Idle Adjustment

[EngineerDoug]

Hello all,

Got a couple of questions about idle mixture adjustment on a mild 383. Cam is 214/224 degrees @ 0.050", with a 650 CFM vacuum secondary Holley. Street use with a Torqueflite. Low compression at 8.5:1. I am running electronic ignition with the following settings: 18 degrees initial timing, 18 degrees in the distributor, and 18 degrees in the vacuum advance. It runs so much better with this much initial advance, compared to 12 degrees, BTW.

I have set my idle mixture screws for max vacuum (16") with the vacuum advance connected to manifold vacuum. I've read all I could find on the ported versus manifold debate and elected to run manifold vacuum. I have seen a few folks post that they adjusted these screws with the vacuum advance disconnected - am I missing something here; I would think it should be connected when adjusting?

Also I do notice the exhaust will make you eyes water a bit. Not as much as other cars I have owned with more radical cams, but just wondering if this is a hallmark of running too rich? Or should I expect even a mild cam like this to smell a bit "watery"?

Thanks!

 

[JimCoronette]

My buddy is going through a similar scenario right now. I would consider myself an intermediate level tuner, so I'm looking for tips from the experts.

 

[Wietse]

The statement "ported or manifold" is a very much described and long discussion.
It went from manifold to ported due to emission regulations changed but the original idea was to have it connected to manifold vacuum....too many opinions on this.
If you think about it, do you need vacuum advance at idle?
At idle you only need a certain amount of timing that keeps the engine running well with sufficient vacuum.
Vacuum advance was integrated to have a variable timing during cruising speed where more ignition advance is beneficial for fuel economy.
Regardless of your choice of ported or manifold, you need to adjust it as how you are going to run it.
Adjusting it with the v-can disconnected from manifold and after drive it with the v-can connected to manifold does not make sense to me. (it is not set on how it is used, on the manifold vacuum the v-can will add advance at idle.)

So in your case, your engine idles with 18* + whatever the v-can adds (12* as example) so in that case around 30* which is way too much timing for idle. (if on ported vacuum, you will maybe get a slight vacuum advance, depends on the ported signal hole location relative to the throttle blade. but maybe you will idle at 20*) Even 20* is high.

The water-your-eyes smell from the exhaust indicates a very lean burn. If it is really bad you need to open the idle mixture screws a bit to richen the mixture a bit.
That smell has nothing to do what cam is in there, it's all about the air/fuel mixture.

 

[Wietse]

18 degrees initial timing, 18 degrees in the distributor, and 18 degrees in the vacuum advance.

With saying that, i assume you set the initial 18* with the v-can disconnected, right?

 

[EngineerDoug]

Wietse,

Your points are well taken. I did not see the sense in adjusting the idle mixture being adjusted under different conditions than the engine would be operated, but I thought I would ask the question anyway.

Interesting that the watery-eyes characteristic suggests a lean mixture. I had thought it was the opposite but apparently not.

Thanks.

 

[Wietse]

Interesting that the watery-eyes characteristic suggests a lean mixture. I had thought it was the opposite but apparently not.

If it runs rich it will have a fuel smell and the exhaust will start to smoke grey/black.

 

[EngineerDoug]

Yes, I have 18 degrees initial timing without vacuum advance.

 

[theman440]

I'd give it what it wants as far as initial timing then adjust your total accordingly. You can buy one of those plates for the mechanical advance to limit the total swing.

 

[WP29440SE]

I always run my vacuum advance off the carburetor and not the intake. why would I want full advance at idle on the street?
if my eyes are watering from the exhaust , it's because the raw fuel from an overly rich mixture is burning them.

 

[EngineerDoug]

Yep, I have a limiter plate set to hold the mechanical advance to 18 degrees.

 

[Ranger16]

if my eyes are watering from the exhaust , it's because the raw fuel from an overly rich mixture is burning them.

I agree. Mine was eye watering when I got it as well. I changed the jets and metering rods in the Edelbrock 1411 (750) to lean it out a bit and that solved the problem.

 

[theman440]

Yes, I have 18 degrees initial timing without vacuum advance.

If you use manifold vacuum you won't need as much initial timing. That is how I have my '66 383 4-bbl set up. It works great and the fuel mileage is much better.

 

[EngineerDoug]

I've done a little fooling around with carb adjustment on this engine. Not sure I agree that it is running lean. Plugs look a bit rich.

Also, I had always understood that idle lope (due to cam overlap/duration) is a sign of incomplete combustion at idle. The cam I am running is mild, but there is still some bit of lope. So mebbe even if I do get the mixture set right, wouldn't you still expect a bit of stink in the exhaust?

 

[Wietse]

From what i know from my cam, 238/244 @ .050" is that due to overlap and duration a part of the exhaust gasses stay behind in the combustion chamber, and in worst case scenario in to the intake.
This leans out your idle mixture, and therefore you need to add fuel at idle to compensate for that.

It is not a really reliable source, but i see average 13.0 AFR at idle on my O2 sensor.
Due to an incomplete burn, and also the possibility of fresh air flowing through the cylinder and into the exhaust the O2 sensor might indicate a false value, therefore i question the reading being right.