CIS runs rich under load, perfect at idle?

[djb25]

Hi everyone,

I've been slowly working through the CIS injection on my '77 911S.

I had a bit of a problem with the WUR, but that has been cleaned and seems resolved.

Last night I hooked up a wideband o2 sensor (LC1) and discovered the following:

1) at (cold) startup - a/f was around 12.5

2) after a few minutes, a/f stayed around 14.5 (it jumps around, 14.3,14.6, etc)

3) driving down the road, at steady throttle, a/f stays around 14.5

4) half-throttle and above, a/f drops to 12 and lower... at full throttle, I found the a/f to drop as low as 11.

Also, and I don't know if this means something, but I found when revving the engine (without a load) the a/f ratio would momentarily jump up really high - as high as 20 or 22... but just momentarily.

Anyone out there have any advice as to where I should start?

Also, sorry that I don't have any graphs, but at this point I don't have any way to log RPM...

- Damion

[GBStu]

Sorry that I don't have an answer for you, but my 1979 CIS is exactly the same. I'm very interested in the responses.

Thanks, Stu

[MikeBogue]

I am far from an expert, but could it be possible that there is an air leak causing the problem? My thought is that if there is an air leak the unmetered air will be a larger percentage of the total air in the system due to higher vacuum in the intake. If the mixture is then set at idle it will tend to run rich at higher speeds (less vacuum=lower percentage of unmetered air).

[timatt]

Are you having driveability problems??

I'm assuming that your car did not come with an installed oxygen sensor.

I'm no expert, but your A/F numbers do not look bad to me. Certainly not as good as a modern injection system with lambda feedback.

Tim
73.5 T

[djb25]

Originally Posted by MikeBogue

I am far from an expert, but could it be possible that there is an air leak causing the problem? My thought is that if there is an air leak the unmetered air will be a larger percentage of the total air in the system due to higher vacuum in the intake. If the mixture is then set at idle it will tend to run rich at higher speeds (less vacuum=lower percentage of unmetered air).

Mike,

That's a really good theory - why didn't I think of that? I think that's where I'll start.

Tim,

I'm not having driveability problems per se - rather I want to have the car properly set up and tuned. And no, my '77 did not come with an o2 sensor - and certainly not a wideband o2 sensor.

Not sure how that's relevant, though - running rich is running rich, regardless of whether I would have been able to log my a/f ratio on my thinkpad in 1977. I'm certainly not an expert, but I don't think a/f ratio dropping below 12:1 at full throttle is "okay." I've heard that CIS cars like to run a little rich, but this seems excessive to me.

Maybe I'm wrong and someone will correct me?

- Damion

[psalt]

Damion wrote,

I'm certainly not an expert, but I don't think a/f ratio dropping below 12:1 at full throttle is "okay." I've heard that CIS cars like to run a little rich, but this seems excessive to me.

Hello Damion,

It is hard to get a steady state readings on the road without datalogging. A 14.7:1 AFR is "ideal" only for emissions and the life of a converter. It neither produces best power or best mileage. Lean best torque occurs at around 13.3, safe best power at WOT 12.2, and rich best torque at WOT around 11.5. You are losing nothing with an AFR of 12 at WOT, especially if it is under acceleration which requires more enrichment. CIS is calibrated for a new engine, if the engine is old and the compression has dropped, it is a less efficient air pump with the same amount of fuel. Try and capture some data a steady rpm.

Paul

[safe]

Whats the problem?
Its 35 year old technology!!

Lambda 1 = AFR 14.7:1, that's good for making the cat work.
Maximum power on the other hand is around lambda 0.86 => AFR 12.6:1

Whats your CO-level at idle?

Maybe you could lean it out a little, but if its running nicely, why mess with it. Your gas is so cheep anyway.