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.

Originally Posted by Mike

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.

Just a little exsperiment for ya ,crank the idle speed up to about1000 -1300 rpm with the screw on the throttle linkage and crank the idle screws all the way in and see what happens
Things were a little different 20 years ago than now ,people used to really have to make do and I was one of those people.I cant count the junk carbs I have used when the previouse owner said they were junk or found them in a pile of junk,bolted them right up and proceeded to drive many miles with it ,to the previouse owners disbelief,sure the idle passages were blocked ,easy fix ,dont idle,sure the accelerator pump was shot,big bog for a sec when you accelerated it ,you just learned to feather the gas or punch it to the floor,sure the filter hold down was busted ,easy fix ,no filter.sure 1 of the four barrels was clogged ,no biggy ,let the other 3 work harder,now ya got a 3 barrel.!!If they were really messed up you had to use the choke about 3/4 of the way on to help increase vaccume to pull fuel thru the partially clogged ports we wont get into the 3 bolts holdin the tranny on or the block of wood with washers and wood screws holdin the starter on from just the top bolt,had to get the philips screw driver out and tighten that up quite often or carry a hammer to beat on the starter to get them last starts out of it before pay day or drivin cars with no reverse,after all you go forwards most of the time anyway I am just a young greenhorn,I cant imagine some of the stories the other fellas have. Your just talking about playin with the screws on the outside of the carb,not the stuff inside you have to peel plugs out to get to If Denny says them idle circuts aint runnin when the throttle blades are opened up,them circuts aint working then,besides that he was carb mechanic ,I think he just might know whats happening inside that fairly simple device ...... Best bet for learning is when you think someones mistaken ,do some research on the internet and find out whats really goin on.I would venture to say there is atleast 10,000 articles on carb tunning on the internet ,just google a few thousand up and start reading ,it will all be clear then. This is a perfect chance for learning .

Lets just say for argument's sake, one was working on a Rodchester Quadrajet (please don't tell me they are a quadrajunk, either you like them or you don't. I happen to like them.) Anyway, on the idle mixture screws, which way is rich/lean, in/out?

Out is rich.