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CIS Idle Speed and Mixture Setting Without an Analyzer

This came up in a thread about CIS idle mixture, idle speed and mixture checking w/o a gas analyzer. So I figure I’d give it a separate thread.

Someone asked about what I call the “field test” for idle mixture. They questioned the method and seem a bit confused about how it works and what it can tell the tuner. This is not something new; it’s been discussed before. As a matter of fact, it may have been Dorki Charlie’s post that I recall seeing a long time ago. Prior to that I just used the “over rich” or “over lean” screw settings (turning it until the car gets upset then back off) method to set mixture w/o an analyzer.

Here goes….

The air meter does not actually meter when the car is idling. The throttle bypass is technically the only way for air to get into the engine. Realistically, there are “leaks” that augment the bypass. There are internal and external leaks. Some cars have small leaks, some not so small, and the idle setting is often times compromised to make a car idle well with the larger than usual leaks. The compromise can show up in a drop in idle speed or a hang up in idle speed (e.g. when you come to a stop sign and the car wants to stall or the idle won’t drop to 900 or 1000 RPM. There can be other causes of the two symptoms, but more often than not the system setting is compromised)

EDIT: Regarding the above paragraph....(and pulled from page 2)

You are correct Jim. Air does have to go through the air meter housing to get to the throttle. There is enough differential across the air plate to move the plate just enough to raise the plunger (and break the contact with the switch on the back of the meter housing) in the fuel distributor. Fine adjustment of the plunger height is then performed with the idle adjustment screw.

My state of mind during that initial post was to conclude that the plate-venturi (meter) is not metering, because the plate is still within the throat of the venturi. But that was a poor conclusion on my part. A portion of the plate (furthest from the fulcrum) does clear the throat, teetering on that ragged edge where the differential pressure holds it. The throttle bypass changes the flow rate around the throttle plate at idle, which inturn affects the differential pressure across the air plate (increase flow---increase differential), determining its height. Adjusting the throttle bypass in conjunction with the idle mixture screw achieves acceptable idle mixture and speed.

So yes, it was misleading. I hope the above (and your post) clears it up and prevents anyone from going down a wrong path.

End edit


So! While at idle and you are adjusting the idle mixture screw, you are ONLY affecting the AFR at idle. Reducing the bypass thus richens the idle AFR because you are reducing the air (disproportionately to the fuel-edit) in the air-to-fuel ratio at idle so you must also reduce the fuel! When you turn the idle mixture screw, you are changing the height of the fuel plunger, not the meter plate (plate height is determined by the flow controlled mostly by the bypass screw at idle-edit)! You haven’t changed anything that will affect the volume of air that drawn into the engine. The fuel plunger rides on a secondary arm of the air meter plate lever. That secondary arm’s angle can be changed relative to the air plate’s lever. The meter plate height is set with the stop screw on the spring under it.

The field air plate lift test is just a quick and dirty way to check if you are rich or lean…You can even pull down on the plate. By pushing up or pulling down on the plate slightly, you are in effect changing the idle mixture. You are raising or lowering the fuel plunger. STOP! Don’t ask about the air meter or change in flow. Stop! Think about the throttle body? Is the throttle body valve (butterfly) mechanically connected to the air plate? NO! If you thought yes, you need to step away from your CIS….just put the 3mm Allen wrench down and walk away. Go call a friend and ask for help…heck call me! My cell phone has a few minutes between rings.

You are only moving the air plate a millimeter or two with this test. As a matter of fact, you can move the plate enough to stall the car by giving it too much gas when you push up on the plate…lets call that a “rich stall,” because you created too rich of a mixture in the engine.

If you move the air plate up slightly with a resulting rough or surge, you are closer to the rich extreme. If you have a 3mm Allen wrench handy, you can turn the mixture screw counter-clockwise to lean it out a little, maybe 1/8 of a turn, then repeat. To get it just right, you’ll have to develop a touch and maybe even a sense of smell. To test for lean, you can pull down on the arm slightly and get the opposite effect of the rich test. That’s it! That’s all there is to it.

Understanding what’s happening when you make adjustment is the key. I often tell folks to read up on the individual components. With out doing that first, one can’t understand how the CIS (S=system!) works. System…if you don’t understand how it all works as a system, you’ll get something out of whack, and then it’ll be a trial and error fest when you go looking for help. Understanding the system will also minimize the expensive “parts swap” trouble shooting that some folks have gone through.

As I hinted at the beginning, you can also test for rich or lean by changing the plunger height using the idle mixture adjustment screw. If you turn the screw clockwise until the engine starts to stumble or surge you are getting too rich. If you want to run rich, say with advance timing, turn the screw back 1/8 or ¼ turn, race the engine and see if you idle is acceptable. If there is no surging and the idle is where you want it, you’ll be OK…rich, but OK. To test for the lean extreme you do the opposite. Turn the screw counter-clockwise until you stumble or surge. If you make a mental not of how many turns of the screw there are between too rich and too lean, you can then put the mixture right in the middle!

This brings us to idle speed and how to lower it. If you paid attention, you’ll know that you can raise or lower you idle speed by changing both the bypass screw setting AND the idle mixture. You have to adjust both, not just one. You can adjust one at a time, and you may find that you will create a lean or rich surge. (That is actually the way you should do it if you are not confident of what you are doing) If you are closing the bypass, and the engine surges, then reduce the fuel. Get the idea? Air and fuel combine to produce energy when ignited. Often a high idle speed is the complaint. So reduce air and fuel to reduce the idle speed. Be wary that you do not have a problem somewhere else which is causing your high idle.

Always make a mental note of the amount of your adjustments so you can return it to the “as-found” setting if you booger things up.

Analyzer Note:

I've made adjustments on my car (w/ a '78 CIS engine) using a 4-gas analyzer, and I think a good tuner should be able to make a good mixture adjustment without the CO tester that everyone thinks is so indispensible. The CO tester only tells you one thing. Sure it's nice to have, but unless you have a 4-gas or 5-gas, you'll just not making all the right changes. Even with a 4-gas I had to make adjustments to the ignition, bypass and mixture screw in order to get the critical EPA components (pollutant) low enough to pass! A friend recently bought a Euro '78 SC. I tuned it for him to make it a little more peppy. Even with the "richer than before" setting on his car, he passed the Illinois EPA test. I'm not saying I can replace a gas analyzer, but when the CIS is operating well, you can make adjustments without a CO analyzer and still be pretty damn close to perfect!

Now the disclaimer:

Don’t go diving into the engine bay and start turning screws. As I said before, learn the components and understand the system! Besides, I’m operating on 2 hours of sleep after a long night in the garage. So if you really screw something up or if I made a mistake or forgot something…blame it on the stubborn bolt I had to wrestle with last night, thus turning my 3 hour session in the garage into a 6 hour swearing session!

Last edited by MotoSook; 04-18-2006 at 08:40 PM..
Old 07-20-2005, 12:07 PM
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Outstanding, Souk. You're the man.
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Old 07-21-2005, 04:43 AM
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When I was at your house last you just tuned by sniffing the tail pipe exhaust gas. Or was that for some other purpose??

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Old 07-21-2005, 04:57 AM
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Oh no Thad...my nose is only calibrated to tell me if it's too rich...and too too rich...which is accompanied by watery eyes For on the fly AFR monitoring, I tie the neighbor's dog to the bumper. If I here him choking...I'm too rich Them yippy yappy poodles can really run after they find out how bad road rash can be!


michigan: glad you got something out of it....usually my ramblings are just that....ramblings
Old 07-21-2005, 07:21 AM
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For a guy who is just STARTING to grasp some of this, and who has no few tools yet (no pressure gauges, etc), this stuff is priceless.

Thanks Souk.
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Old 10-28-2005, 07:52 AM
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Per usual, bravo brotha!
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Old 10-28-2005, 08:25 AM
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nice souk. i learned a FEW things!

have you ever set mixture using a dwell meter and the lambda duty cycle? i read about it all the time, but i can't make sense of it. like the first damn step: "find duty cycle attachment point." ????
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Old 10-28-2005, 09:01 AM
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Never Cliff....but I have a wideband O2 sensor and logger. And I'd disable the lambda circuit on the 80-83 SC's anyhow
Old 10-28-2005, 09:10 AM
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Thanks Souk, good stuff.
Good reminder on understanding the components first and that all parts are working with no vacuum leak.
I have done the AFR without an analizer for years.
Go cw until idle gets rough (Rich), go ccw until coughing (Lean) then set the mixture screw in the middle. It's between the idle speed (Large, slotted by-pass screw) and mixture control (3mm Allen screw)

You may want to edit slightly:
Quote:
If you want to run rich, say with advance timing, turn the screw back 1/8 or ¼ turn, race the engine and see if you idle is acceptable. If there is no surging and the idle is where you want it, you’ll be OK…rich, but OK.

That should be: turn the screw forward (cw) ......No?
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Old 10-28-2005, 09:16 AM
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Although i have a pretty darn good understanding of ther CIS system, I have one question I hope you or someone can answer:

I've read somewhere in my manuals or books (factory, haynes, CIS books, etc) that the mixture adjustment screw should always be adjusted from lean to rich.
In other words, to lean it out, you need to go past the desired point, and then back up in the "rich direction.
WHY?!!!!!
I don't quite get that.
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Old 10-28-2005, 09:17 AM
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Souk,

this is great. I've been using a variety of methods over they years and have found that you can get it running pretty well with your methods.

Question, are there certain telltales of rich vs lean running. I.e. when you lift off the throttle and the revs drop down really low before returning to idle -- is this an indicator of too rich or lean. Or when you stamp on the gas, if you get a split second of hesitation before the car picks up, is this an indication of too rich... There seem to be a number of these sort of telltales which I forget right now.

Finally, the CIS system has components which affect idle vs running mixture. The WUR. This will use vacuum (in certain years/models) to lean the mixture at idle and richen at WOT. Can this throw off your mixture adjustments, such that checking for acceptable idle is not the best way to check for overall mixture?
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Old 10-28-2005, 09:24 AM
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Gunter, I didn't go back to read the first post (yeah I'm lazy), but I believe I was speaking about going to the point of surging rich, then backing off. Basically, running as rich as you can w/o surging...only in the case that the timing is advanced to 35 degs, and in the summer. We're looking for predetonation protection, with no regard for mileage or exhaust quality.

Sammy, most things are better adjusted from low to high. Like a regulator (air, fuel, what ever fluid you want to consider) setpoint. Making the idle mixture adjustment causes the plunger to go up or down while the air meter plate is fixed. So the rising action is better as you are "extending" all factors....I ain't to good at articulating what's in my head unless I have a few Guinness in me, so use your imagination.
Old 10-28-2005, 09:24 AM
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This thread is a worthy addition to my favorites list.
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Old 10-28-2005, 09:29 AM
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Chris, we're talking about idle mixture. I see what you're getting at though, and it's gonna take me some time to address soem of the other factors that affect CIS operation....not while I'm at the office though...too many distractions.

It comes down to understanding how the components operate and how they work together.......
Old 10-28-2005, 09:30 AM
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Chris:
Quote:
Question, are there certain telltales of rich vs lean running. I.e. when you lift off the throttle and the revs drop down really low before returning to idle -- is this an indicator of too rich or lean. Or when you stamp on the gas, if you get a split second of hesitation before the car picks up, is this an indication of too rich... There seem to be a number of these sort of telltales which I forget right now.

I had exactly what you are describing, RPM dip too low when lifting and slight hesitation when accellerating. It was annoying. Had to adjust mix and idle speed several times minutely; I suspect that it was a rich condition that caused the symptoms. But..............let Souk chew on that one.
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Old 10-28-2005, 09:52 AM
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Why would you disable the Lambada on a 80-83SC - And after you do would I need to adjust the mixture.
Thanks
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Old 10-28-2005, 10:07 AM
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This should be a sticky..
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Old 10-28-2005, 11:34 AM
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Re: CIS Idle Speed and Mixture Setting Without an Analyzer

Quote:
Originally posted by Souk
So! While at idle and you are adjusting the idle mixture screw, you are ONLY affecting the AFR at idle.
If this is the case, how do you adjust the cruising/part throttle AFR and WOT AFR?


Quote:
Originally posted by Souk
The meter plate height is set with the stop screw on the spring under it.
Perhaps by adjusting the meter plate height?
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Old 10-28-2005, 12:42 PM
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I'm a little late catching up to this thread, (this fall).
Hi Steve!
Charlie showed me this method last spring and I believe it is a great start.
I have been learning and tweeking my system for the last few years and learn more as time goes on.
I have an 83 Row cab. This is the same k-basic system (no brain) as the 78-79.
I recently passed MD emissions without a cat. I got a little time on a 3 or 4 gas anylizer at a gas station. I had no problem getting the co down below the max but if I got that down the hydrocarbons went up. I was about to give up when I found that If I raised the idle up to about 1100 they both fell below the limits. I don't recall the hydrocarbon # but co was at .89.
The car runs better than I expected on this lean mixture.

With emissions behind me I began flirting with trying to find the right mixture for performance. I barrowed a LM1 from Kurt who is another dorki and PCA tech chair.
My quest is to forget about idle mixture and consentrate on achieving the best mix for the engine's power band.
I have been running arround for the last few days with the LM1 hooked up and watching the "System" in action.
Although I don't have the rpm sensor. I recorded several sessions and tried to remember what happened during those runs. BTW, I did richen the mixture ever so slightly since the emissions test. I am also just learning how and what this new toy and the graphs are telling me.

Idle is currently hovering arround 14:1
Light cruise is arround 13.5 -14
Decellerating with the engine in gear the ratio goes up and I've seen on some occations 16 or 17:1 (Possible air leak?)
While recording acceleration runs the ratio progressivly gets richer and you can actually see the effect when the air flow caused the plate to get to the steep part of the cone.
I have seen and recorded mixtures going from about 12:1 @ 4500 rpm going close to 10:1 at 6000 rpm.

I am loving this toy and the fact that it is confirming what I have read and understand about the CIS system in my car. I wish I could justify getting the AUX box and I would like to record the rpm, map, temp and control pressure. With this info I think I could possibly get closer to my quest.

In the mean time I have a few questions to pose to those following this thread. (And I guess directed to those that have the k-basic system w/out lambda control)
Observation, I am currently set at 14:1 which correlates to 1% Co on my graph. My little spec. book shows 1-2% for 82-83 row SC.

1st: what rpm band should I be tuning for? I can see the point at which porsche/bosch designed the cone to make the mixture richer. (I haven't pinpointed it exactly, but it's about 4000.
2nd: what is the max power mix? I've seen 12.6 - 13 is optimal.
3rd: It seems like the shape of the cone is creating a progressivly richer mixture as the rpm's go up. 4000-6000 at least. What's up with that? Is that designed on purpose? (detonation prevention with the 9.8 cr?
4th: I understand that too rich a mixture actually produces less power. How much is too much?

Finally, the cut-away is great. One thing that is not mentioned is the little adjustment screws near where the 6 lines to the fuel injectors are connected. My assumption is that these are for equilizing the flow to the cylinders?

PS I was recently on the LM-1 site and they are now selling a simplified system that can be used to just display the mixture on a gauge. No datalogging etc.
























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Old 10-28-2005, 07:09 PM
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emac: disabling the O2 sensor dumbs down the CIS so you essentially have the operate like the earlier CIS. It allows you to run a little richer with more advanced timing.

Kaefer: the CIS is designed (via venturi design and fuel distributor design) to handle the other operating conditions. Set the idle and go. If you have issues after a properly set idle mixture...look elsewhere...ignition, decel valve AAV, AAR, vacuum, control pressure, fuel distributor, injectors, leaks..... Each of these should be address separately or rather understood seperately so you know what the results of their failure.

This is a thread about idle adjustment with the assumption that all others are working properly.

Something the manuals and tech references don't address is how they all operate together or more importantly how they DON'T operate together! The trouble shooting matrix that has been developed by the factory is good, but it doesn't contain enough detail/explanations. It's a good start, but as with as with systems that contain many components or sub-systems, it is difficult to cover it all in the matrix.

One of these days, when I run out of projects, I may just write a tech manual on this stuff!
Old 10-28-2005, 07:14 PM
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