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Idle Adjustment w/ Vacuum Guage

Posted By Half-dude 3 Years Ago
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Half-dude
Posted 3 Years Ago
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Hey guys,

So I finally bought myself a vacuum gauge to do some proper idle adjustments on my car's Holley 4000 4brl. Now keep in mind, you're dealing with a 28 year old here who's not experienced in carburetor tuning. I believe I did a good job, but I figured I'd post some before and after readings in video form to see if any of you guys have any important observations.

Some things of note: I had to screw the idle screws in almost all the way before I got any kind of improvement in vacuum and idle smoothness. The right screw is about 1/4 to 2/4s turn out from closed, and the left screw is about 2/4s to 3/4s turn out. I was a bit surprised actually as I was basically able to close both screws and the engine seemed no worse for wear, I expected the engine to die from lack of fuel honestly. I had my carburetor rebuilt by Carburetors and More so I don't know how much of any mechanical modification they did to the inside of the thing.

Anyway, I mostly wanted to do this today because the car, well, has a rough idle. I seems to run smoothly for the most part but have sporadic little hiccups which you can see as vacuum dips on the gauge. After adjusting I've got them mostly eliminated and the hiccups that are still there aren't as strong as they used to and for all I know might just be part of that signature Y-block idle sound people love so much.

My other concern and question is this. Should you take vacuum readings with the car in Neutral or in Drive/Reverse? Or should it not make a difference? I ask because while in Neutral or Park the engine vacuum is right in the middle/high of the green zone 20-30 pounds. But in drive or reverse the vacuum drops into the red where it's labeled "Latent Timing" on my gauge. Is there any sign for concern here?

Anyway enough chat here are the videos:

Initial Vacuum Check in NEUTRAL


Vacuum Check in DRIVE - (check it a bit ways in, that's when it start dipping randomly.)


Vacuum Reading After Idle Adjustments

chris70
Posted 3 Years Ago
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Hi,here is an interesting article that can help you with the vacuum gauge: http://www.secondchancegarage.com/public/186.cfm   It seems to me that your initial reading was very good 22hg is fine,in the last video the vacuum is way to low.If the engine doesn't die with screws all turned in there is something wrong with carb.It's getting fuel somewehre it shouldn't.
charliemccraney
Posted 3 Years Ago
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On modern carbs, the engine should die if even one screw is completely closed.  If it doesn't die, it's most likely that the power valve is blown.  Another possibility is that the throttle blades are open too far and it is actually idling on the primary circuit.  Unless you have a radical cam, the throttle position is probably ok.

I don't know how much of this translates to a Holley 4000.



Lawrenceville, GA
Half-dude
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chris70 (9/19/2016)
Hi,here is an interesting article that can help you with the vacuum gauge: http://www.secondchancegarage.com/public/186.cfm. It seems to me that your initial reading was very good 22hg is fine,in the last video the vacuum is way to low.If the engine doesn't die with screws all turned in there is something wrong with carb.It's getting fuel somewehre it shouldn't.



Thanks for the link! I'll be sure to check it out. As for your comments I just wanted to point out that between those two videos, the one with the good reading was with the engine in Neutral or Park. The other was with it in Drive with the Parking break on. I'm not really sure what gear it should be in when you do a reading and if it should make a difference. I don't know why but my car has always been like that, it idles too fast when out of gear, and drops a lot when in gear.
Half-dude
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charliemccraney (9/19/2016)
On modern carbs, the engine should die if even one screw is completely closed.  If it doesn't die, it's most likely that the power valve is blown.  Another possibility is that the throttle blades are open too far and it is actually idling on the primary circuit.  Unless you have a radical cam, the throttle position is probably ok.

I don't know how much of this translates to a Holley 4000.


Thanks, it sounds like it's time to email Carburetors and More and see if they made any substantial modifications to the thing when they rebuilt it that would explain that.

chris70
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Make your readings in P/N the drop in rpm and vacuum in gear is normal.Mine goes from 16/17 in Park and 9/10 in Drive.You must achieve the highest vacuum by the lower rpm for a correct carburetion.
Kahuna
Posted 3 Years Ago
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You definitly have a carb problem of some sort.
On the Holley 4000 carbs, the power/economizer valve
is held closed by spring tension on the valve itself.
The valve is opened when vacuum is decreased to a small diaphram
with a pin located directly above the power valve. If that part is installed
incorrectly or the diaphram is leaking, the power valve will be open all
the time.
A mentioned earlier, closing the idle adjustment screws should stall
the engine.
Either Mike Suter or Sal Scicala can fix that carb for you.
I have no info about the Carb & More that did the work on your carb
Half-dude
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I had a leaking secondary problem with the carb before I had it rebuilt one of the secondaries was always running.

If the power valve is always open shouldn't i be able to see fuel coming out the secondaries?
Ted
Posted 3 Years Ago
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Half-dude (9/20/2016)
....If the power valve is always open shouldn't i be able to see fuel coming out the secondaries?

The power valve (enrichment circuit) is for the primary side of the carburetor and simply allows the primary fuel mixture to fatten up as the manifold vacuum drops to a given point.  The secondary side jetting is controlled completely by the secondary jets and has nothing to do with the power valve circuit.



Lorena, Texas (South of Waco)


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Is there a way to visually check if that's whats happening? Like can I see a flow of fuel from the power valve through the choke plates or anywhere else?


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