Setting mixture without a gas tester

[echrisconnor]

I've been trying to get my mixture close by using the "lift the plate" method of CIS adjustment. This has proven somewhat effective but I'm curious about other methods for non-lambda cars.

I read the following page yesterday and saw a method which was new to me described and thought I'd ask if anyone else had used this:

http://www.ncr-pca.org/tech/tech-cis.htm

Quote:

CO setting is hard to adjust properly if you don't have an exhaust analyzer but there is a way to get very close. Get your engine to normal operating temperature. If you have an O2 sensor unplug it. Use a vacuum gauge on manifold vacuum, not on the distributor line, and using your 3 mm allen key lean out (counter-clockwise) the mixture until the engine just starts to stumble. The gauge should read about 15 at this point. Now slowly richen the mixture a little at a time, pausing to remove the allen key from the screw until the vacuum is at it's highest point.

[curiousone940]

My grandfather was a mechanic back in the 1920's-1960's...never saw him use anything but his ear when tuning a car. Of course, with all the emission stuff on cars today and how they run relatively coarse even when properly tuned, that method wouldn't work. Vacuum guage ought to get you in the ballpark.

[armandodiaz]

Why not get an air/fuel meter guage. Since you already have the O2 sensor it's an easy plug-in. They sell for about $40 unless you want to go fancy and get a wide-band one for $400>.

[john walker's workshop]

richen until you hear the idle get smoother and faster, then back off until you just hear it begin to drop down. adjust the idle speed to around 1000 if it changes. this is assuming it's already on the lean side. kind of like adjusting a carb.

[echrisconnor]

I've got a 02 sensor and meter, but actually don't have an 02 bung on my headers. Maybe I'll tap one in the metal and temporarily thread it in there.

[armandodiaz]

The o2 should go before the cat? You might have to take apart the exhaust because of all the metal flakes of drilling. Remember if it's a 1 wire O2 then you have to run the car to normal operating temp before the O2 gives reading's (this should be done even with the 3wire).

[Vipergrün]

Can't you use the frequency valve plug and an Ohm meter to do this? I though I had done this once before and got the mixture set to the point where the meter showed 50%, meaning 1/2 on and one half off, which would have set the CO to approx 3.5, IIRC. Maybe I just dreampt it

[echrisconnor]

JW - I suspect I'm rich (mixture-wise). Would you lean out first and then re-richen as you described. Do you use any of the plate-lifting or vacuum methods? You probably have an analyzer though don't you.

Performance and driving wise, what are some of the characteristics of running too lean or rich? For example, when rich, will the car stumble a little with first application of the gas. Etc

[Paulporsche]

Chris,
If you get any popping or backfiring during acceleration (or idling for that matter) then you are too lean. If you get a sudden drop in revs when pulling to a stop, followed by a fluctuation in revs then you are too rich. If you get an oscillating idle speed then you are set too rich.

I like to set the car so that it is just shy (lean) of the fluctuating idle. Then it seems to be just right. Keep in mind that you may have to slightly lean it as the hot weather approaches, and then richen it back as the fall comes.

[echrisconnor]

Paul.

Interesting, I'd always attributed popping & backfiring to being rich -- not lean. Isn't it the unburned fuel that causes that?

I've experienced the sudden drop in revs, vs a slower drop and had attributed the sudden drop to rich and slower to lean. However, in order for me to get fluctuating idle, it seems that the car has to be really rich -- like off by 1/4 turn or more.

At high-rpms (like 5500 +) would lean or rich cause a hesitation or stuttering in acceleration? I've been having a problem where at high altitude this happened to me, and went away at low altitude. I attributed it to mixture.

[john walker's workshop]

if it's rich, just lean it until it really drops off then do what i suggested above. i have a machine, but often when i first get one running on the rack i do this and more often than not it's real close when checked on the machine. i tend to go for 3.5% on CIS. stronger idle, somewhat better low end response.

[echrisconnor]

Is there a good way to see if things are rich or lean at WOT or higher revs? Given that the mixture is controlled by a number of things such as the WUR, how can you tell if your mixture is still on-target off-idle short of a dyno (or onboard mixture gauge)?

[Paulporsche]

Chris,

I think if you get popping through the exhaust it would be caused by excessive fuel. The kind I am referring to is through the intake, say through the airbox or, you would hope, the popoff valve.

Someone (was it you?) had a thread recently where their high vs low altitude performance was different, and had a problem w/ the vacuum line on their WUR. I am wondering if you are getting the right amount of vacuum @ WOT and @ varying altitudes. As I mentioned in that thread, the WURs are suppoosed to vary the CP based on throttle, temp and , apparently on some, altitude. Maybe someone out there knows which, if any, WURs adjust for altitude changes.

Today it was quite hot here in Toronto, much above the usual for this time of year. I could feel my car experience that oscillation in idle/ almost stall symptom. When it gets back to normal in a day or two, I am sure that symptom will stop.

I am wondering that something, like maybe the "curve" of your WUR or the advance curve of your distributor is not quite matching w/ your modded engine.

Have you tried leaning (in small increments?)
Have you tried another WUR?

[echrisconnor]

I think I'm in business now. Yesterday I re-checked everything. I did the vacuum method of mixture checking and it worked great! I found it much more accurate in being able to tell when you reach peak vac. pressure and have the strongest idle. It's nice to be able to look at a gauge, see exactly where things pick up and drop off. After setting mixture this way, the car idled strong and drove hard both at low and high revs. At least for me, this method seemed to allow me to find that sweet spot very nicely.

Paul -- you're right. I'm the one having the altitude issues with the modded engine. I think I found what it was though. I also retarded my timing a little bit. I think what I was experiencing at altitude was knocking. The car was hot, my timing may have been too advanced and it was under very heavy load. With the mixture change and this timing change things are running beautifully. The hesitation I was experiencing may have appeared to have cleared up at lower altitude since I cooled the car down as I was decending 3000 feet and then never got to run it as hard again when down low.

[armandodiaz]

Has anyone tryed the vacuum method on a Motronic system? I know the CIS system was touchy compared to the Motronic, when it came to adjusting mixture.

[oly]

Armando,

Did you get an answer to trying the vacuum method on a Motronic system?

[sailchef]

Originally Posted by curiousone940

My grandfather was a mechanic back in the 1920's-1960's...never saw him use anything but his ear when tuning a car.

Back in the early 60's , when I was a kid, I watched a guy set the valves on a straight 6 overhead cam while the motor was running. I remember him trying to make me laugh with his chattering teeth.

They just don't make Mechanics like they used to.