[MotoSook]

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!