The Carburetor Shop LLC
204 East 15th Street
Eldon, Missouri 65026
Aftermarket updraft carburetors
We receive a lot of questions concerning replacing original carburetors on older vehicles. There are many reasons for this; but the three main reasons seem to be: (A) incompatibility of the “automatic” designs (especially airvalve) carburetors with the lower energy content of modern fuel; (B) the cost of repairing some of the older designs; and (C) the fact that many of the early designs were not as good as others. This article will address in detail the various updraft designs that we sell and or service; and in lesser detail those designs that we will offer parts but will NOT either sell or service.
In selection of an updraft carburetor; one has the normal task of selecting the proper size carburetor, but also in determining what type of fuel delivery system is to be used. MANY updraft carburetors were supplied with gravity feed; thus requiring the installer to fabricate a much larger fuel valve for the carburetor than one designed to operate with a pressure fuel pump. Two examples:
1928 Chevrolet 4 cylinder Carter carb with 0.140 inch inlet orifice.
1972 Cadillac El Dorado with 500 CID engine 0.132 inch inlet orifice
Note that the little 4 cylinder Chevy, with gravity feed, requires a larger orifice than the mighty 500 CID Caddy. It is IMPERATIVE to know what type fuel system will be used; and modify the float system of the carburetor accordingly. Gravity or pressure; the carburetor can be set for either (within limits) but NEVER both. The within limits in the preceding statement refers to the amount of pressure to be used. The mechanical advantage and buoyancy of the float, combined with the orifice of the fuel valve, will determine the maximum pressure that may be used by the carburetor.
As to the selection of the size of the carburetor; many mistakenly simply use what they have available, or what will bolt on with no adapter. This procedure, unless the installer is extremely lucky, will result in less than optimal performance, and generally doesn’t work. Many of the early designs were horribly inefficient, and often a much SMALLER carburetor will perform much better. A good example of this is the Marvel carburetors, where the airvalve restricts the throttle by more than 50 percent! Another issue that many fail to consider is that most of these carburetors have a number of different internal venturi sizes available. Thus two carburetors that look very similar externally may vary as much as 50 percent or more internal airflow.
Carter produced the BB updraft series carburetor beginning with the 1932 model year. These were originally sold to Chrysler Corporation for use on Plymouth, De Soto, and Chrysler engines. However, these carbs were so far advanced that many mechanics would substitute these units on older vehicles that were daily drivers in this era. Carter recognized this fact, and came out with 3 different models of the BB-1 specifically for the aftermarket. These are: 245s (S.A.E. size 1); BB1A (S.A.E. size 2); and 289s (S.A.E. size 3). These models featured cast iron bodies, extended throttle shafts with universal clamp-type lever, a fast idle circuit, an externally adjustable main metering jet, an accelerator pump, an adjustable idle circuit, and a power circuit. These carburetors are fairly compact, and will fit a few applications where other universals will not fit. These units have been in favor for many years with many who participate in the Great American Race, which has significantly driven up the price. Rebuilding kits are readily available, and reasonable. Many other parts are available, but not inexpensive. In all, Carter produced 69 DIFFERENT versions of the BB updraft carburetor; MOST of which were sold as original equipment on trucks. Because of the high price of the aftermarket units; some enthusiasts are trying to make the truck units work. The truck units may be converted; but the following part (not commercially available) must be fabricated: (A) extended throttle shaft, (B) universal throttle arm, (C) adjustable main metering jet, (D) fast idle parts (if the installer wishes the fast idle to function).
These (opinion) are really excellent carburetors. My only complaint (other than the artificially high price) is the mechanical accelerator pump. Accelerator pumps on updraft carburetors come in two types: (A) mechanical, and (B) vacuum. The mechanical pump is attached directly to the throttle and pumps fuel if the throttle is moved. This can create a fire hazard if a novice attempts to start the vehicle, and like with a downdraft, pumps the footfeed several times. This will result in fuel being pumped out of the air intake onto the ground under the engine. The vacuum accelerator pump has a vacuum piston, which is pulled, into a vacuum chamber by engine vacuum after the engine starts. This action charges a heavy spring. When the engine is accelerated, and vacuum is reduced, the energy stored in the spring drives the pump and pumps the fuel. With a vacuum pump, working the footfeed with the engine off results only in the exercise of the operator’s ankle. And vehicles with updraft carburetors should always be started via the use of the choke NOT a pump.
Stromberg produced several series of universal updraft carburetors, including the UV series, the UR series, the SF series, and the SFM series. We will discuss only the SF and SFM series. The SF series were cast iron carburetors produced as universals, for stationary engines, for trucks, and for agricultural applications. There are hundreds of different models. The universal carburetors have all of the circuits described for the Carter BB updraft except the fast idle circuit. Most of the truck carburetors also have these features except for the adjustable main metering jet. Virtually all of the agricultural and stationary engine versions have no adjustable jet, no accelerator pump, and no power circuit. Machining costs to convert the agricultural and stationary engine versions is sufficiently great as to render them (other than original application) useless except for parts. The SFM series is simply a marine version of the SF series. Most have pumps, some have adjustable main metering jets, and most have power circuits. The SF and SFM series were produced in 5 sizes (S.A.E. series 1 through 5). For a better understanding of S.A.E. standard sizes see http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm The SF and SFM series carbs have replaceable venturi. Again, two carburetors, which appear identical externally, may be radically different in airflow. The accelerator pump in these carburetors is vacuum, thus eliminating the fire hazard discussed with the Carter BB updraft.
Of the universal carbs out there, the Stromberg SF and SFM series, along with the Zenith 63 and 263 series, are my choice. They have only two drawbacks: (A) weight (they are heavier than the Carter and the newer light-duty Zenith units), and the cost of parts. Repair kits, while readily available, are quite expensive. However, the carbs are ultra-reliable, and should, with fuel filter maintenance, last virtually indefinitely, thus few parts are ever needed.
The Stromberg UV series are not being discussed as they are constructed from the early zinc alloy, which falls to pieces. The Stromberg UR series are not being discussed as the vast majority of these were for small engine.
Zenith has made a mind-boggling assortment of models over the years. We will discuss the 63 series, 263 series cast iron series; and the newer 68 and 267 zinc alloy series. Both Stromberg and Zenith were divisions of Bendix Aviation. The same engineers did much of the engineering for both divisions. The 63 series is very similar to the Stromberg SF series, with the primary exception being that Zenith used a brass accelerator pump rather than the leather accelerator pump used by Stromberg. As the brass pumps do not normally need changing, they are not included in the rebuilding kits; thus the rebuilding kits for Zenith are less expensive. Similarly to the Stromberg SF and SFM series, the Zenith 263 is the marine version of the Zenith 263 series. Both the 63 and 263 series were produced in sizes from S.A.E. size 1 to 5.
Zenith is currently producing both the 68 and 267 series zinc alloy carburetors. These carburetors, originally designed for agricultural use, may be used on many low compression early vehicles. These carburetors can be ordered with the externally adjustable main metering jet; but do not have an accelerator pump, a power circuit, or a fast idle circuit. Of course, virtually no carburetor produced before 1925 (any many after) had these features; thus many hobbyists opt for the lower cost of these units. As they are current production, rebuilding kits are readily available (often locally), and quite reasonable in price. Unfortunately, these units are not available in sizes larger than S.A.E. size 2.
There have been a number of other makes of replacement carburetors. Some of the better-known makes would be Johnson, Marvel-Schebler, Schebler, Tillotson and Winfield). We will NOT service any of these replacement carburetors; although we do offer rebuilding parts for the Scheblers and most of the Tillotsons. We will service the Marvel-Schebler agricultural original equipment carburetors. A note on Winfield carbs. These are rotary throttle carbs, and were basically designed for all-out racing. I will admit a dislike for any rotary throttle carbs; but we have been able to make Stromberg and Zenith carbs of the same period outperform the Winfields for racing, and rotary throttle carbs have severe drivability issues for all except wide open throttle use. Both Stromberg and Zenith DID offer racing carbs in the day; however, they did not have Mr. Winfield, a highly successful and respected racing driver, as a spokesperson. Winfield carbs were basically assembled at the point of sale. The selling dealer had an assortment of castings, shafts, floats, jets, etc. and a catalogue. The customer would specify and engine, the dealer would look in the catalogue, and assemble a carburetor, which supposedly was calibrated for racing that engine.
Should you wish help in selecting a replacement carburetor, we will be glad to offer our advice. We stock the Zenith 68 and 267 series carbs. We also acquire, and restore, the Carter BB updraft, the Stromberg SF and SFM, and the Zenith 63 and 263 carburetors. We normally have a decent selection of all sizes for S.A.E. 1 through S.A.E. 5. We can modify the float system for your type of fuel delivery system, and determine and furnish the proper internal venturi size. We also make a large selection of adapters (necessary when changing flange sizes, or converting a cross-flange application to a straight-flange carburetor), and will discuss with you the description of adapters if you have access to machine tools. 573-392-7378 (9-4 Mon-Wed central time).