The Stovebolt Site Forums The Shop Area Engine Shop "Off the shelf" carbs.

When someone buys a carb from a part store (lets say a holly or demon) can they expect to bolt it on and fire it up right away? I would assume a few vacuums will need to be plugged and some minor air and fuel mixture adjustments. Will instructions be included with the carb?

I have a 350 in my 66 and I want to replace the Quadra jet carb that’s on it. I don’t want to jump into a major ordeal without being prepared first. I swapped out a carb on an old vehicle that I had (non GM) without any problems.

I want a descent 4 brr to replace it

Yes they are pretty easy to install & generally come with good instructions. You will have to jugle vacuum lines & fuel lines. The throttle linkage will have to be modified. If you want my opinion I would get the Edelbrock performer. The hollet & demon are o.k. if you are going raceing. The performer works good right out of the box but you can tweek it a little with a jet kit. If you have an old manifold like came on the 66 you may have to modify it or get an adapter. I like the aluminum manifolds. You can get a performance manifold or an aluminum one from a early 80,s G.M. car.

Why replace the Q-Jet? It was engineered to work with the 350, and anything else will just be "different" not necessarily better. The aftermarket carb will be approximately right, IF, and it's a big if, you manage to get one that's close to the right CFM rating, and it's somewhere in the ballpark, jet-wise. If it's not, you'll need a flow bench, and a lot of years of experience to make it work. Most people buy WAAAAAY too big a carburetor, and end up with flat spots, backfires, and a lot of other nasty stuff to deal with. Then they make things worse with a single-plane intake designed to make top-end horsepower only. It's much better to stick with what was designed to work right in the first place.
Jerry

I have an aluminum edlebrok intake already installed. The truck has a 78-80 crate motor in it.
Ill probably go with an edlbrok carb.

My current Q-jet looks a lot older than the crate motor and seems a little flat. Plus the choke is not working. I might have a mechanic look at it first.

I tend to agree with Jerry, I would get the Q-jet rebuilt. I'm totally unimpressed with Edelbrock carbs. They package a disclaimer with each carb about flooding out and that's what they do best. First you need a phenolic spacer as they do not disperse the heat properly - unlike the old AFB they copied. Second they are critical of fuel pump pressure. According to the Edelbrock tech rep we called you should not exceed 5 1/2 lbs pressure so you need a pressure regulator. We purchased an Edelbrock fuel pump with the carburetor and found out it had 7 1/2 lbs pressure. So much for them engineering a balanced system. After returning 2 carburetors we finally got one that would run fairly well. You can go to their facility in Torrance and pick up new carbs that have been returned and "reconditioned" fairly cheap. I have talked with others that say they work just fine. You might get lucky. Fred

As a general rule, the aftermarket carbs are going to be calibrated significantly richer than a stock carb; mainly because many who opt for the A/M carb will have a larger than stock cam as well as headers, and the A/M carb makers don't wish to pay for a flamed engine.

So your new A/M carb is going to fire right up, and probably run fairly strong, assuming you buy one of the proper CFM; however "out of the box" fuel economy (other than the Carter TQ) is going to suffer significantly over stock.

Yes, the Demon and Holley gurus can recalibrate these things; but I can never recall anyone seriously comtemplating fuel economy from either.

You mentioned you have a 1966. What year is the carb. The Q-Jet came out in 1965. 7 nanoseconds after the first one was delivered to a dealer it was nicknamed the quadrajunk (and deservedly so). The early Q-Jets had issues. The 1966 carb corrected one issue. The 1967 corrected a second. The 1967 still has a third issue that the Q-Jet gurus will modify, and the modified 1967 is an excellent carburetor. The 1968 is even better. By 1970 the Q-Jet was (opinion) the finest street carburetor available (until the Carter TQ was released for the street).

As Hotrod Lincoln commented, the aluminum manifolds are another story. Few, if any of the A/M aluminum units, will compete successfully with the stock cast iron manifold IN THE LOWER RPM RANGES. So how often do you have your 350 above 6000 RPM to fully utilize the design of the better aluminum intakes?

Personally, my own 350 (Pontiac) has a Carter TQ on a factory intake (but it may be a little warmer than your crate 350).

For those comtemplating the purchase of an A/M carburetor, the information below is copied from our website, and the sales pitch has been removed. Information only is posted.

Jon.

CARBURETOR SELECTION

When choosing a carburetor for your car, a little homework will often pay large dividends. Consider the following questions:

Is your car a 'numbers matching show car'?
If yes, you are generally restricted to the original carburetor or a duplicate thereof.

2. If no, is your car going to be used for sanctioned racing?

If yes, check with the sanctioning body for their regulations.

If no, is your car going to be based on some factory engine (Example. You have a 1966 Pontiac catalina with a regular fuel 389 and two barrel carburetor. You want more horsepower. Consider upgrading THE ENTIRE ENGINE to 1966 Pontiac catalina premium fuel 4 barrel carburetor specifications, or further to GTO specifications.) By duplicating a factory engine, the engineering has been done for you by the factory. YOU KNOW THE THING WILL WORK!
Assuming you answered no to all of the above, you accept responsibility for doing your own engineering, and the following set of questions are designed to help you make your choice. Please remember that any choice you make will be a compromise. You may choose to maximize horsepower, drivability, fuel economy, changability, appearance or initial cost. Maximizing one category will result in reductions in one or more of the other categories.
Is it legal? The Federal Clean Air Act of 1966 took effect with the 1968 model cars. The Federal Clean Air Act applies to all vehicles licensed in all 50 of the United States. Additionally, some states may have emission requirements more stringent than the Federal level, but the Federal level is the minimum in all 50 states. (1998 addition). It is being reported in many publications that California vehicles through the year 1974 are now exempt from smog emission due to California S.B. 42. THIS IS INCORRECT! Remember government 101 (the state cannot override the federal government). SB42 reduced California's more stringent requirements on 1968 to 1974 vehicles back to the Federal level, and removed emissions on 1966 and 1967 California vehicles.
B. Do you want to use a single carburetor or some form of multiple carburetion?

If you wish to use a single carburetor, the following equation can be used for multi-cylinder 4-stroke engines: CFM = (RPM x CID) / 3456. This equation is familiar to most enthusiasts, but understanding the equation seems to be a different story. Most apply the equation for WOT (wide open throttle) to determine the maximum size of the carburetor, which is good, but only half of the story (unless you plan to drive on the street constantly at WOT). For street use, it is also important to use the equation for your normal cruising RPM, and these that CFM figure for the primary side of your 4 barrel carburetor. This will maximize the air velocity (and the primary efficiency) for your cruising RPM. Remember that too large a venturi means too small air velocity, which means a LEAN condition! We have found that, for modern 8 cylinders of 300 CID or larger, almost without exception, a spread-bore carburetor (small primary, and large variable secondary) will perform best on the street. We have a small quantity of high performance Carter spread-bore aftermarket carburetors (with electric choke). These are part number 9800 and are rated 800 CFM (200 primary plus a variable secondary). Also, consider whether the engine is a large displacement high torque, low RPM; or a smaller displacement high RPM engine; and also the breathing capability of the engine. Chevrolet found the small block to like smaller primary and larger secondary. The 1963 Corvette 327/350 used a 575 (225P/350S) CFM unit. Pontiac found the spread-bore 750 did not do as well on a 455 as an 800 (200P/600S). Chrysler used spreadbore 850 (250P/600S) on their 440 engine.

To help you pick the size carburetor for street use, consider the following Carter aftermarket carburetor sizes:

Square-bore (secondary is variable, based on engine demand)

400 CFM – 200 (P), 200 (S)

500 CFM – 225 (P), 275 (S)

600 CFM – 250 (P), 375 (S) (this is NOT a misprint)

625 CFM – 250 (P), 375 (S)

750 CFM – 375 (P), 375 (S)

950 CFM – 375 (P), 575 (S)

Spread-bore (secondary is variable, based on engine demand)

800 CFM – 200 (P), 600 (S)

850 CFM – 250 (P), 600 (S)

1000 CFM – 400 (P), 600 (S)

MULTIPLE CARBURETOR SET-UPS

Assembling a multiple carburetor street set-up can be a tricky situation. Unless you wish to maximize appearance only, FOR BEST RESULTS, the number of carburetors should be a factor of the number of cylinders ie 2, 4, or 8 carburetors on an 8 cylinder, 3 or 6 carburetors on a 6 cylinder. (Four cylinder engines present their own set of problems - CALL). Before all you Chrysler 6-pack and Pontiac tri-power gearheads get your hackles up, please read further! I have tri-power on my own 1964 GTO because Pontiac put it there to maximize appearance, but a properly selected 4 barrel runs better! I didn't say tri-power won't work. It just doesn't work as well as a 4 barrel. When more than one source (carburetor) feeds a cylinder, turbulence exists at the point of join. This turbulence will cause the air fuel mixture delivered to the cylinders to break down at higher RPM. Please note that when the factorys used three two barrel units, they were primarily installed on relatively low RPM undersquare engines. Please also note that Chrysler used dual quads on their race engines with multiple carburetors, as did Pontiac with their Super Duty race engines.

Once you have determined configuration, you must choose carburetors. FOR BEST RESULTS, use:

original carburetors from a similar multi-carb unit
aftermarket carburetors whose manufacturer offers calibration pieces
carburetors with mechanical power systems (especially important if building older units such as 2 x 2 on a flat-head Ford, etc.)
Your carburetors will probably have to be re-calibrated. Having a ready source of calibration pieces is useful. Modern carburetors utilize vacuum to determine auxilliary fuel flow (power valves, metering rods, etc.) A higher lift camshaft will lower the engine vacuum and require recalibration of the carburetor's power system. Many problems on street systems are a direct result of ignoring the vacuum signal, even when the proper size and type of carburetor are used.

Sizing carburetors for multiple units can be very tricky. Remember that two barrel carbs and four barrel carbs are rated with a different scale. Also remember that carburetors too large for your engine will run lean!

Linkage can be another problem. We recommend street dual quad systems run simultaneous linkage. That is, the throttle controls both carburetors as though there were only one. We suggest using the choke and idle circuits on both carburetors. This may mean fabricating your linkage.

Wow, talk about informative. I will have some questions about my Carters, that are on my 216's. Now I know who to ask.

John

Well, I am running a little more HP on my 67 Camaro, but I bought a new Edelbrock 740 (I think, can't remember now), looks like a take off of the traditional Holley DP, vacuum secondary and I am completely impressed. Car runs 12s and will get me 17 mpg when I take it easy...very impressive for sitting atop a stroker 388...and the choke works great! All I know is I am pleased. Again, I have a pretty hi-output motor. They do offer smaller CFM versions. 600 or 650 would probably be fine. Definately get an aluminum manifold if you go this route. Spacer plates are a pain from my experience and take up space. I have probably done things wrong, but I can never seem to get the gaskets to seal at the same time. Always seem to end up with a vacuum leak using a spacer.