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Old 2nd-January-2007, 04:09 PM   #1
Dnova
 
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Holley idle mixture question

Just finish retuning my Holley 4150 HP carb and the Idle mixture screws are at 1/2 turn of being fully seated. The questions is should I go up a size or two on the idle air bleeds so the mixture screws can be adjusted out? Other than a rich idle it's running great. thanks
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Old 2nd-January-2007, 04:29 PM   #2
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I've always started tuning the idle mixture by running each one in all the way and back it out 1.5 turns.Then I adjust each one with a vacume gauge for the highest vacume reading.Do the front set first then the back.You may have to recheck it twice but thats part of tuning.Hope this helps.
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Old 2nd-January-2007, 04:49 PM   #3
jason snyder

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Sounds like you may have to much primary jet!! typically your idel screws will be out much more than a half turn..Lowering the pri. jet may help with the rich idle as well! Try two jet sizes smaller and see if the idle screws tune better when out another turn or so.IF NOT THEN ADD SOME SIZE TO YOUR BLEEDS.
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Old 2nd-January-2007, 05:13 PM   #4
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Do you have 4-corner idle adjustment? If so, then 1/2 turn out is not that unusual.
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Old 2nd-January-2007, 06:36 PM   #5
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Quote:
Originally Posted by Pick View Post
Do you have 4-corner idle adjustment? If so, then 1/2 turn out is not that unusual.
Yes it is and I would agree, if it was not so rich. But something has to change and think I've read somewhere that an increase in the idle air bleed will decrease the fuel flow in the idle circuit. Is this correct?

Jason,
This is after decreasing the primaries.
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Old 2nd-January-2007, 06:42 PM   #6
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Quote:
Originally Posted by Dnova View Post
Yes it is and I would agree, if it was not so rich. But something has to change and think I've read somewhere that an increase in the idle air bleed will decrease the fuel flow in the idle circuit. Is this correct?

Jason,
This is after decreasing the primaries.
I had similar problems when trying to adjust idle mixture with both of my Holley carbs. In both cases it turned out that I had the idle transfer slot over-exposed. After pulling the carbs off and closing the primary blades (via idle speed screw) so that only .020 of the transfer slot was showing, everything magically fell into place.


Is this a possibility?
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Old 2nd-January-2007, 07:03 PM   #7
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I wouldn't use the main jets to try tuning the idle as it will affect the cruise mixture. There are idle feed restrictions you can work with instead.

Earl Parker of Earl Parker Carburetion sent me the following setup memo. We used it on a 4779 Holley used on a 331" road racing motor and it worked very well:

One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.


AED has some good tips on carb setup, also.

http://www.aedperformance.com/Tuning%20Tips.htm

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Last edited by Mike Goble; 2nd-January-2007 at 07:07 PM.
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Old 2nd-January-2007, 07:06 PM   #8
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Quote:
Originally Posted by NovatoriusWrecks View Post
I had similar problems when trying to adjust idle mixture with both of my Holley carbs. In both cases it turned out that I had the idle transfer slot over-exposed. After pulling the carbs off and closing the primary blades (via idle speed screw) so that only .020 of the transfer slot was showing, everything magically fell into place.


Is this a possibility?
Always had trouble get it to idle under 1000 rpm so I checked during the re-tune and the secondary was open too much. It's correct now but I'm still running rich at 700 rpm idle.
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Old 2nd-January-2007, 07:14 PM   #9
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What kind of air/fuel meter are you using?
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Old 2nd-January-2007, 07:41 PM   #10
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Burning eyes and a running nose.
I think i'll run and get some new plugs.
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Old 3rd-January-2007, 12:01 AM   #11
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After a little research I have finally found my answer in Inovative Motorsports wideband air/fuel meter tech section. Thought I'll post it , you never know, someone else might find this useful.

If the idle mixture screws don’t have enough impact on the Air/Fuel Ratio or Vacuum , you can try incrementally changing the carb’s idle feed restrictors and idle air bleeds (if the carb is so equipped). If the car idles with the idle mixture screws closed or nearly closed (less than a half turn from full clockwise / bottomed out), you can lean out the idle by substituting a smaller idle feed restrictor for the one currently in it, or you can try using a larger idle air bleed. If the car requires turning the mixture screws out from fully closed / bottomed, an abnormally large number of turns (4+ turns out from fully closed), you can try richening the idle circuit by installing either a larger diameter idle feed restrictor or a smaller idle air bleed.
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Old 3rd-January-2007, 12:57 AM   #12
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do you have idle air bleeds that unscrew?? if you do i would drill them out to about .74 most come stock with about .70 usually..you can get this drill bit at an ace hardeware or a machinist shop..remember do all corners the outer ones these are your idle bleeds the inners are your high speed air bleeds..they almost look like a miniature jet
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Old 4th-January-2007, 01:19 PM   #13
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If the engine doesn't respond to turning the idle mixture screws, there's a good chance you're running too high of a power valve value and the power valve is opening prematurely. Setting-up Holley's is really pretty easy. Changing the primary jets for an idle problem is a waste of time and you'll just end-up chasing your tail. You will need a vacuum gauge to set-up any Holley carb properly. Without one, you're in the dark.

To start you'll need to get your timing set correctly and then get the idle as good as you can and check the vacuum signal. Then you can go from there systematically and easily solve most issues. Once you get a Holley set-up right, you will never have to mess with it again unless you change something in your engine combo that affects the vacuum signal.

Last edited by 64PRONOVA; 4th-January-2007 at 01:22 PM.
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Old 6th-January-2007, 11:47 PM   #14
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Update

I changed all four idle bleeds from 73 to 75 and follow Mike's idle setup instructions again. The sweet spot for the idle mixture screws are at 1 1/4 turn from bottoming out. I'm now.
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Old 7th-January-2007, 02:38 AM   #15
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thats what i was figuring...those air bleeds are a big tuning tool now a days..mine are at .74 and they are spot on....
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