I hope someone can figure this out better than me!
I have a mildly modified 302 - holley 600 cfm, Eddelbrock Torker II, long tube headers on a stock motor. Not the best parts package, I know...

Problem is, when tuning the passenger side idle mix screw, nothing happens. No change in manifold vacuum or in how it runs. The drivers side seems to carry all the load - vacuum reading is best at 2 full turns out.

I just rebuilt the carb again...no change. I've soaked it in carb cleaner for over a day and rinsed thoroughly, and ran air through every passage. No change.

I'm stumped. Any ideas?

 

You've already done what I would have suggested. (remove the idle screw, spray some carb cleaner in the passage and blow out with compressed air)

It's been a while since I've had my hands on a Holley but is the metering block gasket exactly the same as the original? In other words make sure none of the passages are blocked off by the new gasket.

Don't the mixture screws use an o-ring as well? Maybe there's a problem there?

 

Metering block gasket is good. I verified it is the right one with a book I have, and also verified that it is installed facing the right way.

The idle mix screws have an o-gasket around them. I've checked for a leak, I get nothing.

Does anyone know what is going on internally with the idle mix screws? Doesn't this circuit have something to do with the amount of fuel delivered at idle?

I took the screw on the passenger side out all the way while the engine was running. There is a pretty strong vacuum pull there. So is it trying to pull air or fuel, and in either case where from?

Thanks for help!

 

Did you replace the power valve when you rebuilt the carb? Do you remember what size it was?

What brand was the rebuild kit, genuine Holley or other?

 

Here's something I just found on Holley's website:


QUESTION Why is my engine running rich and my mixture screws do not have effect?
ANSWER The first thing you need to ckeck is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" powervalve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance cam shafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle.

Holley Performance Products Frequently Asked Questions

 

I used a Genuine Holley rebuild kit, comes with a 6.5 power valve. I've rebuilt this thing several times. I'm generally pretty good with Holleys but this one has me stumped.

If neither needle had any effect, it would most likely be the fuel level or the power valve. but in my case, the one needle is capable of adjusting the mixture while the other has no effect. Since both of the primary barrels rely on the same PV and fuel bowl, I don't think that is my problem.

I am only guessing here, and would love for someone to straighten me out: there is an idle air bleed in the throat of the carb for each barrel. This is the outermost one on each side. I am guessing that air is pulled in here, goes into the metering block, where the idle mix screw is used to adjust the amount of air that is allowed to pass through it. In my case, there is a clog in the circuit somewhere between the air bleed and the mix screw. That's just a guess...I'll tear it back down tomorrow and test my theories.

but I welcome everyone's ideas, cuz I'm starting to look like this:

 

Could you be sucking air from an abnormal spot on the carb. I had a defective edelbrock carb under warranty . It would not idle quite right. While it was running I began spraying carb cleaner around all the seals until the idle picked up and the truck ran smoothly. The shaft that the butterflies screw to was wallowed out.

 

You can take your can of jet spray Gumout and direct the spray hose at one of those bleed holes and give it a shot to clear out anything in that path.

Each idle bleed hole leads to a fuel well, where the air drawn in bubbles through and gets mixed with the fuel forming what they call an emulsion; a fuel-saturated vapor. That gets drawn into the venturi through the idle ports.

The idle mixture screws on the non-emissions version of the carburetor controls the amount of fuel that gets through to the emulsion chamber. On emissions versions, the screws control the amount of air that gets drawn into the emulsion wells. If changing the screw setting is having no effect on the idle vacuum, then something is over-riding its control. Holley explains some of them on their web site, such as you found, or some other form of leakage.

This happened to me once: I had a pretty new Holley 600 cfm carb on an Edelbrock F4B intake on top of a newly rebuilt 302. While trying to tune it, I also found that one of the idle screws was having no effect on vacuum. I eventually found a leak in a plug I installed on a port in the intake manifold. The F4B is a dual-plane intake, and the plenum on one side is separated from the other. This port was one of two on the manifold, and this one for some reason was opened only to one side, while the other port was opened to both sides. (I had connected the brake booster line to the other port.) So fixing the leak allowed that one idle screw to control vacuum again, and got the engine to run smoothly.

 

I think I've found it! The idle air bleed feeds into the top of the metering block. On the side of the carb that works, I can direct carb cleaner into this hole (in the metering block) and it will spray out of a small jet near the power valve. On the side of the carb that doesn't seem to work, I spray into either the hole or the small jet and it is not able to continue through. So I believe there is an obstruction in there. So far no luck in getting it out, but I'll try to soften it with a good long soak in the chem dip, and try again to blow it out.
XLT4WD90, I wondered about a similar thing that you are describing with your intake leak. I've checked carefully for a vacuum leak all around the carb and intake and not found one. That's not to say it isn't there! But mine for the moment is a single plane intake, with the common plenum feeding all cylinders.
I really appreciate everyone's input. I hope this solves it...

 

It might be necessary to remove the emulsion well tubes. The tops of these are visible on the top of the metering block. They are made of brass and pressed in.

To remove them, clean the tops well and solder on a copper nail to the larger ones, and a 3/16" brass rod to the smaller ones. Then hold the nail or rod in a drill press chuck or vise and gently tap the metering block away with a lead block.

I had to do this once to fix an old 370cfm 4 barrel. It's really hard to imagine any other way of cleaning the tiny holes in the emulsion tubes. I suppose the old style carb cleaner dip would do it, so long as it wasn't a glass bead or some such.

And people think Holleys are so easy to modify! Some Webers, if I recall correctly, had screws that were removed to allow their emulsion tubes to be readily removed.

 

That did it! A long soak in the parts cleaner bucket and a high pressure shot of compressed air. Cleared it right up!
I've got the carb back on the engine and all dialed in. this is the best this motor has run since I dropped it in. Thanks to everyone for the replies!

 

'60ford, that's the same technique I use to check the passages in the metering block; spray carb cleaner into each passage to see if the stuff comes out where it's supposed to. Just wear safety goggles so it doesn't it your eyes. Good job on fixing it.

Beanscoot, I'm amazed that you can get such a strong soldering joint that you can actually pull out those pressed-in tubes. After that, I'm thinking you really have to clean off the solder, or any remnants will inhibit proper air flow, just like an obstruction.

 

The soldering is barely able to have enough strength. It only works with a copper or brass puller, steel doesn't have the strength. Even so the solder joint often breaks on the first time, so I have to try again.

The solder is only applied to the top of the tubes on the metering blocks where there's no holes, so nothing to plug.