Still haven't figured out the idle issue...



Well-Known Member
Jun 19, 2011
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Jefferson, MD
So a few months ago I had problems with my '68 missing and all kinds of nonsense..turned out that issue was a faulty ignition coil and wonky ballast resistor. Got that figured out, but there has been an ongoing issue with idle quality pertaining to fuel mix. I'm running idle at about 850 rpm, vacuum is 20 at idle, with 5 degrees of advance. Valve lash is .12 and .22. Anyways, if I set the idle screws on the Holley 390 for best idle, there's a huge dead spot as soon as I start pushing on the gas, and the motor leans out enough that it usually stalls. If I set the idle pig rich so that the motor surges, the dead spot obviously goes away. But then the idle sucks. So the gist is that I have to sacrifice idle quality for driveability.

So what should I be looking for? I'm almost sure this is a carb issue, but what in the carb could beccausing this? Is it possible that this is a power valve issue?
Needs WAY more initial timing first. Get the timing nailed down first and then see where it is.
OP, did you mean .012" and .022" on the lash? (You listed .12 and .22 so I assume that is a mistake LOL).

Well, my /6 idles good at 5 degrees BTDC initial ignition timing BUT that is a 1 BBL Holley 1920. not a 390 cfm 4 BBL. Yes, add 5 degrees of ignition timing and see what happens; if it improves this problem then work over the ignition timing setup.

Also, check:
- Float level in the carb... triple-check it! This is critical to all phases of carb operation.
- The secondary stop screw and adjust it so that the bottom edge of the transition slots in the secondaries are just BARELY visible. Then reset up the carb and try again. This secondary stop screw setting is important to make the idle-to-cruise transition work right. It may take several iterations to optimize this so work this issue thoroughly.

While you are doing all of this, it can be useful to put a vacuum gauge on the manifold vacuum port and monitor it while the car acts up; knowing the vacuum levels when things act up can be a useful tool for knowing what it likely going on.
5* initial and 20 inches idle-vacuum, does not compute in my head.That's a lotta vacuum for just 5*

But that may be no never mind, cuz a tip in problem is usually a pumpshot issue.
It is not a PV issue.
I assume it is a stock cam, AJ, and idle RPM's are a bit up there. My stone stock Dart Lite idled at 21" at a lower RPM.
^^I understand. But what timing were you running?

I assumed the 850 was a requirement to satisfy a big cam.
And to have a big cam, 5* idle timing, and still pull 20inches;Hmmmmmmmmmmmm
That's a pretty good ring-seal

I've had a few stone stock slantys that wouldn't pull that off. Not oil burners either.
My last slanty does tho. But it's not stock.
And no, I'm not arguing; I just can't see it, based on my very limited experience.

FWIW;stock slantys can idle real good at 450 to 500 in gear. I love it when you can count the shots, ,and those shots start to shake the car,and the sound in the pipes go "thoop,thoop,thoop."
Oh and I've had better results at 13/23 lash;splitting hairs I know.
Stock idle settings for the Dart Lite setup....whatever that was! Maybe 5 degrees BTDC? I never did anything to that car except put in gas and fix what broke! I would also put it in a lower gear going down a mountain and see how high could get the vacuum readings to go with the throttle closed.....27" was not uncommon. So part of the 20" idle vacuum for the OP may be just due to the high idle RPM.

My '62 /6 with a mild torque cam and an .050" head shave from Doug Dutra idles at 17-18" vacuum with a simple 1920 carb, which is low to me for an almost stock /6.
Primary jets? still has not told us what he is running. Stock /6 with a 4-barrel? probably not. How does the engine run at low throttle? Primary jets are involved with low rpm operation and if you are leaning out and stalling you may not be delivering enough fuel. If the motor picks up when you add more pedal that is because the secondaries are assisting. A pair of larger primaries may be the easy and cost effective solution. I think .51 was standard I wound up at .60 before getting decent initial throttle/engine response

Also to quote nm9stheham double check the floats in the bowls!!
IMO....while the primaries feed the idle wells in Holley's, that would be the last thing I would work on. My order would be:
- Float settings right
- Timing changes to see if that helps
- Secondary stop adjustments and a couple of iterations
- Small float setting increases
- Accelerator pump
- Primary jets

BTW, OP, do you still have the points in the distributor? Make sure the peak points gap is around .018" and that the 'nubs' on the distributor shaft that operate the points are not worn down. A dwell meter would be ideal to check points operation.

The reason to look at the ignition system a bit more is that the energy required to make a spark jump the plug gap will vary considerably with load, exact A/F mixture, and operating phase of the carb. You might find it very good to do an HEI conversion.

And as 3spd6 said, what else is on/in this engine?
Woops, that's what I get for starting this topic without checking the car first!;)

Here are the car vitals:
Base timing:12 deg advanced
Idle rpm:around 800, drops when in gear to about 600
Valve lash:12/22
Points gap:.018

Yes, this is a stock motor aside from the carb. The dist was just rebuilt 2 months ago and the running issue was unaffected by that. interesting suggestions! So, PV is out, float level and main jets are in! I'll check them once I get dug out from the blizzard...been on a tractor 2 days straight moving snow!
Ok...carb tuning day!;) primary jet size is 51, so stock. Float was a little high, but not terrible. The accelerator pump is a that too small? Ive got the carb apart abd am not finding any dirt or anything if that sort.
Ok...carb tuning day!;) primary jet size is 51, so stock. Float was a little high, but not terrible. The accelerator pump is a that too small? Ive got the carb apart abd am not finding any dirt or anything if that sort.

Check out how much transfer slot is exposed, it worked for me anyway on my small block.