Setting the mixture.

[Benny771]

I have a 89 930 and im wondering how to set the mixture screw? It seems to run rich (by smell) at idle.
Is it true its only at idle that you need to adjust the mixture screw? Thats all it effects?
I have a O2 sensor on it, so it should self adjust......no?
I take it i unplug it to set it (warm eng.). If so what range am i looking for. I have a Gunson gastester to assist in this. Seems to run fine, but id hate to miss out on some HP.

Thx in advance for the 930 brains that respond.

[mark houghton]

Benny, I run with the O2 sensor unplugged, thus open loop, and used toadjust the mixture with my Gunson to about 2.5%CO (I don't use the Gunson anymore, in favor of my AFR gauge). I get best idle there, but yours may differ somewhat. Adjust, listen to the engine, and set where she idles best. Most of these cars like it a bit on the rich side, which 2.5% is. Unless you absolutely must have the O2 hooked up for emissions testing, I recommend going without it all together.
That method will get you in the ball park and work just fine most times. On-board AFR will confirm where you want to be. Yes, the mixture screw has its largest impact on idle AFR's, but also has a smaller affect across the mid range power band as well. I emphasize small....your WUR, control and system pressures, are what really sets the mix across the power band.

[Benny771]

Must be doing something wrong?
O2 sensor unplugged.
Gunson tester set at 2.0
Engine warm, idle approx.950
Insert tube...... slowly goes to 1.0..... What does this tell me ???
rich without to O2 sensor?

[Benny771]

no one ??

[jsveb]

What happens if you try to lean it out? Just a bit at a time. You should be able to hear the change on the engine, also see it on the gunson. If it incr rpm then it was to rich, if. It drops then it was to lean.
There are threads on tuning without a tester all together. They expand more on this. This, I think, will tell you more about your car at idle, and your tester.

Good luck

[Richard930]

Ben - you need to get an AFR meter if you're going to play around with mixture setting. Setting the idle mixture only has a minor impact on cruise and on-boost mixture though, so you won't be able to optimize mixture for power and economy without an adjustable WUR and an AFR meter.

I'd offer to lend you my innovate LM-2 but I sold it a while ago and replaced it with a permanently installed one so I always know what's going on mixture-wise. There are plenty of AFR options out there.

If it runs well you're not going to be able to improve it without the right tool and the adjustable WUR

[Benny771]

I made my WUR adjustable and happy with the job i did. I guess i need is a AFR meter to check higher rmp's.
But for the time being i am just trying to set the idle mixture.
I'm not convinced that the 1.0 i'm getting is richer, i am leaning towards leaner.
Just trying to confirm this.

As this is stating..... http://www.mgexperience.net/article/co-afr.html

[mark houghton]

Ben,

I would have to drag out the Gunson instructions for calibrating in free-air (well alright, I just did that) and setting at 2.0% is the correct procedure for calibration.

If when inserting the probe into your exhaust you are reading 1.0%. then you are leaner (at idle) than these cars like to run (make sure to stuff that probe as far up her bum as possible to prevent ambient air from diluting things). Turn your mix adjusting screw down (clockwise) just a scan degree or two, and shoot for a reading of 2.5 to 3.0%. That should get your idle mix where it needs to be for best running. This will get you in the general ballpark for idle and minor impact on cruise, as Richard930 has already stated. Your AFR meter and adjustable WUR will take you to the next steps.

[Benny771]

Is there a special tool for this or just a allan key? Mine seems to be a spring loaded shaft.
Also, because i have a O2 sensor will this correct the idle mixture anyway, or just a small amount?

[jsveb]

Yes it is a spring loaded shaft.
The end of that shaft is shaped like an Allen key. You need another Allen key to engage and push down on that shaft.
If you can imagine, this will then seat into the CO adjuster that is below and downstream of the airflow meter plate. This is how you do it.
It only take small adjustments.

[Benny771]

Anyone remember the size? If not I'm sure I'll figure it out

Thanks again all.

[mark houghton]

Push down until you feel the slightest resistance, which would be when the adjuster shaft contacts and engages internally. If you feel no resistance when rotating the shaft with your allen wrench, then you're not quite far down enough to engage the adjuster. Push down too far and you will deflect the metering plate arm and go instantly rich (rpms will most likely drop). A little trial and error will get you the "feel" you're looking for. Turn very slightly (as in a degree or so) at a time. It doesn't take much to affect AFR's (or CO%) readings.

4mm wrench if my memory is working tonight. I left my wrench permanently in place.

[JFairman]

3mm

[Benny771]

Finished !!
Thanks again to all.