Flipping Manifolds

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.

1929 Chevrolet 194-cubic-inch (3.2L) overhead-valve inline six-cylinder engine, rated at 46 horsepower and approximately 120 lb-ft of torque, matched with three-speed manual transmission
1929 Chevrolet 216 with updraft carburetor
1955 Chevrolet 235 with downdraft carburetor

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.


Carter Brass Bowl Carb – Chevrolet up to 1931

Carter W-1 carburetor – Chevrolet from 1932-1949

Rochester Model B Carburetor – Chevrolet from 1950-1967




Rochester Model B Carburetors

Model B History and Identification

1958 Chevrolet Truck Rochester Model B Carburetor – Restored by The Carburetor Doctor

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)
1st Design Pump – Slotted Stem, Leather Cup
2nd Design Pump – Round Stem, Rubber Cup

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.

Rochester Model B Design Features Summary

1st Design

2nd Design

early late long pump (2 13/16″) short pump (2 3/4″)
years 1950-51 1952-1962 1957-up 1963-1968
pump type slotted stem, leather slotted stem, leather round stem, rubber round stem, rubber
airhorn screws 10-32 12-28 12-28 12-28
choke type manual (B), integral choke (BC) manual (B), integral choke (BC) manual (B), integral choke (BC) manual (B), remote choke (BV)
standard kit CK10 CK10 CK33 CK31
master kit MK10A MK10B MK33 MK31
1st Design Model B Manual Choke Carburetor, 1950-51 Models (10-32 Airhorn Screws), Clamp-On Oil Bath Air Cleaner
2nd Design Automatic Choke Carburetor Model BC With Wing Nut Air Cleaner Attachment (1957-up)
2nd Design Automatic Choke Carburetor, Model BV, 1963-up


Master Kits

Our master kits for Rochester Model B carburetors include the following items:

  • correct airhorn screws and lockwashers
  • complete accelerator pump assembly (ethanol compatible)
  • pump duration spring
  • pump return spring
  • idle mixture screw and spring
  • brass float
  • needle/seat (ethanol compatible)
  • gaskets, clips, seals, check balls
  • mounting gasket
  • instructions with exploded diagram
MK10A: 1st Design, 1950-51, 10-32 screws, leather slotted pump
MK10B: 1st Design, 1952-1962, 12-28 Screws, Leather Slotted Pump
MK31, 2nd Design, 1963-1968, 12-28 Screws, 2 13/16″ Round Stem Pump w/ Viton Cup
MK33: 2nd Design, 1957-1962, 12-28 Screws, 2 3/4″ Round Stem Pump w/ Viton Cup


Standard Kits

Standard kits are an economical way to rebuild a carburetor in good condition. They don’t include floats, springs or screws but all of ours feature:

  • complete accelerator pump assembly (ethanol compatible)
  • brass float
  • needle/seat (ethanol compatible)
  • gaskets, clips, seals, check balls
  • mounting gasket
  • instructions with exploded diagram
CK10 Rebuild Kit for All 1st Design Rochester Model B Carbs
CK31 Rebuild Kit for 1963-1968 Rochester Model B Carbs With 2 13/16″ Long Pump
CK33 Rebuild Kit for Rochester B Carbs With 2 3/4″ Accelerator Pumps


Pull Through Turbo Carbs

The Question…

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.

The Answer…

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.

Pontiac turbo QJet 17080274
Pontiac Turbo Carb – Rochester QJet # 17080274

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.

Stromberg WW Carburetors for GMC Trucks

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.

Stromberg WWC for GMC
Top view of a Stromberg WWC carburetor.
Stromberg WW for GMC
Front view of the smaller Stromberg WW carburetor

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

Year Engine Type GM Part No. Vendor No. Code No. Notes Carb Kit Float
1960 305C V6 WW-2 2391843
CK4491 F77
1960 351 V6 WW-2 2382770
CK4491 F77
1960 401 V6 WWC-3 2382772 380978 23-125 CK738
1960 702 V12 WW-2 2382773 380979 23-126B uses 2
CK49 F77
1962 305C V6
305D V6
WW 2427805 381031 23-133A except
CK4491 F77
1962 305C V6 WW 2427807 381033 23-135A L-4000 CK4491 F77
1962 351 V6 WW 2433601 381038 23-137 except
CK4491 F77
1962 351 V6 WW 2433602 381039 23-138 L-5000 CK4491 F77
1962 351 V6 WW 2433603 381040 23-139 SP-5000 CK4491 F77
1962 401 V6 WW 2433608 381045 23-144 F77
1963 305E V6 WW 2445281 381076 23-151 1000 to
CK4491 F77
1963 305C V6 WW 2445281 381076 23-151 4000 except
L & LH-4000
CK4491 F77
1963 305C V6 WW 2445282 381077 23-152 L & LH-4000 CK4491 F77
1963 351 V6 WW 2433601 381038 23-137 5000 except
L & LH-5000
CK4491 F77
1963 351 V6 WW 2433602 381039 23-138 L & LH-5000 CK4491 F77
1963 351 V6 WW 2433603 381040 23-139 SP-5000 CK4491 F77
1963 351 V6 WW 2433606 381043 23-142 Hydraulic
CK4491 F77
1963 401 V6 WW 2440817 381064 23-147 CK738 F77
1963 478 V6 WW 2440818 381065 23-148 CK738 F77
1963 702 V12 WW 2433607 381044 23-143 uses 2
CK49 F77
305E V6 WW 2453827 381094 23-153A except
CK529 F77
305C V6 WW 2453827 381094 23-153A except
CK529 F77
305C V6 WW 2456769 381103 23-158 L3500
CK529 F77
351 V6 WW 2433601 381038 23-137A except
L & SP series
CK4491 F77
351 V6 WW 2433602 381039 23-138A L4000
CK4491 F77
351 V6 WW 2433603 381040 23-139 SP5000 CK4491 F77
351 V6 WW 2433606 381043 23-142 Hydraulic
CK4491 F77
1964 401 V6 WWC 2440817 381064 23-147 except W5000
1964 401 V6 WWC 2433601 381038 23-137A W5000 CK4491
1964 401 V6 WWC 2441975 381066 23-149 SP5000 CK738
1964 478 V6 WWC 2441975 381066 23-149 SP5000 CK738
1964 478 V6 WWC 2440818 381065 23-148 6500 CK738
1964 702 V12 WW 2433607 381044 23-143 uses 2
CK49 F77
1965 305E V6
305C V6
WW 2472952 381123 23-160 except
L Series
CK50 F77
1965 305E V6
305C V6
WW 2472953 381124 23-161 L3500
CK50 F77
1965 351 V6 WW 2472954 381126 23-163 except
L & SP series
CK526 F77
1965 351 V6 WW 2472955 381127 23-164 L5000 CK526 F77
1965 351 V6 WW 2472956 381128 23-165 SP5000 CK526 F77
1965 351 V6 WW 2472958 381131 23-168 Hydraulic
CK526 F77
1965 401 V6 WWC 2472960 381134 23-171 All H Series CK738
1965 401 V6
478 V6
WWC 2472961 381135 23-172 SP5000 CK738
1966 305C V6 WW 2485214 381164 23-181 HM-5500 CK50 F77
1966 351C V6 WW 2485215 381165 23-182 HM-5500
CK526 F77
1966 351C V6 WW 2472955 381127 23-164 TM-7500 CK526 F77
1966 351M V6 WW 2487147 381168 23-185 RM-7500 CK738 F77
1966 351M V6 WW 2483900 381167 23-184 HM/JM/TM-7500
CK738 F77
1966 401M V6 WW 2483900 381167 23-184 HM/JM/TM-7500
CK738 F77
1966 401M V6 WW 2487147 381168 23-188 RM-7500 CK738 F77
1966 478M V6 WW 2487147 381168 23-185 RM-7500 CK738 F77
1966 478M V6 WW 2483900 381167 23-184 HM/JM/TM/WM-8500 CK738 F77
1966 637 V8 WW 2492166 381177 ~ All-9500 F77

The above table was adapted from the excellent 60-66 GMC Truck Site at http://6066gmcguy.com


Correct Size AFB for a Stock Chevy 350

Edelbrock AFB Carburetor
Carter Competition AFB


The Question…

How would a stock 290 hp 350 handle a 650 CFM carb? Would it be to much fuel?

The Answer…


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:

CK294 Carter and Edelbrock AFB kit
Carburetor Rebuild Kit for Edelbrock and Carter Performance AFB Carburetors


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.

Holley 885FFC cfm

Holley 885FFC carburetor for 1949-51 Mercury

The question…


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.

The answer…

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:

CK409 carburetor rebuild kit for Holley 885FFC
CK409 Carburetor Rebuild Kit for Holley 885FFC Carburetors

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.

1949-51 Holley 885FFC specifications

I have also posted the specification sheets from the Holley carburetor manual for these carburetors on The Old Car Manual Project site.