Is my mixture too lean?

[Aurel]

Car is 1978SC.

I am assuming my mixture is too lean because I never readjusted it after switching from 8.5 to 9.3:1 compression, backdated exhaust and sport muffler.
It runs fine but has a few symptoms below:

- Cold starts are difficult, usually starts on 4 or 5 cylinders, then backfires and then runs on all cylinders just fine.
- If colder, I usually have to lift the pressure plate to get it started.
- If I set the idle air bypass screw to be 950rpm when hot, it oscillates when cold.
- Backfires a little bit on deceleration
- Spark plugs are light grey, very clean.

So my plan is to richen the mixture a tiny bit. Other suggestions?

[walt]

The first post of this thread might give you a clue as to whether you are too lean. Anyone local that might have a CO analyzer?
And I would check for any possible vacuum leaks first.

http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

[mikedsilva]

Originally Posted by Aurel

Car is 1978SC.

I am assuming my mixture is too lean because I never readjusted it after switching from 8.5 to 9.3:1 compression, backdated exhaust and sport muffler.
It runs fine but has a few symptoms below:

- Cold starts are difficult, usually starts on 4 or 5 cylinders, then backfires and then runs on all cylinders just fine.
- If colder, I usually have to lift the pressure plate to get it started.
- If I set the idle air bypass screw to be 950rpm when hot, it oscillates when cold.
- Backfires a little bit on deceleration
- Spark plugs are light grey, very clean.

So my plan is to richen the mixture a tiny bit. Other suggestions?

Sounds like an 1/8th turn clockwise, might get you close. But best to check with a wideband to be sure.

[yelcab1]

I bought an AFR (air fuel ratio) meter after I messed around unsuccessfully with this. That gets me right on the money.

[eastbay]

Originally Posted by yelcab1

I bought an AFR (air fuel ratio) meter after I messed around unsuccessfully with this. That gets me right on the money.

Every old car needs this........

[mike sampsel]

Originally Posted by yelcab1

I bought an AFR (air fuel ratio) meter after I messed around unsuccessfully with this. That gets me right on the money.

+ 1, already had an AFR meter for my carb cars. Just used this on my sport cat bypass which already had the appropriate connection.
You are likely fine if you have an SC with lamda sensor, 78 not so good with OEM cat connection. Gets it right to 13.5 AFR at 1100 rpm with web cam 20/21.

[mike sampsel]

Originally Posted by Aurel

Car is 1978SC.

I am assuming my mixture is too lean because I never readjusted it after switching from 8.5 to 9.3:1 compression, backdated exhaust and sport muffler.
It runs fine but has a few symptoms below:

- Cold starts are difficult, usually starts on 4 or 5 cylinders, then backfires and then runs on all cylinders just fine.
- If colder, I usually have to lift the pressure plate to get it started.
- If I set the idle air bypass screw to be 950rpm when hot, it oscillates when cold.
- Backfires a little bit on deceleration
- Spark plugs are light grey, very clean.

So my plan is to richen the mixture a tiny bit. Other suggestions?

You can raise the plate ever so slightly and judge to lean or rich. I think if you move your lever up just a tad and the RPM increases you are too lean.


Or here

911 CIS Primer - Testing:

Although this is based on 14.7 AFR? Nothing beats the AFR meter.

Mixtures

[mike sampsel]

PS there is a nice video of a RUF slant nose Turbo here in the upper right corner. We can assume the mixture is correct,
I think

https://www.rarelyl8.com/cis.html

[PeteKz]

+1 on AFR meter. It's so much more useful to know what your AFR is doing while actually driving it.

[proporsche]

back in the late 1970 in Czechoslovakia an oldtimer show me how to check emission , he used a larger march and put it behind the muffler pipe .If the flame went in it was lean to ok ,if it went away from the pipe -it was to rich.For tuning we actually use short way VKV radio..sounds

Ivan

[Aurel]

Thanks everyone. I will start with 1/8 turn to the right and take it from there.
AFR gauge is good advice, but this ain’t a turbo or high compression race motor, so I will play with the mixture screw first, and if I mess everything up then will reconsider AFR gauge.

[walt]

I found that adjustment to be quite sensitive. I would start with 15-20 degrees and take note of your starting position.

[Aurel]

I added about 1/8 turn richer. The car seems to run smoother, and does not backfire at all on decceleration. I am a the point as if I lift the air flow pressure plate it will stall, while before the idle would surge if I lifted the pressure plate. The true test will be a cold restart. If that is improved, then mission accomplished .

[unclebilly]

You did what I would have suggested. If it’s oscillating, it’s too lean. I wouldn’t have gone more than 1/8 of a turn.

[pmax]

Decel valve adds air on decel which contributes to the lean backfire.

[Zuffenwerker]

The best way to dial it in is with a gas analyzer. It’s amazing what the gas analyzer can pick up on. The analyzer is much more sensitive than an o2 sensor it’s easy to see if you have both and play around with the mixture. But hey if you don’t want to buy one or don’t want to take your car to a professional an o2 sensor is the way to go

[Gretz]

1982 SC here. Lambda unplugged, Wideband O2 sensor and AFR installed. I am good with mixture / idle adjustment.

Just wondering what idle AFR people are running?

Obviously I will be monitoring while driving to analyze AFR under WOT and other conditions to ensure the enrichment mechanisms within CIS are functioning correctly.

[PeteKz]

Ethan: As you may know, the Lambda CIS WUR does not have vacuum enrichment. It only enriches when you push the throttle past about halfway and trigger the switch on the throttle body that tells the OXS box to enrich to full load.

Please post your AFRs running open loop with the lambda unplugged. I'm curious what you end up with.