CO mixture setting for 80SC

my Haynes manual says between 1.5 and 2.5 but an earlier post said 3.5
The car was running very badly but is ok now around 1.5...would I expect to see better running at a higher setting? my neighbour has an analyser so it isnt a problem to adjust it.

Hello Malcolm,

JWW said 3.5% in the earlier post, & that sounds about right.
There is tune up data for all the models in the back of the Bruce Anderson book. Let me know the year & model of your car.

The CO% is checked with the air pump disconnected.

I'm interested in this one as well, my 2.7 has a different CO% listed for each of the 4 model years.
The exhaust was changed in '75, but 75-77 I would expect to be identical they're not.(?)

Regards, Jon

Jon...model is a 1980 SC. When you say the air pump what do you mean? Air conditioning? If so check where I live ...no need for air conditioning in Scotland.

No not the A/C, originally the car would be fitted with an air injection pump driven off the alternator pulley.

The pump is a small compressor that feeds air to the exhaust ports. This means the measured CO% is actually a lot lower than it actually is.

Most of us Europeans remove this pump & blank off the ports in the cylinder head, so it may not be there, mine wasn't.

Anyway if the pump is doing it's job it will lower the measured CO value considerably.
Disconnect the drive belt, measure, adjust & go.

The book is at home so I'll tell you tomorrow what the CO should be (at least according to Bruce Anderson).
Unless anyone else can help sooner.

Regards, Jon

it may not pass the emission test in your area, but regardless of what the factory spec is, around 3.5% CO improves the idle and off idle response. that's with the oxy sensor unplugged. leave it unplugged if you don't want a cruising surge.

Thanks for that,

the UK spec. for a vehicle of that age is less than 4% so that's okay.

Is 3.5% good for my '76 2.7 (no lambda sensor) too or a different figure?

JWW, thanks for all the good advice on earlier threads, you made removing my air box a breeze.

JG

I don't expect to get a reply here, but what the heck. What O2 dwell (similar to O2 Duty Cycle) setting will give me the best performance without cooking my CAT?

I set the (O2) dwell at 45, because that is correct for my car (factory spec). Then I unplug my O2 sensor. The car runs rough for a few seconds each morning and if I open the throttle, even a little on startup, the engine gives my popoff valve some real excitement.

Would it be better if I set the (O2) dwell at 40 or 35 (a little richer), then unplug the O2 sensor? John? I don't have a gas analyzer.

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'83 SC

3.5% seems to be the magic number for all CIS engines, although 930s can use 4%. superman, you,re still pretty lean at 40%. try adjusting the CO like a carb. lean it until it falters a bit then richen it until it peps up and sounds good. the idle will go up and smooth out as you richen. just go only as far as necc. to get the smoothest, fastest idle. turn down the idle screw as needed. if you turn down the idle screw a lot after this, the remaining mix will be richer, and may need to be leaned. if it still spits and pops cold, the warmup reg may be the culprit. you've seen the posts about making them adjustable. the pin needs to go down a hair to richen cold running. problem is when you tap the pin down, it usually goes too far, resulting in a surging warmup period. hence the need to make them adjustable to pull the pin back up.

John, this is very interesting to me so I hope you don't mind my new question. If I unhook the O2 sensor (which I have) and richen the mixture too much, can't I overheat my CAT? I've tuned CIS by ear before but never left them like that for long. I guess I'm looking for a dwell (duty cycle) setting that is safe. Spec is 45. Maybe I should quit worrying about my CAT. A pair of SSIs is the answer.

Like all my CIS cars, this one seems to start better when it's cold. The colder the better, so I don't suspect my warmup regulator, but I've never looked at fuel pressures.

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'83 SC

Malcolm,

I looked up the data last night, wrote it down.
Now I'm at work, can I find it??

From memory it was 0.6 - 1%.
I will double check tonight but I do remember 0.8% being in the middle.

Also your model SC does not have the air injector pump (76-79), so that clears up the earlier confusion.

The later CIS system had a plug near the CDI box that you connected a dwell meter to & adjust the mixture screw until the duty cycle of the signal at the plug was 50/50 (set dwell meter to 4cyl & adjust to 45 degrees) for the correct mixture.

I found by accident an online tutorial on how to set up CIS for Audi technicians, If I still have the bookmark I will forward on the link.

I would give 3.5% a go with the O2 sensor unplugged.
Regards, Jon

Superman,

Are you sure that a lower dwell number indicates a richer mixture? My experience has been that the opposite is true (contrary to the diagrams in the Probst book). A clockwise turn of the adjustment screw (which I'm pretty sure richens the mixture)increases the dwell. I find that about 60-65% (or 55-60 degrees) with the O2 sensor functioning gives no starting issues.

I have the same CIS system on my SAAB (hey I have to drive something in the winter) and a similar setup works well there too.

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Graham Archer
83 911SC Cab

Jon

1.0% seems very low but I will wait till you check again. As for a dwell meter, its hard enough getting hold of a gas analyser!!!

if a cat car gets too rich, they get real stinky, so ever so slight stink gets you close to the right mix. the old cat will live. it's lost a lot of it's effectiveness long ago, so it's not real reactive.

The PO replaced my CAT just a couple of years ago, so it's not old enough to give it the broom handle treatment. I'll mess with it tonight and see if the CAT glows.

I agree that turning the screw clockwise richens the mixture, but I still believe that this lowers the dwell reading.

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'83 SC

Malcolm,

The Bruce Anderson books atates CO 0.4-0.8% with the O2 sensor disconnected.

Regards, Jon

Thanks Jon. I will need to borrow the gas analyser again and reset it.

Jon...I don't think I have an O2 sensor to disconnect (but then I have been known to be wrong on the odd occasion!!). Is there a different setting for cars without the sensor or is that why they are disconnected?

set it at .8% if you want but idle will be rougher and off idle response will not be as good.