CIS mixture adjustment

[arrivederci]

I adjusted the mixture on my 930 in preparation of an emissions inspection (leaned it out...turned it CCW). It was running at 2% CO on the dyno with a cat, hopefully it'll be lower now. The idle hasn't gotten lumpy, though, and i've turned it about 1/3 of the way.

I've seen mention of not adjusting when the engine is running...but haven't tried it yet. Can anyone explain why? When hooked up to a gas analyzer, that would seem the easiest way to get the CO exactly where you want it...

[targa80]

I have adjusted my mixture with the car running but DON'T hit the gas pedal with the allen key in the adjustment slot. Remove the allen key from the adjustment slot then you can hit the gas pedal. If you don't remove the allen key you will damage the mechanism in the throttle body.

[john walker's workshop]

another old wives tale.

[targa80]

J.W. are you trying to say that you can leave the allen key in the slot attached to the mixture adjustment and hit the accelerator without the possibility of bending damaging the adjustment mechanism inside the throttle body. If you are please elaborate for the ill informed people like myself since I do respect your opinion and I know you have done this many more times than I have.

[john walker's workshop]

i was commenting on jpinkert's question about not adjusting the CO with the car running. also, if the engine is revved up accidently with the allen wrench in the hole, like we've all done before, there really isn't any damage done. the sensor plate just won't move much, and the RPM will be limited. there isn't any delicate mechanism down there that will get all twisted up. try driving one down the street like that. they just don't go.

[targa80]

J.W. thanks for the explanation I did not know that it would do that.

[arrivederci]

Thanks...I figured that it would be much easier to adjust (next time, I guess) when the car is running.

[Jdub]

Well nobody has commented on the 2% CO and reducing from that number. That seems pretty scary low numbers to me...is this to pass emissions?

John

[Doug Zielke]

Originally posted by Jdub
Well nobody has commented on the 2% CO and reducing from that number. That seems pretty scary low numbers to me...is this to pass emissions?

John

OK..here's another data point.....

I've been playing around with my Gunson Gastester, seeing just where the 'Bomber likes her mixture.
As an experiment, I did set it to around 0.6-0.8% CO. That's pretty close to the Porsche spec for my car, I think.

The motor ran OK with that setting, but seemed to lack some throttle response. I then tried a setting of 3-3.5%. The motor seemed to be a bit lazy at lower rpms with that setting.
I finally settled on 2.5%, and the motor runs very strong from idle to redline. That's where I'm keeping it for now.

[Jbabic]

Ditto!! I have read that SC's run best at 2.5-3.5%...have to find your own sweet spot. I am at 3.0%..

[fred cook]

I have the CO set on my '80 911SC at 2%. It starts easily, runs well and gets about 18 mpg in town and 25-27 mpg on the highway. Also have the O2 sensor unplugged. I use the Gunnison single gas analyser to make adjustments.

Good Luck

Fred Cook

[arrivederci]

I have to get below 1% CO (under load) to pass an emissions test. Afterward, I'll set it back to 3%-ish.

[Jdub]

JohnW is a big believer in the 3.5% for the SC. That's what we set my baby at.

I think it's a good idea to change the CO twice a year at the same time you swap out the fuel filter. For the summer, I'll have to reduce CO a bit I would think to handle the lack of air density (warmer air).

John

[Doug Zielke]

Originally posted by Jdub
JohnW is a big believer in the 3.5% for the SC.

Actually, I've been consulting with JW during my mixture experiments. He told me the "seat-o-the-pants" is the final indicator of performance. He said every motor is different....some like it a bit richer/leaner than others.

That is the good thing about having the Gunson. I can do a setting, and then go out on a run immediately and check to see how it feels.