# Holley carbs



## bearddd29 (Nov 15, 2009)

Hows it going to everyone im new on her hope i meet some nice friends on here to get help of .anyway i have a 350 chev in a hotrod and went in second hand worked a little had a 780 holley on it i think or 750 i decided to go for a tunnel ram for the looks and installed this as well as two holley 600 4 barrells which came of sep worked 6 cyl jetted nearly the same i thought they might have been ok but i think there way to big for the engine because i cant get the idle down it must be pouring in fuel even if you try and lean it out its no good so i went through my old mags and i noticed some blokes put two 390 holleys on there tunnel rams is there anyway to modify the 600s to 390 4 barrell cfm so they will suit the car would this solve my problem cheers.peter


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## Wrench97 (May 10, 2008)

They are too much CFM but that would give you an issue with lag when the pedal is dumped, they should still idle back ok providing all 4 throttle shafts are fully closing the CFM would be 0. 
Do you have a progressive or synchronized linkage setup? 
Are the floats adjusted correctly?
Have you checked for vacuum leaks?


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## bearddd29 (Nov 15, 2009)

thnks wrench do you mean the butterflies ive got adjusted about the same just this side of the idle flow holes there abouts i do not think there is a vacuam leak its has new mannifold gaskets etc and one carb had been recoed the other i dont know seems ok where will i start looking for leaks . dont you still think there to big for the engine size . i have the both throttles adjusted roughly the same where they join the linkage the jets are 65 66 they seem ok dont they for the carbs there nearly standard size for them .


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## bearddd29 (Nov 15, 2009)

sorry wrench what did you mean by progressive and syncronized if synchronized yes adjusted to roughly the same opening position and 1 joining linkage from one carb to another cheers buddy


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## Wrench97 (May 10, 2008)

Progressive linkage opens one carb first anywhere from 1/4 to 1/2(it's adjustable) before the second starts to open(kind of like how the secondaries on a Qjet work) Synchronized is just joining the linkage together both cards open equally when the pedal is pressed.
The idle issue is not a matter of jet size, leaking power valves or butterflies that are not fully closing would keep the idle up.

To check for vacuum leaks I usually use a can of carb cleaner and spray on suspect areas while listening for idle speed changes, if you have concerns about painted areas and staining with the card cleaner a spray bottle of fantastic, 409 etc will also work.

Try pulling the linkage off the rear carb and run the idle screw all the way out, but first make sure the floats are adjusted properly* for the installation angle on the new engine there on.

*Do they have the window or the screw you take out on the side to set them?


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## pat mcgroin (May 1, 2009)

I have to agree with Wrench concerning the vacumn leak but you may have more than one issue going on here as well.

Many Holleys have a small screw in the side of the float bowl to help getting the float set to roughly the correct level.

If you are using the standard jets in the 600's they will supply too much fuel for idle due to the 2 carb setup. Combined with a possible vacumn leak the idle cant be slowed.
As stated above make certain that the throttle plates are as close to closed as possible.

Another thing to be aware of is if using the standard jet size on twin 600's the fuel will effectivly pour out of the jet and not be atomized correctly due to the lack of air flow velocity through each carb in the dual setup as compared to a single carb setup.
This will cause poor idle but will also cause VERY poor part throttle response.
Basically the standard jet size on the 2 carbs will provide enough fuel for a 950CI engine and 1500 RPM. Anything less than that will need an appropiate jet size.

Think of it this way:
If running a single 600 CFM carb you will have roughly 100 CFM of use at idle and the standard jetting will be fine.
If the carbs are synchronized each carb will have roughly 50CFM of use at idle and require smaller jets to compensate for the reduced airflow and be able to provide the correct amount of fuel and the ability to atomize it correctly.


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## bearddd29 (Nov 15, 2009)

thanks wrench they have the screw type where you run the elect pump and check floats i didnt think of powr valves il give the spray ago and go from there cheers buddy


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## bearddd29 (Nov 15, 2009)

thanks pat il take them of and try an syncronize them both on the bench throttle opening etc and reinstall them i think the jets were standard or thereabouts about size 65-66 what size will i go down to about 60


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## pat mcgroin (May 1, 2009)

Im not positive about the correct jet size but I think a call to a local speed shop that deals
in perfomance issues like this they would have a better idea.
Off of the top of my head I would guess it to be nearer to 50.
It wouldnt be 1/2 size but it would be significantly smaller.

A buddy of mine has 2 650's on top of a 427 Ford and I expect to see him today so I will see if I can find out what he is running.


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## pat mcgroin (May 1, 2009)

Well I saw him yesterday and he is running 60s.


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## bearddd29 (Nov 15, 2009)

thanks pat for the help il be in touch let you know what happens im going to start at 50 jets and see what its like then and put a vacuam guage on the carbs and see where im at


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## pat mcgroin (May 1, 2009)

That just the thing.
As wrench stated above 2 600s is a lot of CFM on a 350.
It may well trun out that your vacumn reading could turn out to be on the low side.
Generally larger CFM carbs are used for higher RPM engines.
The larger opening will allow more airflow but less air velocity.
They are always inverse of each other.
Give the 50s a try and I hope it works but Im not sure that you will be completely pleased with the performance. Im sure that it can be made to run but you may still need to run smaller CFM carbs in order to achieve partial throttle response.
A smaller CFM carb will create more air velocity through the venturi and thus help to meter the correct amount of fuel as well as fill the cylinders faster at a smaller throttle opening.

Though it isnt for your application here is an article to help understand the principle.
http://www.aircooled.net/gnrlsite/resource/articles/jetting.htm

Keep us posted on how you get along.


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## bearddd29 (Nov 15, 2009)

pat i think i probably prefer to drop cfm size in the carbs am i able to convert he 600s to 390 cfm in any way or are the carbs not similar i think for the engine size there still to big for the engine they,d be ok on a blown engine where ther sucking some air do you no anyone over there that can help with this question .cheers peter


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## pat mcgroin (May 1, 2009)

I dont know of any way to do that.
The thing that makes them different is the giant hole in the middle of the carb.
The 600 hole is roughly 10cm in diameter and can allow 600CFM of air.
Where as the 390 is roughly 6-7 cm and can only allow 400CFM of air.
The smaller opening will also increase the velocity of the air and atomize the fuel air mixture better.

Is there possible a way for you to sell or swap the carbs on Ebay or something like that?
Possibly for 2 500s.
Or possibly get a different intake and use one of the carbs.
I think dual carbs would be much cooler though.


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## Wrench97 (May 10, 2008)

I do know of a bucket T running around here with the rear carb dummied out, blocked off fuel line and block off plate under a spacer on top of the manifold it appears to work with all the linkage hooked up and fuel lines plumbed in.


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## pat mcgroin (May 1, 2009)

That is a interseting idea.
I just have to wonder what performance impact that it could cause.
I would be interested to see the plugs to find out if the rears were getting enough fuel.

Just because you made me wonder.
I wonder if a manifold with 3 4BBLs and only the center one working (assuming the right manifold) would work well.
I have never considerd that but it sounds awesome.
Who would know>


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## Wrench97 (May 10, 2008)

I think it depends on the manifold if the plenum is open enough it should work , this one sits on top of a fake blower btw so there is a lot of custom work involved here there isn't a lot of weight so anytime you stomp on the pedal the tires break loose any gear/speed.


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## bearddd29 (Nov 15, 2009)

thats not a bad idea wrench might see how it runs like pat said i dot no wether the rear plugs would be running to lean i might try with only one carb plumbed i dont want to leave it like that or might give the smaller jets ago as you and pat metioned reset levels and so on and see how it is for now il keep an eye out for a couple of smaller carbs if this doesnt work il let you know


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## bearddd29 (Nov 15, 2009)

that would look pretty tough although it works the same as a single 4 barrell on a tunnel ram and front and rear blocked of except 3 carbs would look better i might start it on one carb and check the plugs see if the rears get enough fuel ive never herd of anyone ever having one dummy carb on a ram


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## bearddd29 (Nov 15, 2009)

i forgot to tell you pat and wrench i took the bowls of the carbs . 1 of the carbs on front manifold had size 66 jets and the other size 60 back manifold


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## Wrench97 (May 10, 2008)

66 is a little large, been too long but I thing I only ran 64's on big block many years ago.


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## bearddd29 (Nov 15, 2009)

il drop them down thanks wrench to about 55s for a start on the small block c how i go


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## pat mcgroin (May 1, 2009)

55 seems like a good place to start.
60 and 66 is a lot of fuel and may be a large part of th idle issue.


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