Turning the mixture screw too much will cause exactly what you have.
One clue your too rich is you say on startup it stumbles around 600.that's because it's loaded up. When warm it hangs up ,both are signs of over rich mixture setting.
The more i think about it ,it's a small problem/minor adjustment.
Also the extreme temp diff where you are morning to afternoon also has to be taken into consideration. I think it just hangs up a little longer in the 100F is because it's a lot easier to spin an engine at 100f than at 65f+ 100f less air so richer again.

Idle and mixture screws adjustments.......
 

Bruce,

Adjusting the idle screw affects the engine idle speed (RPM). It by-passes metered air and has nothing to do with fuel mixture ratio. The mixture screw in the metering unit does affect the fuel ratio and idle speed. I never tinker the mixture screw unless I have checked and confirmed the absence of vacuum/air leak/s. Without knowing the fuel pressures and vacuum plus the change in the fuel mixture, it is not easy to diagnose the problem and identify the culprit/s. At this stage, we will be all guessing.

Tony

Yes ,your right about the mixture screw.
When talking about the bypass I should have said counterclockwise more air,clockwise less air.

Matt says his mechanic only touched the mixture screw and now he has a problem.
If the mechanic did nothing else and the car started and ran fine before than is it not logical the the problem stems from the mixture adjustment.

If I saw my mechanic hammering a nail into my tire and on the drive home I got a flat would I look for a muriad of reasons why or think maybe it had something to do with the mechanic hammering a nail into the tire.
Sorry ,I'm having a little fun.
Something else may be causing the problem but if it was me I'de start with the mixture screw if that's the ONLY thing that was touched.

OK guys - looks like I will have to bring the car to a Porsche tech and have them re-adjust the mixture using their gauges and equipment. I am tempted to turn the mixture screw myself, but I have no idea what effect of which turn of the screw will do what so I better have someone do it professionally. Will update the thread when I get that checked out next week. Thanks for all the suggestions! :)

Matt, I am saying your car should die because it should be (with no vacuum leaks) too rich to run. With vacuum leaks it can run. (Hold that thought because I am going to literally check it on my 81 to make absolutely sure - Back in a bit) Edit: Car warmed for 5 minutes and I turned idle bypass screw all the way in. 600 RPM. Had to wait over ten minutes to make it stall. That thing even spun at 200rpm.

What I do not understand is the interrelationship between the single change made by your mechanic and a preexisting air leak - assuming there is one.

lonewolf - I am glad you brought up the idle bypass screw because it makes me think harder about it. It will not change the (a/f) mixture mechanical setting, but it will impact the exhaust analyzer results, surely, at idle. The exhaust is a function of the a/f into the power cell - not the air passing over the air sensor.

Matt.
If you wanted to give it a try figuring nothing to loose at this point and lets assume nothing else was touched then here's what I would try.
first get the car warm and RPMs hanging up just as you described but make adjustment when at idle.
You have the right size and length allen key? think it's 5in long..
drop the key into the mixture screw .
where -ever the right angle part of the key points lets call that 6oclock. In other words it's pointing to the number 6 on a clock face.mark it so you can return there if need be..from that it's easy to tell where 3 and 12 and 9 would be in relationship to 6.on a clock face.
I would turn the key counterclockwise to what would be 5.30. That's a very small amount. Hold the key firmly when turning it so you can control a small turn .
reach over to the throttle linkage snap the throttle so it revs to at least 3000 and see how the revs drop.You should notice a difference. If not at the 5.30 setting then go to 5oclock and repeat.
By doing this you are slowly leaning it out .
I would first go to 5.30 or 5.0 and leave it and let car cool down ,maybe sit overnight .
how the car starts up when stone cold will also tell you if your adjutments were in the right direction.
should fire right up and be right at 15-1600 right away and slowly (within 2-3min or sonner depending on ambiant temp over 70f)go down to 950.
yours was chugging on startup . I think because it's too rich.by leaning it out a CH you should notice a different and better startup as well as the hangup problem slowly going away.

lonewolf - are you thinking he has no pre-existing air leak - or leaky aav/aar? How should he address this?

Matt can perform the mixture screw adjustment and his drivability will improve. Just don't want to lose sight of another problem that has not been addressed.

Also, adjustments always should end with a CW rotation of the screw after backing off CCW.

I keep seeing this advice about ending with CW turn. Can someone explain why that would be necessary?

Yes.. I'm assuming that car ran fine before adjustment and had no air leaks or idle problems.Tony asked why Matt took the car in.
From what i read it was fine just running lean to pass smog test.
Matt.Was there was another issue that you thought adjusting the mixture would fix ?

I 'm not sure I'm in agreement on the ending in a clock wise rotation.
I know what's been written on the subject but we are only moving it back a ch or so and I'm trying to make it simple for some one who has never done it

Just another myth........
 

People believe what they like to believe and continue on from generation to the next........this is like adding a little sugar on your coffee after mixing it. What if you didn't add that little bit of sugar? You have coffee with less sugar!!!!!

Tony

Hey, I just read the stuff, too. Regarding threads, pitch, etc - CW vs. CCW. I think I read it in the Charles Probst book in addition to numerous posts. Probst is pretty good about explaining the reason for doing stuff. I'll see if I can find it and post what he says about the final tweak.

Stop hurting my sensitive feelings with your crafty metaphors.

Edit - I did find my 101 projects book and Wayne does not mention the final CW rotation of the screw. I am still looking for my Probst book.

Thought I would give an update about the problem. Bob Kontakt has been an immense help and he has been emailing me photos of places to check for vacuum leaks and even tweaking his car to imitate my symptoms. Thanks Bob. And thanks to the rest of you for your suggestions. Over the past couple of weeks I have been looking for air leaks and found out that the back of the pop off valve had a giant air leak. I ended up sealing that with JB Weld and epoxy. Tonight, I attempted to adjust the mixture and I got mixed results. I turned the mixture screw CCW and nothing really changed. I tried adjusting the idle at the same time and finally got the idle stable at 950.. unfortunately the car still hangs up at 1600 but since I leaned the mixture, it hangs up for a shorter period of time. I think its time for a Porsche tech to get the right gauges so we can figure out the A/F ratio numerically with the right equipment.

Matt,

I've been telling people that being unable to find a vacuum/air leak in a CIS engine does not mean you don't have one!!!!! So a test to verify and confirm this anomaly is mandatory. A simple pressure test is all you need to do. But people still insist using flammable gas or solvent for detecting air leak/s which is not very effective compared to pressure test. Aside from ignition and compression, fuel pressures and vacuum/air leaks are the two primary sources of CIS problem/s. Control these two parameters and you'll have a very reliable and dependable car/engine. Just my two-cents.

BTW, we need more 'Bob K.' in this forum. He's always willing to give you a hand and extend help when needed.


Bob,

I'll be away from this forum for 3 weeks and when I get back, I'll read all your posts and read them with great interest. Good job.

Tony

Got it fixed!! Turns out it was the deceleration valve that needed adjustment from the nut at the top of it. The mechanic re-adjusted the mixture and then adjusted the decel valve and the problem went away. So I guess for the future guys who read this: If all else fails to improve the condition, try adjusting the decel valve (only applies to early 911 SC engine since the design of the decel valve changed in 1980).

Thanks to all who helped and happy driving! SmileWavy

Thanks, Matt and Tony.

Matt, the 80 onward valve can be adjusted but it's not scientific. You squish the spaceship shaped thing near to make it a little skinnier. Some do it in a vice, some do it with channel locks but it probably has roughly the same impact on the diaphragm, brings it back into operating tolerances.

I know it's an old post, but from the Matt's first post, I thought "decal valve". For those of you/us with an 80-83 911SC, the decel valve isn't visually adjustable--but it IS! A TSB came out showing that you can put sockets on both sides in a vice and compress it 1mm.

The way I do it is a bit more scientifically--take a vacuum reading of your manifold vacuum at idle / 950 RPM (any port on the throttle body BELOW the throttle plate), that's your idle vacuum. Now, with your decal valve out of the engine and in your hand, connect a vacuum hand pump to the nipple. While pulling vacuum, simultaneously suck on one of the two large ports and STOP pumping as SOON as you can feel air passing the into your mouth. This is your decal valve trigger HG. If your decal valve's trigger/open point is LOWER than your idle vacuum HG, it will open at idle! Folks get it mixed up all the time--manifold vacuum drops on acceleration, NOT increase.

Let's say your idle vacuum is 18HG. Then when you accelerate, your manifold vacuum drops to, say 15 HG. On it's way back down to idle vacuum, it should hit a point where it triggers the deceleration valve to open just for enough time to slow down your deceleration, preventing an ultra-low RPM situation and possible stall. So in this situation, a decal valve that opens at 16-17HG would be perfect--as long as it's a little less than your idea manifold vacuum. Otherwise it creates an endless loop where the decal valve thinks, "Hey, it's time to do your thing!" but it's NOT. That's why when you clutch the motor, you create a high vacuum situation by putting a load on the motor, thus closing the decal valve momentarily.

The only way this valve could have be improved is by tying an electronic valve into the 35 degree throttle switch: Close valve when below 35 degrees. That's my opinion, that it for what it's worth.

Aloha, Keoni