Any carb will need some adjustment. I have used Edelbrock 1406 (600cfm) and 1906 (650cfm) on a few motors. They were both basically plug & play with slight fuel and idle adjustments.
Timing light with rpm and a vacuum gauge and a screw driver is all you need to adjust them
Just went through this. Called Holley tech line and they were a great help. They have "too" many choices. If my rebuild on my 750 DP Demon does not work after 30 years properly, I will go with a Holly 750 DP on my 427. I have another with a 770 Avenger that performed right out of the box. Give them a call.
If you're running a factory intake you'll need a spread bore carb. That starts limiting
your choices fast. I used a Holley vacuum secondary spread bore carb and had all
kinds of problems. Turned out to be a factory defect. Holley was of no help so I ended
up with an Edlebrock 1906 AVS2. It required an adapter to use on my cast iron spread
bore manifold. So far I've been very pleased with it. Minor adjustments were required
to start with. I've gone a bit further with it working on better overall performance and
mileage but as far as "off the shelf" it worked just fine.
And as Allan and Ed have said your spread bore, square cornered, manifold will require a spread bore carb. I have used the 650 Holley dirrect replacement carb in place of the QuadraJet many time without issue. Holley does make the occasional bad carb (they do make millions of them so statistically there will be a bad one now and then); but they usually replace it after you call and explain what is wrong with it.
I don't sell Holley products, and no longer own stock in the company (Colt industries, the gun manufacture), but I have been depending upon Holley carburetors for a half century to make a good product. Holley is popular with racers not because it makes any additional power compared to any other brand. A 650 cfm Holley makes the same power on a dyno as a 650 Carter AFB (Edelbrock) or a Rochester Quadrajet, The preference racers have for a Holley is the ease in completely being able to tune it at the track with a screw driver. You can not say that for a Rochester QuadraJet.
If you have to tune the carb after you install it then one of two things are different from what the carburetor company was expecting compared to the rest of the world. Either you live on top of a mountain (3,200 to 4,500 feet up will require a retune); or you have a cam that is adversely affecting idle quality. With a stock engine and you don't live at high elevation then the carb will work just fine out of the box assuming you bought the correct size carb for your application.
It is sometimes hard to pick a carb. The Z/28 engine only displaces 302 cubes yet it has an 850 from the factory. The LS-6 454 shipped with only a 750 cfm carb? Carb size is based upon intended usage. The factory built the Z to be a race car that twists routinely to 8,200 RPM. The 454 was a street bruiser that rarely saw beyond 5,800 RPM Though bigger it didn't move that much more air compared tho the smaller 302 running flat out.
If you own a Z and don't want it to fall flat on its face every time you hit the gas it will need a smaller 650 cab for street driving. You can bolt on the 850 when you get to the track. A 350 should be happiest on the street with a 650 cfm carb. If you don;t want to buy a a bigger carb for the track, then a 750 is a good compromise; but it is just that a compromise.
It is sometimes hard to pick a carb. The Z/28 engine only displaces 302 cubes yet it has an 850 from the factory. The LS-6 454 shipped with only a 750 cfm carb? Carb size is based upon intended usage. The factory built the Z to be a race car that twists routinely to 8,200 RPM. The 454 was a street bruiser that rarely saw beyond 5,800 RPM Though bigger it didn't move that much more air compared tho the smaller 302 running flat out.
If you own a Z and don't want it to fall flat on its face every time you hit the gas it will need a smaller 650 cab for street driving. You can bolt on the 850 when you get to the track. A 350 should be happiest on the street with a 650 cfm carb. If you don;t want to buy a a bigger carb for the track, then a 750 is a good compromise; but it is just that a compromise.
Good comments. Just for the record, the 1969 Z/28 had a780 CFM dual-feed with vacuum secondaries, but your points are well taken. I swapped my 780 for a 670 vac secondary Holley making the car more streetable.
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