You left out a few details - what carb?  what is your curb idle speed?  

You can never tell until you try so two things I would look at areL  The position of the throttle plate in the transition slot if a Holley.  This can cause the mixture to be uneven at idle and cause vacuum to drift.  Second, you may have too much initial advance.  If the total mechanical is limited to 16 or 18 (which is added to your initial), you may end up with an initial advance that is too high when trying to reach an ideal 34 to 36 total.  For a quick experiement, drop the initial down to 12 or 14 degrees and readjust your idle speed and mixture for this setting.  The car may seem a little weak on the high end but if your lopey idle improves then you may want to make some changes to build in more mechanical advance depending on your distributor.  I have had some cars that when there was too much initial it tended to get a little lopy.  All engines are different and some will idle happily with 18 degrees, others will not - one tune does not fit all.

If you are trying to run a low idle speed, you may have it too low based on the manifold and carb you are using as well as the lift on that cam.  So, as another experiment, increase idle speed by 200 RPM, adjust the mixture, etc., and watch your vaccum.

Wandering vacuum can also occur with a blown power valve on a Holley but usually you will be running way rich if this happens, and if it isn't a Holley (or some variant) you may not have a power valve.

Until you see how the car responds to a change (either better or worse) you are just guessing about the cause.

Sorry about the ommisions, I was multi-tasking between drafting my question and putting together an pink IKEA bed for an impatient 8 yo girl, guess which one demanded more attention.

The carb is an 1.12 Autolite 4100 with the primaries sleeved down to 1" (Pony carbs spreadbore)
Headlights on and trans in D the idle sits at 700 rpm
I limited the mechanical advance to 18* so total is 36* @ 2900 rpm

I have one of those Moroso PV testers so I can check if mine is blown out, and I am trying to run a lower idle as I have manual disc and it makes waiting at the stop light less fatiging. I'll look again but I thought the throttle plates where fairly well closed, I had wondered about the transition but kind of wrote it off on account of the throttle stop being adjusted low to get the idle speed I wanted- I'll take a second look, if anything theres something to what you're on to and I for sure want to map my fuel curve once I get my O2 sensor and gauge back in.

That's all good stuff GP, I know I have more tuning and tweeking so early in the game but I was concerned I may be looking for ghosts, I'll keep on it and follow up.

 

I am trying to run a lower idle as I have manual disc and it makes waiting at the stop light less fatiging.

Seems like another symptom here.....
 

6sally6 wrote:

Brents65 wrote:

I am trying to run a lower idle as I have manual disc and it makes waiting at the stop light less fatiging.


 

 'Splain what this means^^^^   Manual discs and idle speed???? Lost me.
6s6

 
It's part of Jon Richards "splainin" .   Sounds like his torque converter is partially locking up causing him to have to wrestle the brakes at stop lights.

I have a T-5z awaiting a Chevy S-10 forward position tailshaft swap, The T-5z would have gone in originally but Modern Driveline wasn't able to help when I reached out to them and I was (and still am) dead set on keeping the bench seat in the Falcon.

Point being idle speed at a stop won't be an issue for long. 

Rsmach1 wrote:

If it's possible the exhaust lift of that cam is 496 and not 498 it would be a commonly used older grind I have used in the past. Specs were 214-224 duration @ .050 lift, 472/496 lift, typically LSA of 112, ICL 107.

Edit; in a 289 that cam will have a slight lope, it was a good midrange cam.

Thanks for that. I was pretty uptight about measuring the lift so I'm not sure I'd be off by .002" but it's possible, at the very least it tells me it would not be uncommon for a cam with these lift numbers to have a less than smooth idle.

I'll get that A/F gauge hooked up, re-test the manifold vacuum with a different vacuum gauge, and mess with the idle screws and base timing a bit to see how she reacts.

Downey eh, you shop at Team-C? 

Rsmach1 wrote:

It's longer duration that causes the lopey idle, not lift.
Edit; LSA has an effect on idle also.

I understand, was just asking those with experience to speculate-

Jon Richard wrote:

Thanks 6Sally6, I knew the lift wasn't the culprit but I thought it may provide some insight in to what other grind properties one would expect in that range.

I am in SoCal, Just moved to Long Beach from Santa Ana. Team-C is near me and seems one of the last brick and mortar shops around.

Well, I re-adjusted the valves, raised the idle, turned the idle screws out an 1/8" turn, installed the A/F gauge, re-torqued the intake manifold, checked the plugs and gaped them to .040" and all seemed good.

Took it out and was headed to a friends for some BBQ when after I got 'er good and warm decided to get on the wood after which the motor started klacking hard-

Yeah, I'm begining to think I might have been onto something with my earlier suspicions