Chris_Xtreme
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| posted on 10/6/13 at 12:23 PM |
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how many idle mixure screws should I have - Holley 8007 / 390cfm
Hi,
I have a holley 390cfm / 4160 / 8007 in my kit.
It is running lean on idle so I wanted to get it sorted. I've got a vacuum gauge on order and after reading the linked doc, I think I should be
having 2 idle mix screws on the primary and 2 at the back on the secondary.
On mine the secondary points are there, but no screws in them ! does this mean they are open to air? or from some internet viewing I think I saw that
some carbs may only have 2?
any thoughts? should I be buying two more screws?
http://holley.com/data/Products/Technical/199R8108-2rev2.pdf
cheers,
Chris
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mark chandler
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| posted on 10/6/13 at 01:08 PM |
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If its a vacumn secondary then no screws as all idle controls are on the primary circuits.
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britishtrident
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| posted on 10/6/13 at 01:14 PM |
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On any carb where the secondary throttles are progressive that is the secondary butterflies don't open until the primary throttle butterflies
are almost fully open then you only have idle mixture adjustment on the primary(s)
The same applies where the carbs such as the old Solex carbs that had depression operated secondary throttles..
[Edited on 10/6/13 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Chris_Xtreme
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| posted on 10/6/13 at 01:47 PM |
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interesting that you both think that - tho it does contradict what it in the manual
snippet:
IDLE MIXTURE NEEDLES:
Idle mixture needles control the air/fuel mixture at idle. The amount of air fuel mixture used at idle is controlled by engine vacuum. So when tuning
the
idle mixture, you are actually tuning for best manifold vacuum. Idle mixture needles are found on the metering blocks. Your carburetor will have
four
idle mixture needles, one for each venturi, this is known as four corner idle. If you change one idle mixture needle, you are required to change
the other
idle mixture needles the same amount. Here are the proper steps for setting the idle mixture needles.
1. Attach the vacuum gauge to the manifold vacuum port usually at the rear of the carburetor and on the throttle body.
2. Adjust each idle mixture screw the same amount to achieve the highest possible vacuum reading without increasing the idle speed screw.
3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed screw. Continue back and forth between the tuning of the
idle mixture needles and idle speed screws until little change is noticed in manifold vacuum and idle speed is correct.
this is a vacuum operated secondary carb by design which the instructions explain alot about and why it can't / shouldn't be changed to
mechanical...
cheers
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Canada EH!
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| posted on 10/6/13 at 02:08 PM |
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I haven't played with a 4 barrel Holly in years, but I had several and all were vacuum secondaries (this is done so that the rear venturies
don't open at low speed and drown the engine, most don't open until 3/4 throttle).
There should be no fuel going through the secondaries at idle.
If you think of a Weber DGV the Holley is just a 4 barrel version, but with a vacuum controlled secondary instead of a linkage.
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mark chandler
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| posted on 10/6/13 at 02:15 PM |
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All I can refer to is my memory, I had a 3litre Capri with a 390cfm Holley with vacumn operated secondaries.
It only had idle circuits on the primaries, there was a throttle stop for the secondaries that you set to fully closed.
Each primary had its own air bleed screw and where you set the throttle plate very slightly open, something like 10 thou and then fine tuned the idle
just on the primary adjusters.
The carb had blanked off holes on the secondaries, you could purchase 390's that all butterflies opened together, in this case you would set the
idle on all 4 screws.
Regards Mark
[Edited on 10/6/13 by mark chandler]
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Chris_Xtreme
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| posted on 10/6/13 at 03:04 PM |
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all experiences welcome!
http://www.bob2000.com/carb.htm
this guys site says something a bit different too:
'The idle circuit supplies fuel when the throttle plates are open only slightly. Mixture screws let you adjust the mixture of this circuit. On a
Holley, the fuel comes out a little hole below the throttle plates in the baseplate. There is an idle circuit in the rear barrels of your Holley, too.
Most are not adjustable, but Holley put it there to keep the fuel flowing through the rear bowls so it doesn't get stale just sitting there.
This is why you can't turn off the fuel flow to the secondaries. Doing so will mess up the metering system by causing a lean
condition.'
reads like there are different potential options on the setup of the same carb ! as per the holley doc, your thinking and this guys..
and they can be converted to have secondary metering or not..
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britishtrident
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| posted on 10/6/13 at 04:19 PM |
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Just to clear up a point before anybody beginner gets on the wrong idea, secondary throttles can't be operated by engine vacuum, they are
operated by the drop of pressure in the primary barrels venturi created by the air velocity at the venturi restriction
This makes the secondary barrels difficult to test as it/they will only open under load at full throttle.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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