Idle Hunting and Lower Vacuum

[bigvette1]

This is primarily for Cliff, but all else please comment.  I had a Q-Jet (from a 77 w/apt) and set up for a 73 Vette 454" rebuilt and modified by Cliff and for the most part it runs well.  Here is the issue:

-  At startup and during warmup I have almost 20" of vacuum.  Once the engine warms up fully it drops to 15-16" tops and tends to hunt the 1".  Adjusting the idle screws has minimal if any affect and are set for 4 turns out.

-  When the engine is running and warm you can hear a slight surge in exhaust sound (gets throater than normal)  and I would bet it follows the filling of the float bowl.  Or  

-  I have a Edelbrock 750cfm and when warm it pulls 18" and is fairly steady, maybe hunts 1".

-  I have a Holley Street Avenger 770cfm and it also pulls 18-19" when fully warm and reacts the same as the Edelbrock, maybe a 1" hunt but pretty stable. 

As for feedback to Cliff, when you went over the Q-Jet you had to bore out the idle bleeds larger than what it normally should have been.

After we got done shipping back and forth, I said something is affecting the idle.  I found exact TDC and found I was 1 tooth off with a MSD distributor at top dead center.  I reset the dist. in the block and that  caused me to move the wires 1 post to get the reluctor gear to line up exactly with #1 tower when at a true TDC.  Then set normal timing for the engine. 

My question are:
1.  Did we mess something up with the large air bleeds causing lower vacuum at tidle than the other two carbs?

2.  Is there somthing amiss or is this to be expected?

Thanks Jim

[Cliff Ruggles]

Jim, are you using ported or manifold vacuum to the distributor vacuum advance?

Have you watching the timing with a light at idle speed?

Hunting at idle is most often caused by weak distributor advance springs, or play in the distributor internals, etc, which cause or allow the timing to move around some at or very close to idle speed.

Just spent a couple of weeks diagnosing a very simliar problem, and it ended up being the distributor, not the carburetor.  The customer had a Holley carb and it "hunted" as well, and when the distributor was replaced, the problems immediately stopped.

I have no idea what you are talking about for "airbleeds".  Your carburetor required a much richer idle calibration than would be typical for the combination of parts.  We see this on occassion, and there are a lot of things that can cause this.  Most likely cause is cam timing, intake closing late, and/or relativley "low" static compression ratio.

We've ran into a few vacuum leaks as well with the factory intakes thru 1969 that used the channel under the carburetor fed by the exhaust crossover.....Cliff

[bigvette1]

Thanks Cliff,

I will study the timing of the engine when warm and see if it is wandering.  Also, time to pull out the propane cylinder or carb cleaner and do some squirts for leaks.  With a vette and the immense amt of vacuum lines, you can have a leak 5 feet from the engine and not know it. 

After I spend some time, I'll get you some feedback.  Thanks again for your assistance.  Still waiting to get through Mt. Vernon and stop and visit.  Some time I will.

Regards,

Jim

[bigvette1]

Status.......

Cliff, I checked the timing and darn if it was not changed.  I suspect that when I had the car into a shop for a tranny rebuild and other work they changed it or nudged the distributor.  Anyway I reset the timing to about 11 degrees from 4 advanced.  This improved the vacuum from 15-16 to almost 18".  Next I started playing with the idle mixture screws and here is where I am a little confused.  I had them out about 4 1/4 turns.  Remember these are the fine thread screws on this carb and that does not seem like too much. 

Anyway, I turned them in to about 3 turns and the vacuum rose slightly and stabilized.  This with the timing made a difference in the sound of the exhaust from a pulsing to more of a steady flow. 

Question - leaning out the idle on this carb does not seem like it should increase vacuum?  Why on this one?

[77cruiser]

When you increased the timing & the vacuum, it increased the pull on the circuit. By leaning the idle screws you probably have about the same AFR as you were at with the late timing. So it was too rich after the timing increase.

[bigvette1]

Makes sense.  Advanced timing increased vacuum.  Now to get the AFR correct so it gets burnt and not a smelly car.  Thanks

[Cliff Ruggles]

In all cases, with idle mixture screw adjustments, verify the initial timing setting first.  Make sure that the timing is not moving around at idle speed as well.  If you have an auto trans car, have someone place it in gear to verify that the timing is rock steady at whatever you have it set at.

If you are using ported vacuum to the advance, verify that it is not adding a small amount of timing by pulling off the hose to the vacuum unit.

Once fully warmed up, and well heat soaked, set the idle mixture screws for the best idle quality at the leanest setting(s). 

If the manifold in is dual plane, and fully divided with a fully divided gasket, set each mixture screw out for maximum idle rpm, then trim then in one at time until you get a speed changed, then back out about 1/2 to 3/4 turns.  If an open gasket is being used, same procedure but you may have to spend some additional time balancing out the mixture screws for your final settings.

Keep in mind that a lean mixtures is often very "stinky", as quite a bit of unburnt fuel makes it out the tailpipes when we don't put in enough fuel for complete combustion.  This is often confusing for many tuners, as they think the mixture is too rich.  In any case, the engine will tell you what it wants, and in any and all cases, set the mixture screws for the best idle quality no matter how many turns in or out they end up at.....Cliff