CHOOSING THE RIGHT CARBURETOR
FOR YOU & YOUR MG
- Tim Baxter

As of:  16 April  2006 


        Contrary to what a lot of folks will tell you, there is no ďbestĒ carb for the MG.  Quite a few very different carbs are available for our MGs, and which one is ďbestĒ really comes down to what you want out of your particular MG. 

        Armed with some knowledge about what the differences  are among the various carb choices, you should pick the one that best suits your needs.  So, let's see wht's out there:

Zenith Stromberg (ZS):

        We'll get this one out of the way quick because - frankly - itís probably the worst carb you could put on an MG.  Fitted stock to í75-on rubber bumper MG's, the Zenith Stromberg setup in the B had problems from day one.

        The problem isnít necessarily the ZS carb itself (although the ZS has several weaknesses in comparison to the alternatives) but rather with its application on the MG.  Itís too small to effectively supply the amount of air the B-series engine wants; and the combination intake and exhaust manifold strangles it even further . This combination resulted in a 30-horsepower drop over earlier MG's.   In addition, the ZS is probably the least reliable carb setup available for an MG B.  It can be made to work, and work reliably, but itís always at a disadvantage.

        This setup is only desirable if youíre adamant about originality, your local laws wonít allow you to change your fuel/intake system, or youíre out to prove something.

Note:   Any change from the ZS to another carb requires either an intake and exhaust manifold from an early MG B or a matched intake manifold and set of headers for the carb  you choose; unless you want to make major (and pointless) modifications to the horrible stock combination manifold.

SU:

        First, a brief explanation of the various types of SU's that were fitted to MG's is in order.   SU named their carbs according to the size of the throat, in eighths of an inch over one inch.  So, an HS2 would be two-eighths of an inch over one inch, or a 1- 1/4Ē throat; thus, an HS4 would be four-eighths, or 1- Ĺ inches.  MGB's were usually fitted with HS4's.
  The " H" stands for horizontal, as all SU's mounted on MG's are side draft designs.  

        There were also two different types of  SU's fitted to MG's:  HS (with an external float beside the carb) and HIF (integral float).  Although there are detail differences, both HS4 and HIF4 SU's work  pretty much the same.  For most people, twin SU's are the carb of choice on an MG B, and for good reason:   SU's look the part and offer a tough-to-beat combination of performance, reliability, and originality.

        Theyíre not perfect, though.

        Heavily built-up engines will eventually overtax the SU's, requiring more air and/or fuel than they can provide.  Worn  SU's are a nightmare, also.  The throttle shafts begin to leak air, and theyíre virtually untuneable.  SU's also require the owner to check the level of oil in the dashpot from time to time, inspect the linkages, and generally give them a once-over to keep everything in top-notch condition.  They can also be difficult to fit on rubber-bumper MGB's without fouling the brake booster.  A good set of SU's will also usually (but not always) cost more than other carb options.  Still, for the vast majority of owners, SU's probably provide the best all-around carb choice for an MGB.

        Larger SU's, in particular HS6's (thatís a 1.75 inch throat if you donít want to do the math) are available for the MG B, although theyíre only necessary (or desirable) on an engine thatís overpowering the standard SU's.   Theyíre not cheap, though, and you must have the appropriate intake manifold to use them.

Weber's:

We'll separate the various Weber carburetors into types, as they are very different in operation and design.

Weber DGV:  


 

For years the DGV was sold as a performance replacement carb for the MG. Itís not. While it will produce almost as much power as a pair of SUs, itís   much less sportscar-like about doing it. Actually, DGV clones were sold on   about a gazillion different cars from the factory, not because of their performance,   but for their other attributes: good economy and rock solid reliability.

The reason itís not really a performance carb has to do with its   basic design. The common 32/36 DGV downdraft carb has two barrels, a 32mm barrel     and a secondary, 36mm barrel (which explains the 32/36 designation). An SUís   1 1/2 inch barrel equals 37.5mm, so both of the DGV throats are smaller than     a standard SU. The 32 and 36mm throats together, though, are considerably   bigger than the single ZS.

Itís not just throat size, though. Under  partial throttle, only the smaller throat of a DGV is open. This contributes  to itís excellent economy,     but does nothing for the thrill of the ride. In fact until the secondary   opens, DGVs are a bit sluggish.
They do have many virtues, though. They are a beautifully easy swap on any   MG, even those with a brake booster. They can be set up in no time flat (many  work beautifully right out of the box) and once theyíre set up, youíll   never touch it again. They stay in tune, donít wear out, and are one  less thing to tinker with as long as theyíre on the car.     

 

Thereís actually several different types of DGVs available, differing   primarily in their choke type. The standard DGV uses a manual choke. The DGEV   has an electric choke, and the DGAV uses a water choke. Choose the kind that   suits you. Personally, I think the electric choke is ideal. It works flawlessly   every time, and you never touch the choke again either. DGVs are ideal if you   just want to drive the car and not worry about your carburetion, if economy   is a concern, or if cost/ease if fitment in a conversion is important.

A 38/38 DGS downdraft is now being offered for MGBs as well, which offers   throat sizes equal to the SUs and synchronous operation of both throats. In   theory, it promises the best of all worlds, but field reports are scarce. Firsthand   information is welcomed.

DCOE:

Weberís sidedraft DCOE has become legendary, but like many legends itís   largely misunderstood. While the DCOE has powered all types of race cars, from   tiny 1000cc screamers to big honking V8s, itís not necessarily a performance/racing   carburetor.

 

The DCOEís great strength, and great weakness, is that it can be infinitely   customized. Every part of it can be swapped or adjusted to achieve the desired   results. You can optimize torque, horsepower, economy, or any combination of   them.

 The downside is that it can require a lot of know-how and experience to do  so. A DCOE may meet your requirements right out of the box, or it may not,  depending on exactly what you want it to do.

 From a design standpoint, the DCOE is somewhat like two single throat sidedraft   carburetors sharing a single float bowl and throttle shaft. From that standpoint,    they share some similarities with SUs. Most commonly a 45 DCOE is offered  for MGBs, but the smaller 40DCOE is sometimes found. The 40 and the 45    DCOE share       most parts, but the primary difference is in the venturis. DCOEs allow  you to set the actual venturi size, something DGVs and SUs canít do, and  the 40 will accept very venturis, while the 45 while take very large ones.   As a practical matter it doesnít make much difference which kind you  have on an MGB, as most B motors want a 34 or 36mm venture, which either type  will comfortably handle.

 While properly setting up a DCOE and all of its varied tubes and jets can   be a daunting task, it does have several advantages over other carbs.  Like the  DGV, once itís set up, itís 100-percent reliable and requires little  or no attention at all. And because of its modular nature, it can grow with your motor. With one set of jets and chokes, it can provide trouble free motoring  on a stock motor. With another set, it can fully exploit the hottest B motors  long after a set of SUs have been pushed past their limits.

 Itís also a bit better made than SUs: The throttle shaft rides in bearings,  so wear is virtually non-existent, and the overall casting and design is a  masterpiece of smart engineering.

  Clones:

A brief mention of the clones is in order. These are carbs that occasionally  turn up on MGs or could be used on an MG with ease. I say clones because all   tend to follow one of the preceding designs very closely, and could be matched   to a readily available manifold with little trouble.

 The most common SU clone is the Mikuni, built under a license from SU and usually fitted to older Japanese cars. Theyíre not often found on MGs, and when  they are itís usually the result of some junkyard engineering, but theyíre  almost identical to standard SUs and can work quite well.

  The Holley 5200 is almost identical to the DGV (and even closer to the   Weber DFV), and was used on a ton of 4 and six-cylinder Dodge and Ford products,  among other things. Be careful with these Ė the castings are reportedly   inferior to the Weber part, and some have much smaller throats than the 32/36. Solex 2-barrel downdrafts have also appeared on MGs occasionally.

 The DCOE has tons of clones, too many to list quickly. Many  can share most parts with the original.     

 

Again, the point is that there is no ďbestĒ carb. Think about   what you want out of your MG. How you drive it, how much you like to tinker   with it, what kind of economy you want, whether you like a torquey motor or   more on the top end, and make your choice accordingly. All of them, properly set up, work well.

Ultimately, itís your MG, and your decision. Make it a wise and informed decision