Thread: Idle circuit, Timing Related??

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Does ignition timing directly affect the idle air/fuel mixture? I set the timing (by ear) on my '71 440. No starter drag, smooth idle. I then adjusted the idle circuit on the carb using a vacuum gauge. Everything seemed ok, but sometimes I can't leave well enough alone. I borrowed a timing light and set the timing to the factory specs. (My ear was way off) If I remember correctly, it was 10 deg BTDC. Now, the carb seems to be running rich. Is this somehow related to the timing? I can readjust the carb, but I would rather not start tweaking if there may be another issue.

Originally Posted by 65ny

Does ignition timing directly affect the idle air/fuel mixture? I set the timing (by ear) on my '71 440. No starter drag, smooth idle. I then adjusted the idle circuit on the carb using a vacuum gauge. Everything seemed ok, but sometimes I can't leave well enough alone. I borrowed a timing light and set the timing to the factory specs. (My ear was way off) If I remember correctly, it was 10 deg BTDC. Now, the carb seems to be running rich. Is this somehow related to the timing? I can readjust the carb, but I would rather not start tweaking if there may be another issue.

Yes, the ignition timing effects the idle circuit. Once you have the timing set correctly, you will need to readjust the idle mixture.

I am inclined to agree that the timing and idle circuit are related. I don't have anything other than personal experience to base this assumption on though. I hope someone will be able to educate me a little on this.

Originally Posted by TyphoonZR

Yes, the ignition timing effects the idle circuit. Once you have the timing set correctly, you will need to readjust the idle mixture.

Can you elaborate on the theory behind this? I have a friend with the exact same problem. Does the reverse apply? Adjusting carb necessitates adjusting the timing?

If the idle circuit is correct, it shouldn't be effected by the timing. Idle speed on the other hand will be effected. The idle circuit needs to be adjusted at a speed below any mechanical advance. It may be nessessary to change idle speed as you adjust the idle screws. If you don't have the idle circuit right its hard to get the rest correct. The idle circuit is always in play. What you want is the throttle plates closed as much as possible at idle rpm. Also each idle screw is its own circuit. They don't have to be adjust in and out the same. One may be out 1 turn and the other will be out 2 turns. I usually start out 5 turns (rich) and screw them in 1/4 at a time. I am looking for a 20 rpm per screw.
Timing should be set with no vaccum and low enough so you don't have any mechanical advance effecting it. If it is not stock, stock setting don't work the best. Trial and error.

Which part is not correct?

yes, please. I would welcome some insight on procedure. I think I know what and how, but someone else is sure to have better ideas. I have set the idle mixture again, everything seems ok, just ok though. It seemed to run smoother when I had it timed by ear. Perhaps it has something to do with the fact that this engine has several zillion miles on it.

Originally Posted by 65ny

It seemed to run smoother when I had it timed by ear. Perhaps it has something to do with the fact that this engine has several zillion miles on it.

Maybe your timing marks are off or your damper has slipped? I dont know much about mopars but I know my Chev has a two part (inner/outer) damper with an elastomer insulator between them. Have heard that the outer ring can slip. Check your timing mark, correct any problems, and use a timing light. Forget your ear...at your age, your hearing may not be as reliable as it once was...

Cayne, thanks for the diagnosis of my ears. I think they work well enough. In conjunction with my "time by ear" process, I also use a vacuum gauge. Perhaps if my engine were fresh, and operating at peak performance, your methods would be ideal. For now, I am just trying to make this tired old beast run as good as it possibly can. (which is still pretty good )

Thanks Denny, I'll give that a try. I appreciate the advice.

Originally Posted by DennyW

In that case, warm up the engine to operating temp. Get the highest and steadiest read on the vacuum gauge you can by adjusting the idle mixture. Then, turn the distributor (with any vacuum advance dis-connected) until you reach the highest reading, and back down 2" of vacuum. This is about the best spot for the overall breathing of the engine.

I don't mind being wrong but you need to tell me what is wrong.

So basicly you are doing the same thing with a vaccum guage as I do with a tach. 2" on the lean side for you or 20 rpm to the lean per side for me. If its because I don't set each screw the same, its because each needle and seat are different. If you use a tach you will see it. This was how I was taught at automotive school. We didn't use a vaccum guage but I can understand how it works. What I'm after is the proper adjustment of the throttle plates and the idle mixture screw. After that any adjustment of the timing that effects the engines rpm can be compensated for by a small adjustment to the throttle plates. No need to adjust the idle mixture screws.

If you start overly rich, you will see the rpm's increase as it leans out. If you go from lean to rich this will not happen. You will going overly rich then have to come back. I stand by the what said about the needles not being the same also. No needle and seat are the same. We are talking small differences here. The full turn was just an example.
I believe you're mistaken about the idle circuit. It draws fuel all the time. That is why it is important to get it right before you adjust the primary and secondary circuits.
There is always more than one way to do things. If you don't have a vaccum guage, you can use just a tach.

What about 4-corner idle on a BBC 454, hydraulic roller 218/226 @.0502, 112 lc, mid-500 lift? One person advised a good starting point is all 4 screws open 1/2 turn, another suggested 1 turn. I have just set the timing (total 38 deg) which is where the best dyno results were, and 1/2 turn seems better than a whole turn. Guess I'll have to get the vacuum gauge back on it. Do you turn one primary screw to get best vacuum, then do the other 3 the same? I'm assuming they should all be more or less even?
Chris

Sorry guys,
DennyW already answered my questions on an earlier thread! Short memory!!
Chris

This is from the edelbrock web site:

The Edelbrock Performer Series carburetor has conventional Idle Mixture Screws (IMS) that provide a leaner A/F when turned clockwise and richer A/F when turned counter clockwise. The idle air flow is controlled by a conventional screw that opens the Primary Throttles. The following procedure should be used to set the idle mixture and speeds.
1. Fully warm engine and ensure choke is fully open.

2. Air cleaner in place.

3. Set desired speed with the air screw.

4. Adjust the IMS on ONE side to get the maximum possible RPM. Do not go rich beyond the maximum speed point.

5. If the above changed the idle speed more than 40 RPM, then readjust the speed.

6. Adjust the side OPPOSITE of that in Step 4 to get maximum RPM.

7. Reset the speed.

8. Carefully trim each IMS to again get the maximum idle RPM.

9. Go leaner just enough to get a 20 RPM drop in speed.

10. Reset the speed to the desired RPM.

11. This is a Lean-Best Idle Set. Setting richer than this will not improve idle quality or performance, but could tend to foul plugs.