Back in the day when downdraft carburetors first came out, the hot trick was to flip your old updraft intake manifold upside down so you could fit one of the new-fangled downdrafts. On many models the manifolds – which were simpler then – were the same back to front and up and down, or nearly so, so this could be accomplished fairly easily. Once in a while we still see this today. Sometimes this was done much later in the life of the car because the old updraft carb was worn out and no replacement was available. Depending on the engine model, the main drawback would be the loss of exhaust heat for the intake, which would make cold operation more difficult.
Most manufacturers switched to downdraft carburetors around 1932. Chevrolet used a Carter Brass Bowl updraft carburetor up to 1931 and switched to a Carter W-1 downdraft in 1932. These were used through to 1949, when GM came out with it’s own carburetor, the Rochester Model B – shown above on a ‘55 Chevy.
Starting in 1949, the Rochester Products Division of General Motors produced carburetors for GM products and other makes as well. The first Rochester carburetor was the AA two barrel used on the new Oldsmobile Rocket V-8 of 1949, followed by the model B one barrel in 1950 on Chevrolets. Model B’s were used from 1950 to 1967 on GM 4 and 6 cylinder applications. They were also sold as replacement carburetors for 1932 and up Chevrolets originally equipped with Carter W-1 or Stromberg BX series carburetors.
Model B carbs are named according to the type of choke they used:
B: manual choke carburetor
BC: automatic choke, choke coil mounted on the side of the carburetor (1950-1962)
BV: automatic choke, choke coil on the intake manifold (1963-1967)
Model B Designs
Internally, there are two main designs: the 1st design with a non-rebuildable leather pump that has a flat stem, used on cars up to 1956 and on trucks up to 1962, and the 2nd design with a rebuildable pump with a rubber cup and a round stem, used on cars from 1957 and on trucks after 1962. To complicate things there have been leather pump kits sold for 2nd design carbs, but still with the round stem. Also, the 2nd design, round stem pumps, come in two different lengths, 2 3/4″ and 2 13/16″. We have both 2nd design pumps available in our kits. Cheaper kits provide just the pump cup. We don’t sell these in our store since the pumps themselves are often worn or damaged – changing the cup won’t help in those cases. Neither will it help if you have one of the non-rebuildable leather replacement cups that were sold at one time.
After two years of production the 1st design carburetors were revised to deal with leaks from the top gasket between the float bowl and airhorn. Starting in 1952, the airhorn casting is heavier and is retained with larger 12-28 Fillister head screws until the end of production in 1967. Today these screws are nearly obsolete, so replacing chewed up old screws has been problematic. To fix this, we’ve introduced a line of master kits with the correct screws and other hard-to-find parts to service all Model B carbs.
Hi, I have a “built” Corvair 180 hp turbo. It has a Crown turbo and no muffler. I’m currently running a Rochester QJet on an Upflow adapter. It runs rich at idle, and has a miserable lean hesitation upon acceleration. It’s a brand new Carter, or Edelbrock QJet clone, or something.
Anyways, it’s WAY too much carb, so I bought an adapter for a WW Stromberg.
What do you reccomend? This is a strictly street driven show car.
As a rule, you do need a lot smaller carb when using a turbo. The WW would probably be in the ball park, but it’s an oddball that it’s hard to get parts for. The issue with pull through turbos is that the vacuum signals are mixed up relative to a naturally aspirated application. At wide open throttle (WOT) in a regular set up, vacuum is low and the power circuit is fully open. With a turbo, vacuum is high at WOT, so that the power circuit closes. That’s why you’ve got a lean hesitation. The power circuit needs to work opposite to that of a non-turbo. The solution is to reference the power circuit to another place in the plenum where vacuum is ‘normal’, which would be after the turbo. This is how it’s done in the Pontiac 301 turbo carb. The vacuum for the power piston is taken not from under the carb, but from after the turbo. It’s actually a little more complicated because the vacuum is modulated by the PEVR – Power Enrichment Valve Regulator, but the point is that manifold vacuum is the opposite of what you would expect for a non-turbo set up.
For the Corvair, you might consider digging up a 301 carb and trying that out. The primary side of the QJet is pretty small, so it might work, as long as you set the linkage up right. One trick is to rig the QJet air valve to only open about halfway, limiting the cfm at WOT.
As an aside, Stromberg WWs were used with some supercharged Studebakers, but this was a blow-through, not pull-through, set up. As long as the entire carb is pressurized, the power system works the same way as on a normally aspirated system. However, the carb either has to be sealed (gas will come out everywhere it’s not) or be in an air box. The Studebaker Golden Hawk used an air box on top of a Stromberg WW carburetor in 1957-58.
The vacuum port on the Pontiac turbo carb that supplies vacuum to the power piston is under the fuel inlet fitting – you can’t see it in the picture. You can tell regular Pontiac 301 QJets from the turbo versions by the extra vacuum port under the fuel inlet. Turbo carbs have three fittings on the front of the throttle body, non-turbos have only two.
Here’s an excerpt from the 1980 New Product Info from Rochester Products Division:
The 4.9 liter Turbocharger V8 application uses a unique power system due to Turbocharger operation. The power system provides extra mixture enrichment for heavy acceleration or high speed operation.
The vacuum power enrichment system consists of a spring-loaded power piston operated by a remote vacuum source. The power piston is controlled by a Power Enrichment Valve which supplies vacuum, according to engine load, to the power piston to position the main metering rods in the jets for sensitive control of air/fuel ratios for power requirements.
The Power System operates as follows:
During part throttle and cruising ranges, engine load is light and vacuum, from the Power Enrichment Valve, is sufficient to hold the power piston down against spring tension and the larger diameter of the metering rod tip is held in the metering jet for leaner mixtures.
As engine load is increased to a point where extra fuel enrichment is required and the intake manifold is pressurized by the exhaust gas driven Turbocharger, the Power Enrichment Valve “switches” and reduces vacuum to the power piston to zero. At this point, spring tension operating on the power piston lifts the main metering rods for increased fuel delivery.
The remote power enrichment feature, through the power enrichment valve, provides richer mixtures during heavy engine loads and wide-open throttle power requirements when the intake manifold is pressurized by the exhaust gas driven Turbocharger at a time when manifold vacuum is high enough tending to operate the power piston in the normally “lean” position. In this way, the power system controls fuel metering during light and heavy power requirements.
The 17080274 carb is the one year where a non-computer controlled carburetor was used on the 301 Turbo in the US. Canadian production used a non-computer carb in 1981 as well, but US cars used an E4ME computer controlled QJet.
We have rebuilding parts for the Pontiac turbo carbs including the electric choke thermostat – a Carburetor Doctor exclusive – available here.
Until 1966 GMC light trucks used mostly GMC engines; after that they shared drive trains with Chevrolet. Medium and heavy duty GMC trucks used their own gasoline engines until 1974.
In the 1960s and 70s these were mostly V6 engines ranging in size from 305 CID to 478 CID; there was also a 503 CID straight 6, a 637 CID V8 and a 702 CID V12. All of the V6 and V12 engines used Stromberg WW or WWC carburetors; the V12 used two of them. We have complete listings of GMC carburetor kits and parts here.
The Stromberg WW-series came in two main variations, the WW and WWC. The WW is the smaller of the two, and is often mistaken for a Carter BBD carburetor. In fact, Stromberg WWs and Carter BBDs were used interchangeably on Mopar applications.
One quick way to tell a WW carburetor is that it has a large accelerator pump lever across the top of the carb. WWC carbs don’t have this lever.
There are many versions of these carbs that look similar, so it’s essential to check the carb number when ordering a kit. The carb number appears on these in a couple of different places. As shown in the pictures, it’s often on a round tag that looks like a washer on the top of the carb. This will usually have the vendor number shown in the table below. Sometimes there is a conventional tag with a GMC number. In many cases there is a ‘Code number’ stamped onto the top part of the carburetor – on the flat area on the float bowl cover.
All WW carburetors use our F77 float, listed here. Unfortunately, there aren’t any new floats for the larger WWC carburetor.
Stromberg WW/WWC Carburetors for 1960-66 GMC Applications
How would a stock 290 hp 350 handle a 650 CFM carb? Would it be to much fuel?
No, the usual size on a stock engine like that is the 625-650 carburetor. (Carter calls it 625, Edelbrock says 650 – same carb). In fact, CFM refers to the maximum Cubic Feet per Minute of air/fuel mixture that a carburetor will flow. In other words, when the engine is operating at high speed at Wide Open Throttle (WOT), how much will the carburetor allow the engine to breathe. So, it is not that too big a carb (too many CFM) will provide too much fuel, rather that it will provide too much air and fuel. Now, if a carburetor provides too much fuel for the amount of air, then it will run too rich, but that’s not caused directly by the carburetor being too big.
As for the common Carter Competition AFBs (made up to 1984 by Carter and sold up to about 1998 made by Weber) and the current Edelbrock AFB, the primary side is the same size in the Carter 625/Edelbrock 650 as in the 500 cfm version so there wouldn’t be any difference in part throttle driveability between the 500 and 625. Maximum air flow for a stock engine like a 290 HP Chevy 350 is less than 500 cfm; if the secondary on the carb was a plain venturi type, you would have to use a small carb. However, on the AFBs there is a velocity valve which limits the secondary air flow so that you can use a slightly larger carb. It allows the 625 carb to work on a range of engine sizes. That’s why Edelbrock sells the 625/650 cfm carb for 350s in general.
By the way, the velocity valve is the counter weighted flapper below the secondary venturi clusters in many AFB carbs. Some, mostly smaller AFBs, didn’t use a velocity valve. The AVS carburetor – both the Carter AVS from the 1960s to the early 1970s and the new Edlebrock Thunder AVS – use a valve above the venturis called an air valve, giving more carefully controlled airflow on the secondary side. This will be for another post.
At The Carburetor Doctor we’ve got a full range of rebuild kits for all Carter AFB carburetors here.
Edelbrock AFBs and Carter Competition AFBs from 400 to 750 cfm use our kit CK294 and F27:
Our CK294 kit features a complete ethanol-resistant Viton accelerator pump assembly (not just the cup!), needles/seats with the solid needle and optional spring-loaded needle for off-road use as well as quality gaskets, instructions and other parts as shown.
I have a 1951 Lincoln, with a 337 Ci Flathead, and am interested in rebuilding the original carb, a Holley 885 FFC
Before I do this, I am interested in knowing the CFM of said carb. It has been suggested that I have a 500-600 cfm for that size of engine, which is 100 % stock
Can you help me out with the CFM.
As far as I know, there aren’t any good published cfm figures for this carburetor. Even if there were, it’s difficult to compare 2 bbl figures to the numbers for common 4 bbl carburetors, since the standard pressure drop for measuring cfm is different for 2 bbls and 4 bbls. So… you can compare 4 bbl numbers to each other, and 2 bbl numbers to each other, but cross comparison is harder.
Having said that, the 885 FFC is about the same size as later model 2300 350-500 cfm Holley 2bbls.
The Holley 885 FFC is the version of the 885 used on 1949-51 Mercury engines and Lincolns. There are other 885s, such as the 885 FFG that was used on trucks for a number of years. More details, along with carb kits and parts are on our site here.
If you’re rebuilding one of these carburetors, you’ll want our kit CK409:
This kit is made in the USA, ethanol-compatible and features hard-to-find parts such as the idle mixture screws and springs, float bumper spring and more. It also includes instructions and an exploded diagram. We also have a detailed factory service manual – CM409.
I have also posted the specification sheets from the Holley carburetor manual for these carburetors on The Old Car Manual Project site.