Carb too big?

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Well...Actually an Edelbrock "is" a vacuum secondary carb Bill.

As stated, 750 is way to big of a carb.
If you want to run an Edelbrock find yourself a 500.
Holley, go with the 390.
 
do you recommend the Holley or edelbrock. I am going to keep the 6. but I want a really nice cruzer.
 
Either seems to work. I prefer Holley but that is just 'cuz I heve used them and not Edelbrocks.
 
Brown8404 said:
do you recommend the Holley or edelbrock. I am going to keep the 6. but I want a really nice cruzer.
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It depends on what you are familiar with and can work on.

If you want to just bolt it on out of the box and go, then I would go Edelbrock.

If you are familiar with Holleys and tuning them in, then I would go Holley...
 
krazykuda said:
It depends on what you are familiar with and can work on.

If you want to just bolt it on out of the box and go, then I would go Edelbrock.

If you are familiar with Holleys and tuning them in, then I would go Holley...
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That's it in a nutshell. Depends what your comfortable with. Both will work with great success.

I run an Eddie 500.:burnout:
 

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Johnny Dart said:
...Actually an Edelbrock "is" a vacuum secondary carb Bill.
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Excellent point and I didn't know that. I saw the mechanical linkage between primary and secondaries and assumed the Carter/Edelbrock design was fully mechanical. This article explains it well: www.hotrod.com/how-to/engine/1306-vacuum-or-mechanical-secondary-carburetor/
To summarize, the generic term is "controlled-secondary". Holley uses a large vacuum diaphragm actuator. Edelbrock uses an "air valve" or "auxiliary throttle valve", which is actuated by the air flow and vacuum. Quadrajet and Thermoquad are similar. The mechanical linkage does open the secondary throttle plates, but that just "enables" the air door above that automatically restricts air flow thru the secondaries to what is needed. There are carbs w/ fully mechanical secondaries (Holley makes some). For those, the driver should not "floor it" at low rpm or the secondaries will go wide open and give poorer performance than if they modulate the pedal properly.

More sophisticated is a "variable venturi" design. Modern motorcycles that are still carbureted (ex. Ninja 250 in U.S.) have "sliders" for that. The Predator was an after-market one for cars. Ford used a variable venturi in early 1990's large cars which worked well but proved problematic.
 
BillGrissom said:
Excellent point and I didn't know that. I saw the mechanical linkage between primary and secondaries and assumed the Carter/Edelbrock design was fully mechanical. This article explains it well: www.hotrod.com/how-to/engine/1306-vacuum-or-mechanical-secondary-carburetor/
To summarize, the generic term is "controlled-secondary". Holley uses a large vacuum diaphragm actuator. Edelbrock uses an "air valve" or "auxiliary throttle valve", which is actuated by the air flow and vacuum. Quadrajet and Thermoquad are similar. The mechanical linkage does open the secondary throttle plates, but that just "enables" the air door above that automatically restricts air flow thru the secondaries to what is needed. There are carbs w/ fully mechanical secondaries (Holley makes some). For those, the driver should not "floor it" at low rpm or the secondaries will go wide open and give poorer performance than if they modulate the pedal properly.

More sophisticated is a "variable venturi" design. Modern motorcycles that are still carbureted (ex. Ninja 250 in U.S.) have "sliders" for that. The Predator was an after-market one for cars. Ford used a variable venturi in early 1990's large cars which worked well but proved problematic.
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Ok.
 
yeah, I would call a Carter-brock a vacuum secondary either. Vacuum secondary used a venturi in the primary bore to actuate the secondary linkage via a vacuum pot. The faster the velocity through the primary venturi, the stronger the vacuum signal that was sent to the vacuum pot to open the secondary side, giving a progrsssive controlled opening. Carter-brocks used a weighted door to similar results so the motor pulled open the secondary doors that are on top of the mechanical secondaries. You can be WOT on a carter-brock and still be only on the primaries if the motor cant pull the secondary air doors open. Same on a Holley, you can be WOT on a Holley and not have enough velocity through the primaries to generate enough to open the secindaries via the vacuum pot. You can change springs on a holley to tailor the opening, you cant easily change that on a Carter-brock. The AVS had a better system as it used a spring door that had some adjustability built into it but still is not the best choice for a slant. Good news is you can buy venturis for a Carter that will neck down the primary side but the 750 is just too big for a NA slant. Sell it or trade it for a 500 or a 390. velocity is the key and to alter that you need to narrow the primaries for the other circuits to work properly.
 
You can only have one of two types of eddy-AFB carbs,one that runs great
on your engine,or one that is a total disaster.I have seen both.If it is simply a matter of jet/
needle tuning etc. to fine tune one,your good to go,anything more and you can waste hours
of fiddle f***ing trying put a baby elephant into a bushel basket.If the secondary air valve
weights need modded,it's a royal PITA,not a fan.
AVS/Thunder,Quadrajunk,and Thermobog,are all preferred air valve carbs IMO
as an alternative to vac. secondary Holleys. Oh, and yes :D that carb is way to freakin big
LMAO !!! It would look impressive tho', and you could prob. just not supply fuel to the sec-
ondary side, and lock the blades shut lol !! I suppose a 370cfm 2bbl would run fine ! :):)
 
A holley 1920 is like 160-235cfm depending on if you had a 170 or a 225 model. even a 350cfm Holley 2bbl is pretty big unless you are cammed and winding it up. The 390 vacuum can be had with a stiff spring so its secondaries will just barely open, keeping the velocity high and the demand in line with the fuel curve.
 
RustyRatRod said:
I agree 100%.

It's funny though, whenever I throw that formula up here, I get railroaded. lol
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All CFM's are not the same,that is the problem with discussing them,a basic lack
of understanding the differences.There are two main problems, carb. ratings,and engine
demand. Pish threw out those OE carb ratings 1bbl,2bbl,and 4bbl holley together,which
is a misleading cluster of numbers. Most OE & aftermarket 1&2bbl carbs are flow rated at
3.0"Hg,but a couple of OE's were rated @2.0"Hg,4bbls are rated @1.5"Hg,apples,oranges
and pears!Not to mention some OE ratings were "wet" flowed then dry flowed later chang-
ing the rating for the same carb!Add to that a very non-standardized industry-wide test
equipment method and all those numbers from the last 50 yrs. can be more than a little
misleading. Max perf. efforts target no more than .75"Hg intake depression at peak RPM,
(think engine masters challenge),which means it takes a 1050 dominator to supply 750CFM
at .75"Hg. The problem w/that is the tuning becomes critical and difficult,it is not for the
avg. driver/hotrod.
 
BillGrissom said:
Modern motorcycles that are still carbureted (ex. Ninja 250 in U.S.) have "sliders" for that. The Predator was an after-market one for cars. Ford used a variable venturi in early 1990's large cars which worked well but proved problematic.
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I don't know what you consider modern,but my '72 CB/CL based honda custom
hardtail is a 350 w/two carbs that have diaphragm operated slides,twisting the throttle just
controls how far they can lift.You open the blades,and vacuum strength opens the slides.
I'm not sure how far back those were in use on bikes but.....
I actually have one of the Motorcraft VV 2bbl carbs. It needs some TLC,but looks
serviceable,not sure it's worth fooling with tho'. I forget what max CFM(and at what in.Hg)
the're rated at,and I believe i read they are problematic w/any significant cams etc.
 
As killer6 stated there are different standards when measuring cfm ratings of carbs. A 500 CFM Holley 4412 two bbl flows almost exactly the same as a 390 CFM 4bbl.
Believe it or not, the 390 four bbl would be better on a street car. Even though both carbs have the same total CFM, the smaller primary barrels on the 390, will give better throttle response. Theoretcly the 4 bbl should get better fuel mileage, also. At part throttle, the higher air flow velocity in the smaller primarys, willl result in better fuel atomasation(sp). Plus the 390 has a smaller accelorater pump (30 cc as compared to 50 cc) so small movements of the throttle will not shoot as much fuel.
 
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