope from these pictures. I'll try to investigate more tomorrow when I got a full day in the garage.

Also really good to know, that it seems highly unlikely I've "un-screwed" it. But also from the pictures is obvoius the screw can only be very few places, so that I can search there, if I find out its "un-screwed". But it seems unlikely, and also what I hope, since I really would like not to have to disassemble it all to fix it, it's bigger task.
I'll keep you all posted on the thread hopefully tomorrow night. :-)

Schulisco, on the mobile phone picture, are you sure it so easy? I can't hardly have room for my hand due to the insulating mat, so I doubt it will be easy to use the phone trying to take a picture that will show anything. Remember, the engine is in the car, so the mass air flow meter is not on my work bench. But I can try to tomorrow...

Cheers and once again many thanks to all of you...I keep you posted.

[Schulisco]

Depends on the phone.
Use trigger delay or make a video instead if there's not enough space fot your fingers...

[PeteKz]

If it's running, the screw is still there. It would not run without the screw in place. And, your hex keys would drop down through the hole where the screw was. As Tony said the adjustment is a very narrow range. IME, usually less than a quarter turn.

I think you are just having difficulty finding the hex hole in the top of the screw. As John Walker suggested, put a narrow pick down in there and find the hex hole in the top of the screw, and scrape it around, then blow it out. Then try to insert the hex key. You will have to feel around to find the top of it and rotate the key until it drops in. Using a hex key with a BALL END will help too.

[icarp]

Try removing the air filter , then aim a flashlight in towards the area where you think the allen screw should be . Then look down from the top, between the peanut and fuel dist and the light will be visible through the access hole. This should help to guide you get the 3mm into the hole , then make sure is straight up, no tilt, it should drop into the allen .

[boyt911sc]

Insert the plastic tube/straw (vertically straight) to the access hole and spray a couple of times. Don’t forget to place some paper towel in the air box to collect the wash liquid.



With the 3-mm Allen installed to the fuel mixture screw, you could lift the sensor plate with a gentle twist upward. This will confirm the presence of the mysterious fuel mixture adjuster screw.

Tony

[PorscheCollecto]

Hi all,

Back for a promised update to all of you.
And the update is good :-)

All help from all of you help me get to the goal.But it wasn't without a lot of headaches.
I tried to take pictures and video, it didn't help much, at least not good enough to confirm the screw was there.
I tried the "clean dirt" trick, both with stiff wires of different thickness and spraying brake cleaner into the hole. Didn't manage to get proof of any dirt getting out, but perhaps it worked.
With a very short unbracho allen key with.out rounded tip I was able to hit the hold, but that key was so short it only work when air boot was off.
I then re-manufactored one of my Allen unbracho key so it was possible to bot hold it well and press it down at the same time. (it was with round tip, though I preferred without the rounded tip, since it made it more difficult what one would believe). After spending some time making a "perfect" allen key, I finally mange to be able to both hit the mixture screw and turn it, BUT while pressing the allen key down while turning. It seems like, either there's still dirt in the screw or else the screw internal edge not good anymore. I think most likely it's the first, so dirt, since only few days ago I could hit the screw firmly without problem and I was sticking into the screw so I could lift the "pedal" with the allen key.
Anyway, by usage of the "new" allen key and pressing (not too hard either since this moves the pedal) and turning; I manage to do the adjustment process. Leaning it out while also adjust Idle screw, and not the car starts and runs nice, at 950 rpms in Idle warm. Yahoooooo! Great to have it solve, though I preferred the mixture screw was without dirt, but must be for a little fix when full rebuild the motor where all stuff is out for cleaning. :-)

I though still have another weird, but think I've got another thread about that.
It's that the car is bucking at low rpms, below 3000 rpms. Eg driving with small or nearly nothing pressure on throttle will make it buck... also I noticed ion the test drives, that eg being in 2nd gear accellerating to 5000-5500 amps and then fast releasing the throttle completely, would make the car drop rpms fast (logically), but below idle level, so down to eg 500-600 rpms, and then die. And this scenario is reproducible.
I'm wondering if it can be the decal valve that defective? Any suggestions/recommandations?

Btw: I know all fuel pressures are perfect, they are all just tested. System pressure, Control pressure (cold and warm) and residual pressure. They are all good and within specs according to Bentley.

Anyway, guys, many thanks for the help on the mixture screw problem, couldn't have done it without all you help. So I'm very grateful. :-) :-) :-)

[Schulisco]

PorscheCollecto,

glad to hear that you finally made it.

Concerning to the bucking car: You already tested the fuel accumulator correctly as described in the manuals - it holds up the pressure as designed. But this is in my eyes only half the truth! I think that the FA is the culprit. Why? The FA works continuously, against the general opinion! This means it's not only holding up the pressure when engine shut off! Also it silences the pump vibrations (you can read exactly this in the Bosch literature). I other words it flattens the pressure vibrations and smoothes the fuel flow to the injectors to let them spray much more continuously the fuel.

Modern fuel contains ethanol which attracts water and let the fuel detoriate and corroding the CIS components, like fuel tank, steel lines, fuel distributor, fuel injectors etc. And even the FA. But: The FA is mostly overseen of that because they still hold up the pressure and being claimed as OK. But they also corrode within and this corrosion prevents the diaphragm/the plunger moving smoothly. I heavy cases they get even stuck.
These hesitations of the FA cause an inapproriate fuel disposal in several situations (esp. low speed rpm) and therefore the car is bucking. On WOT the fuel amount is that high that this behviour won''t be noticed...
You can test the FA external from the car by pressurizing the FA with air (closed output and overflow, if present) and suddenly depressurizing it (important!) by opening the hose quickly. If the spring inside hesitates to follow the pressure relief, then you got the culprit...

To prevent such damages drive the car regularly and ahead of longer sitting times use a fuel stabilizer when fillup and drive the car afterwards at least one hour, to make sure the fuel stabilizer mixes completely with the fuel. I use a fuel stabilizer on every fillup, so I don't have any hazzle on that any more, even when the car accidentially sits longer than some days or weeks.

Other tasks needed to be done now (in this order):
1. Check the sensor plate height meticulously accurate as the Porsche/Bosch manuals say for your car.
2. In conjunction to 1. check the "injection begin" (my wording), this means the start of injecting fuel into the cylinders by slightly lifting the sensor plate, this setting is *the* basic setup for CIS, made as well with the mixture screw. This setting is very, very important for the whole rpm band. If this setting does not correspond with the sensor plate height, your CIS will never work as it should: Either the engine starts good and run poor or vice versa.

3. Check the mixture on a certified gas tester! Follow the instructions from the BoschPorsche workshop manuals and only turn the mixture screw from lean to rich mixture as Porsche and Bosch recommended in their manuals. This is because the mixture screw is that sensitive that a senseful adjustment can be done only when turning it from lean to rich very cautious. We're talking about 1/8 turns every step...and always take the allen key out of the hole / from the mixture screw, it's weight also heavily affects the setting...

See some threads with more information:
https://forums.pelicanparts.com/porsche-911-technical-forum/1108835-cis-sensor-plate-height-3.html
https://forums.pelicanparts.com/porsche-911-technical-forum/1119098-911-sc-cis-residual-pressure.html
See also the 911 CIS 2.7series of YT Channel KlassikATS:
https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj
Especially the last two of them are meaningful for CIS basic adjustment.

Also not to forget to check the ignition timing which also have a huge impact on smooth low speed driving, fuel economy and performance! The whole CIS system is quite sensitive about faults and changes. And check the timing advance mechanism of your distributor, if it's stuck, the ignition is also suspicious for any driving issues...

Thomas

[Alan L]

air leaks will cause the bucking too.
You may have set the idle mix - and maybe correctly - for the 950 rpm. But if you have airleaks in the CIS system, as soon as you move off the idle mix, the mixture will be erroneous. Likely lean. You need to confirm no airleaks. There are many potential places for a split vac hose etc.
Also improper injector flows - esp at low openings.
And imbalance in the flows at the FD. Spec is not more than 10% variance. All these things will cause the symptoms.
Alan

[pmax]

Ah yes, the bucking bronco at parking lot speeds ...

always check for vacuum leaks first before touching the 3mm mixture.

[Alan L]

It may not be just one thing - but an accumulation of many small things -eg a couple of substandard injectors, an FD slightly out on a couple of ports, throw in an air leak or two. So you may see incremental improvement as you nail the culprits. Another place to look is the injector O rings. If the car has not had recent attention it could be many of these things.
After paying attention to all these things I have got my car down to a very slight hesitation around 1800-2000 rpm. recalibrated the FD was the last effort to nail it completely. Still there. may be the injectors. 40k miles old now.
Alan