911SC timing and idle adjustment

[83_Silberpfeil]

After I validated the TDC mark and the case half/split are aligned, I started the engine --- first time in 2 weeks.

Again, the cold idle started ~ 500 or 600 RPM, and seem to struggle. So, once again, I turned the idle screw CC to get it up to ~ 900. Then, once the temp engine started to warm up, it settled ~ 950.

That is when the engine starts to do a small hunting/surging cycle. Each cycle is ~ 10 seconds and the idle moves just a few hundred RPMs, and comes back down. Up/down between the 1K mark and the one just under it on the tach.

It is certainly not the up/down by 500+ rpm every second or two type of surging/hunting like what you see here: https://www.youtube.com/watch?v=amC1UBQKZE8&list=PLemxdZbLL_s8wYIUSzgRO9L26k5lC5n1o&index=61

I will to a cold start again tomorrow to check if the idle again starts at 500 ~ 600.

Originally Posted by 83_Silberpfeil

For the rest of afternoon, I was able to get the idle near 950 RPM, and set timing (gun/light @ 5* BTDC) and align the marks Z1 and alt housing TDC marks.

@Bruce - thanks for the comments and posting your fotos. They do help me visualize how the housing is mounted.

I will go back and look in the engine bay - check alt housing TDC mark & how it aligns w case center line.

I must say beforehand, it is so crowded back three, and dark. Really hard to find stuff let alone take a clear foto. But, I'll look for them and try.

[Alan L]

When you change the idle speed it changes the idle mix. You may need to adjust once you have settled where you want to be - after you are sure you have your timing and idle speed where you want.
Alan

[83_Silberpfeil]

@AlanL - "idle mix", do you mean the air/fuel mixture, the 3 mm allen at the fuel distributor?

Originally Posted by Alan L

When you change the idle speed it changes the idle mix. You may need to adjust once you have settled where you want to be - after you are sure you have your timing and idle speed where you want.
Alan

[Alan L]

Correct.
Alan

[PeteKz]

Not to cast aspersions, but changing the idle speed via the idle screw on the throttle housing does not change AFR on my CIS car. I would expect it to make even less difference on a lambda CIS system.

Either way, set the timing first, then adjust the idle screw, then make final adjustments to CO screw for CO%/AFR

[Alan L]

I am open to alternate opinions on this. But I usually find I need to tweek the idle mix if the adjustment is more than minor. It is an air bleed screw that is bypassing the closed butterfly on the intake. So it is passing more air into the intake manifold.
What would your take on it be?
Regards
Alan

[PeteKz]

The screw lets more/less air around the throttle plate, but that is after it was metered through the air metering unit, so it should not change the mixture, and does not on my CIS car. It is effectively the same as opening the throttle plate a tiny bit more, just more precise than an old-style throttle stop screw. And a lambda system will constantly adjust the AFR/CO% to the setting the OXS system was designed to produce.

That's if everything is ideal. However, differences between our engines, due to air leaks, parts out of calibration, etc. may result in measured differences due to idle adjustment.

[Flat6pac]

You have to disconnect the Oxygen sensor from the cat in order to keep the lambda from correcting your changes. Once you’re set replug in your OX sensor
The seeking of the injection is from running too rich which is controlled by the 3 mil Allen. Once warm and running, disconnect ox sensor
3 mil Allen turning slightly CCW will cause more seeking idle, back it CW slightly to smooth idle.
Once smooth, adjust the idle speed knob to set. Replug OX sensor.
Bruce

[Alan L]

Originally Posted by PeteKz

The screw lets more/less air around the throttle plate, but that is after it was metered through the air metering unit, so it should not change the mixture, and does not on my CIS car. It is effectively the same as opening the throttle plate a tiny bit more, just more precise than an old-style throttle stop screw. And a lambda system will constantly adjust the AFR/CO% to the setting the OXS system was designed to produce.

That's if everything is ideal. However, differences between our engines, due to air leaks, parts out of calibration, etc. may result in measured differences due to idle adjustment.

Yep - thinking on it after, it is metered air. So should make no difference.
I have no experience of lambda - both my cars without. But invariably I find if I take the idle from say a lugging 600rpm to 950, I need to tweek the idle mix.
But it must be for a reason other than what I had in mind. My best guess would be with the valve overlap the engine efficiency is different at 600 rpm to 950.
Just went thru this process a few days ago, and did have to tweek the idle mix.
Regards
Alan

[David Inc.]

Originally Posted by Alan L

Yep - thinking on it after, it is metered air. So should make no difference.
I have no experience of lambda - both my cars without. But invariably I find if I take the idle from say a lugging 600rpm to 950, I need to tweek the idle mix.
But it must be for a reason other than what I had in mind. My best guess would be with the valve overlap the engine efficiency is different at 600 rpm to 950.
Just went thru this process a few days ago, and did have to tweek the idle mix.
Regards
Alan

Sounds more like your idle is going off because of a vacuum leak messing with the mix, you adjust the idle back to standard and then you're bandaiding the mix to cover the vacuum leak. I've been in the same boat, and found vacuum leaks later.

[83_Silberpfeil]

In all likelihood, there probably is a vacuum leak somewhere. My car is an 83 SC, stock. Owned it since 2010 and never done smoke test or chased after any hoses/connectors other than the AAR. A few weeks ago, I hand adjusted the rubber hose connecting into the AAR, and tightened the clamp a bit.

I'll need to plan for a proper smoke test one of these days.

Originally Posted by David Inc.

Sounds more like your idle is going off because of a vacuum leak messing with the mix, you adjust the idle back to standard and then you're bandaiding the mix to cover the vacuum leak. I've been in the same boat, and found vacuum leaks later.

[83_Silberpfeil]

@Alan L and @PeteKz - thank you both for chiming in and providing your thoughts on what's happening, and how to remdedy. I'll mark these posts and revisit them as I work on this issue.

I have Bentley's, Wayne's two books, and Jim's Basement Garage as resources. So will continue to study the CIS fuel system and revisit your comments.

Originally Posted by Alan L

Yep - thinking on it after, it is metered air. So should make no difference.
I have no experience of lambda - both my cars without. But invariably I find if I take the idle from say a lugging 600rpm to 950, I need to tweek the idle mix.
But it must be for a reason other than what I had in mind. My best guess would be with the valve overlap the engine efficiency is different at 600 rpm to 950.
Just went thru this process a few days ago, and did have to tweek the idle mix.
Regards
Alan

[83_Silberpfeil]

@Bruce - thanks for chiming in and providing instructions. Very helpful. I will book mark this and revisit as I fine tune.

Originally Posted by Flat6pac

You have to disconnect the Oxygen sensor from the cat in order to keep the lambda from correcting your changes. Once you’re set replug in your OX sensor
The seeking of the injection is from running too rich which is controlled by the 3 mil Allen. Once warm and running, disconnect ox sensor
3 mil Allen turning slightly CCW will cause more seeking idle, back it CW slightly to smooth idle.
Once smooth, adjust the idle speed knob to set. Replug OX sensor.
Bruce

[Alan L]

Originally Posted by David Inc.

Sounds more like your idle is going off because of a vacuum leak messing with the mix, you adjust the idle back to standard and then you're bandaiding the mix to cover the vacuum leak. I've been in the same boat, and found vacuum leaks later.

Always possible. But last I looked, there were none. It is running like a swiss watch - from cold start right thru. I have the same experience with the 930, and def no leaks there.
It has an AFR gauge, altho I have not concsiously followed this via the AFR.
I will give it a shot and see what happens on the AFR. Hopefully get the car back today - getting some minor panel repair. Off o/seas tomorrow, so will try and get time to check today.
Alan