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Old 06-25-2013, 03:20 PM
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Low fuel pressure and holley style carb

OK, I installed a new Quick Fuel ss 680 vs carb last night and I cant get the idle down below about 1200-1300 rpm. Ive done the usual checks, vacuum leaks, linkage hanging up, choke etc... Curb idle screw is all the way up. I cant seem to get it. One thing I did notice is that my fuel pressure bounces wildly between 3- 4 psi. What are the effects of low pressure (below 6 psi)? every where I search deals with high pressure.

Timing is set at 12 btdc and a total of 34. Timing was set with the old edelbrock on it.

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Old 06-25-2013, 04:01 PM
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fuel volume is the important part.I used to use a regulator with stock pumps just to mellow the pulses(mechanical pump)You need to have a good line from tank,7/16 is better than 3/8.You might have to take carb off and check secondary throttle blades,stop might be open too much from factory
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Old 06-25-2013, 04:05 PM
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Never have delt with a holley style carb. Is there a tutorial on how to adjust the throttle blades? Screw adjust? located? Whats the best way to do it?

THANKS
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Old 06-25-2013, 04:14 PM
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there is a tutorial posted somewhere,dont have the link.someone will post soon I hope.browse the wiki,the picture saves a 1000 words
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Old 06-25-2013, 04:31 PM
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Low fuel Pressure

Stea, i had the same issue of the low fuel pressure and it turned out to be the liquid filled gauge. When the motor warmed up the pressure would show 3-4 psi. I also installed a non fluid filled gauge on the out put side of my regulator and that showed the real pressure of 6.5 vs the filled gauge showing 3-4 when the engine was warmed up. Just something to check out
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Old 06-25-2013, 04:34 PM
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Its a non fluid filled gauge and it shows the 3-4 psi right at cold start up and doesnt change when engine warms up.
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Old 06-25-2013, 04:44 PM
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3-4 psi should be fine. Bigger line=less needle bounce.
Try this link:
How To Adjust The Idle Mixture On Holley Carbs - Blogs - Holley Performance Products Forums

some of it is in .PDF
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Old 06-25-2013, 04:51 PM
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Ive read all of that, I think is where i'm off is the throttle plates are too far open. I need a tutorial on how to actually adjust the throttle blades. Primary and Secondary on a 4 corner idle carb. at this point the idle mixture screw dont do a thing and the idle curb is backed all the way off.
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Old 06-25-2013, 04:57 PM
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Carburetor Tuning the Scientific Way
Carburetor Instruction Sheets, and other Technical Information
This last one should give you your answer, good luck
http://forums.holley.com/showthread....on-t-idle-down
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Old 06-25-2013, 05:32 PM
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OK, so its just the curb idle screw that adjusts the primaries and the screw on bottom of base plate for the secondary adjustment?
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Old 06-25-2013, 06:47 PM
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Quote:
Originally Posted by stea View Post
OK, so its just the curb idle screw that adjusts the primaries and the screw on bottom of base plate for the secondary adjustment?
Yes sir, upside down screw, depending on intakes you can actually adjust it from the upside down position using a 1/4 inch rachet and bit. Rpm with a 1" spacer gives lots of room to do this
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Old 06-25-2013, 09:17 PM
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Got it pretty good tonight. I tried everything I could think of. Plugging pcv, pinching off hose to booster. No vacuum leaks. I then decided to lower the floats. Took the primary from 1/2 site glass down to about 1/8 site glass. Dont know why but it did help. Idle is still a bit high in park and good in gear. Now I just need to get the secondaries just right and it has a small off idle bog. Also idle screws are only out about 1/2 turn. Is it a candidate for larger IABs ? Currently has .072 primary and secondary.

Last edited by stea; 06-25-2013 at 09:24 PM.
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Old 06-26-2013, 12:16 AM
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adjust the pump cam one notch for bog,as a first try. air bleeds need to be done on a dyno with a sniffer
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Old 06-26-2013, 10:23 AM
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I have a quick fuel 600 street slayer carburetor and the quick fuel carbs have a very rich fuel curve for the idle circuit and mine was always 1/2 a turn from the seated position on the idle mixture screw and from what I did on research most of them are about a half turn out cause they use a rich fuel curve.

Don't mess with the air bleeds as they are only a last thing to change sort of deal and most of the time don't need changed for the most part. You quick fuel carb has what is called idle feed restrictor's in the main metering and secondary metering blocks and quick fuel makes them screw in design and you can change them to smaller if you want to lean your idle fuel curve out some.

Changing things like these you need to really know your holley good before doing so. What is nice about the adjustable metering blocks you can always put back to stock setting if you need to do so.

Your carb should have a allen screw for the secondary's and you can use a 3/32 allen wrench to adjust it without taking the carburetor off. Quick fuel makes some very good carburetors.

Amazon.com: Holley Carburetors (9781884089282): Dave Emanuel: Books Amazon.com: Holley Carburetors (9781884089282): Dave Emanuel: Books

check out that book on amazon.com and it has a lot about tuning holley style carburetors and you will learn a lot of info from it.
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Old 06-26-2013, 03:32 PM
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TAHNKS Guys, Im going to mess with the pump cam later in the week. Hopefully that will get rid of the off idle bog.
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