VW Carburetors 101: Carb Options and Selection
VW engines are notoriously under carbureted by nature of the original design. A stock VW carburetor is proportionally small in relation to VW engine size when compared to most (if not all) other car makes. Your first consideration should be to note how many cylinders each carburetor throat is “feeding”. If it’s only feeding one cylinder, you have complete control of tunability since you can adjust the air/fuel mixture perfectly for each cylinder. Carb throats that feed multiple cylinders (the case in stock engines) have to be adjusted to a compromise setting, and the result is that all the fed cylinders run at a less than optimal air/fuel mixture. This is one reason why dual carbs provide better mileage than centermounted carb kits (including stock) assuming they are driven similarly.
Jetting for Centermount v. Dual Carburetor Systems
A key difference to note between a centermount application and a dual carb set-up using the “same” carburetor is that, by comparison, the carb in the centermount is jetted slightly richer on the idle circuit. This is because of fuel condensation issues. The gasoline has a hard time staying as a vapor on the long run to the cylinders in a centermount set-up. You must take into account that carburetor jetting requirements differ slightly for dual and single applications when dialing in your set-up. Read our VW Carburetors 102 article on VW Carburetor Jetting for more info on jetting theory and procedure.
Carburetor System Options
When faced with deciding on the type of carburetor set-up you should run, most people could use some clear-cut details and general clarification. Choices include:
- Stock (Solex)
- Stock Replacement (Solex 30/31, and Weber)
- Progressive (Weber DCAV, DCEV – “E” stands for electric choke)
- Non-Progressive (Weber IDF, DCNF or Dellorto DRLA)
Dual 1BBL (Kadron, Solex, Weber ICT, Dellorto FRD)
Dual 2BBL (Weber IDF, IDA, and DCNF, Dellorto DRLA, and Solex 40P11)
Centermount 1 Barrel Systems
STOCK: As you might expect, a stock carburetor will make your vehicle perform as it did when new, assuming that the rest of your engine is also up to the task or in almost new condition. Many vehicles have carburetors that are simply worn out, that have been ignored to a point far beyond what could remotely be considered “acceptable condition” by reasonable standards. Wear commonly results in a vacuum leak at the throttle shaft bushing (air sucks in around the worn bushing). Quality machine shops like RIMCO can repair this condition, at a reasonable price, but some carburetors (specifically the 34 PICT) just don’t rebuild well, and you are better off to replace them. Stock carburetors are still available new from many vendors, and the installation of a new carburetor on a well-maintained engine can transform your bucking vehicle into a smooth operator.
STOCK REPLACEMENT: As you probably guessed from the name, replacement carburetors are not original equipment. But, they are still appropriate for stock-type use. The Solex 30/31 is one of these carbs. The Solex 30/31 is jetted very lean, and often require a main jet upgrade to get them to run properly. Quality control for replacements is not as good as that which was on the original Solex carbs, so you take your chances and should expect a lemon to pop up now and again. Make sure you deal with a vendor who will stand behind their products in case you get one of the lemons…
Weber also made a replacement carburetor, and it was quite popular and available in the 80s’ – much more so than now. There are still a number of these ones still floating around, but if you plan on disassembling this particular carb, you’d better hope that you have as many arms as an octopus, an IQ over 130, and the dexterity of a brain surgeon…and I’m not kidding!
Centermount 2 Barrel Systems
PROGRESSIVE carburetors used on VW’s are mechanical secondary carburetors. A mechanical secondary carburetor opens in relation to throttle position. The Progressive set-up uses a small primary barrel, allowing for excellent drivability and mileage. These also offer a larger secondary barrel for more power when you open the throttle. These carburetor kits can take a lot of time – up to 8 hours – to dial in for your particular car. This is partly because they are supplied from the factory with generic jetting often not suited for the VW engine, and also partly because the jets can be difficult to access. A good kit won’t be too far off straight out of the box, but even a close one will need some tweaking before it is perfect! The centermount progressive is a fantastic carburetor once it’s jetted properly (but remember that it’s no small task). Most are also available with an electric choke, making it easier to start and drive when the engine is cold and when air temperatures are colder. Most stock carburetors came with electric chokes, and they definitely made life easier for the driver of a vehicle with a centermount set-up. The Progressive set-up is an excellent combination of performance, drivability, and economy. The main downfall of this system is the time it takes to get it set up properly.
NON-PROGRESSIVE systems usually use a Dellorto DRLA or Weber IDF or DCNF 2BBL carburetor. The Non-Progressive carburetor set-up tends to sacrifice drivability and economy in favor of performance.
You may hear about the Holley BugSpray carburetor periodically. It was a decent performer in it’s day, but is essentially obsolete due to age and wear, and you aren’t likely to find one in good enough shape to rebuild or run.
Centermounts and Manifold Heat
ALL centermount applications require intake manifold heat to perform properly in cool weather. Original VW carburetion relied on it because centermount set-ups can ice up if manifold heat isn’t available! Manifold heat also “assists” mileage and makes centermounts considerably more driveable, since fuel is more likely to stay atomized, since it is not as likely to condense on the way to the cylinders. Be careful when buying your carb kit, since many kits have inadequate manifold heating, or none at all! Proper manifold heat will make or break your set-up when it comes to functionality and drivability. A “bargain” kit is hardly a bargain when it interferes with your driving experience by making your car temperamental! I strongly suggest you pay the little bit extra for a good kit, unless you live in the hot desert where the manifold heat won’t be needed as often.
Centermounts in TYPE 3 and 4 VWs
Because of the need for manifold heat, I do not recommend centermounts for the type 3 or 4 engine, since the kits available for these engines do not have provisions for manifold heat. For the type 3 or 4 engine, I recommend dual carbs (more info to follow). Also, the exhaust system you have on your engine will affect manifold heat. Many headers do not have the proper intake pre-heater flanges required to accomplish adequate intake manifold pre-heating. Even headers that do have the pre-heater flanges must be manually drilled through to make them functional. Don’t assume yours is ready to roll out of the box!
Centermounts in Off Road VW Applications
90% of the time, off road applications should use a centermount system. Why? There are a couple reasons. The first is simplicity. Dual carbs require more complex linkage, and when you are talking about off-road use, this is just more parts to break or give you problems. In addition to this, dual carbs are mounted on the outside of the car, where they are vulnerable to getting hit with debris. Lastly, a carburetor mounted in the center is going to get bounced around less then if it’s mounted on the outside of the car as the car is pitched side to side. Therefore, fuel control is easier (less chance of flooding) with a centermounted carb. The power difference between a centermount and dual set-up is very small, and with the above noted advantages of running a single, it’s an easy decision to justify. If you want corroboration, just have a look at the winners of the SCORE races and then check out the induction systems installed on the winning cars…
Dual Carburetor Systems
Aluminum Intake Manifolds
Intake manifolds come as part of a dual carburetor kits (in almost all cases). When you order your kit, I strongly recommend you opt for aluminum (as opposed to steel) intake manifolds. Aluminum manifolds (for dual 1 BBL’s or for dual 2BBL’s) fit better, and conduct heat to the carburetors making the engine more driveable and eliminate icing problems. Effectively, this means that all dual carburetor kits with aluminum intake manifolds do not need intake manifold heat like the centermount kits! I have routinely run my car at well below zero degrees F, with no problems at startup or during operation with dual carbs. Because of this, if you live in a region that hits colder temps, dual carbs are the way to go!
Intake Manifold Length
Here’s a little side note about intake manifold length. Usually, you will not have a choice on manifold length since your linkage clearing the generator/alternator and the space available in the engine bay dictates what you must use. Racecars are the exception! You have a choice on a racecar because of the larger engine bay and/or no engine tin and generator/alternator. In general, short intake manifolds produce more power at high RPMs, and long intake manifolds produce more power at low RPMs. The power peak between “shorter” and “longer” is between 300-500 RPM’s — not enough to lose sleep over. Long intakes require a shorter air cleaner which is a limitation for air filter area. Short intake manifolds make it difficult to get your plug wires on and off, but they do allow more room for larger air cleaners. So choose your poison.
When choosing your carburetor kit, you also have the type of linkage to consider. Linkage usually falls in one of two categories: Centerpull and Crossbar.
- Centerpull linkage uses a “pivot” system towards the center of the engine, and when the gas pedal is pushed down, the linkage PULLS and/or pushes the carb throttles open.
- Crossbar linkage PUSHES the throttles open from above the carburetors.
It’s important to understand that VW engines get wider as you run your vehicle, due to metal expansion upon engine heating. This usually isn’t a consideration until you contemplate the two different styles of linkage. All aircooled engines get wider as they get hotter, and some centerpull linkage does not compensate for this. In these cases your idle speed can depend on how hot your engine is (with poorly designed linkage, the linkage gets tight as the engine warms up)! With the crossbar linkage, throttle position is virtually unaffected by engine temperature and the associated growth/contraction, so it’s more consistent in it’s operation. I favor the crossbar linkage style for all applications, if you have a choice. Oh! And when purchasing your carb kit, ask whether the linkage is made of aluminum or steel. In my experience, the aluminum linkage can seize or strip, since the down rod threads are so small and aluminum is soft. Steel is King when it comes to linkage!
Carburetor Brand and Model
Dual 1 Barrel Carburetor Systems
Kadrons: These are Brazilian built Solex carbs. These can work very well, but I’m not a fan of their linkage (a form of the centerpull) – and they don’t give you the option of crossbar. The linkage ball-joints have been known to break or pop off, and if you pay attention to cars you see with Kadrons, you will note that bread ties, zip-ties, and rubber-bands are frequently resorted to as a quick fix for keeping the linkage together. Only buy Kadrons if you are also willing to keep your glovebox stocked with these Kadron essentials. On the positive side, Kadrons are the largest of the dual 1BBL carb kits, and produce the most power. They are very tunable, and the kits are reasonably priced at under $400, but we don’t carry them because of their linkage design faults.
Dual 35 mm Solex: These kits are only available for upright engines, and only available for Dual Port heads. The first thing that strikes you about this kit is that they are inexpensive! They come with FACTORY intake manifolds (which mean they fit), and factory carbs, since the entire kit is based on factory components made in Brazil. These carbs are outstanding for mild engines, from semi-stockers to mild street engines. You are limited to around 100 HP with these carbs. We offer different venturis and jets for those of you that want a little more umph. The linkage is very nicely done, we modify it here to make it work the way it should, and minimize your headaches.
Dellorto FRDs and Weber ICTs: These two brands and models are pretty much equivalent. The Dellorto FRDs are no longer available, and parts availability is sketchy. The Weber ICT is a good carburetor (not great), but because of the 35mm Solex kit availability, we only recommend the ICT for Single Port Upright, Type 4, and Type 3 engines. Again, make sure you get crossbar linkage (all of our kits have this). Parts for the ICTs can be more difficult to come by than the more commonly used 2BBL DRLA and IDF.
Electric Choke Solex: Solex’s work well, but these use center-pull linkage. And there is almost no power increase compared to a stock single carburetor because these carburetors are so small! These are not widely used because chokes are not necessary on VW dual carburetor set-ups, and the inclusion of the electric choke makes this kit expensive. Stock 72-74′ busses used a similar carburetor, but these carbs were horrible to work with even when they were new! Only a masochist should consider trying to adapt the bus carbs to some intakes you have lying around.
Dual 2 Barrel Carburetor Systems
Weber DCNF: These were fantastic carbs in their day, but the Weber IDF has obsoleted the DCNF for aircooled vw applications. Gene Berg utilized these carburetors and made them very popular with VW hot-rodders. They are quite small, so fitting them into the VW engine compartment is pretty easy, unlike many other larger carb sets. You should not use them for an off road car, even in a centermount application, as they will flood easily (their fuel level control is not very effective). Street and race use only.
Weber IDAs were designed for race applications, but contrary to popular opinion, these can work very well on the street but have some inherent limitations. They do not have much of a progression circuit (they only have two progression holes where the IDF has four), so in unmodified form, they are either off (idle) or on (full throttle)! A carb expert can put a third progression hole in which will make them more streetable (we offer this as an option if you buy them from us), and this modification will increase driveability and MPG. 26-28 MPG on the highway is attainable, even with huge engines, when they are properly set up. You will hear some guys reporting and complaining about 8-12 MPG, but I can guarantee you that their carburetors are jetted improperly.
Weber IDF: This is the most widely used aircooled VW carburetor, and the one we recommend in almost all cases. The IDF’s come in 40, 44, and 48mm sizes, and are excellent carbs for street cars. These are also the carb of choice for off road vehicles due to an excellent progression circuit and superior fuel control! These will not flood like the DCNF. The IDF’s have many advantages over the other carb sets available:
- they will run on a wide range of engines (from stockers to full race engines)
- they fit nicely in the engine compartment
- they are easy to fit air filter assemblies onto
- they have a modern float design which prevents flooding
- they have 4 progression holes for smooth driveability under light throttle conditions
- parts availability is excellent because they are still being made
- they have a vacuum advance port (see SVDA and Unilite)
Dellorto DRLA: is essentially the same carb as the Weber IDF, just made by Dellorto. It comes in 36, 40, 45, and 48 mm sizes. It’s important to note that the DRLA carb sizing is 1 step larger than the equivalent Weber. A 36 mm Dellorto is equivalent toa 40 mm Weber; a 45mm Dellorto is equivalent to a 48mm Weber; etc.
Solex 40P11: These are old carbs (they used to use them on old Porsches), and they work very well IF they aren’t worn out (and a non worn out one would be a rare find). They are similar to the IDF and DRLA in appearance and function.
Carburetion and Air Filters
There is one golden rule for carburetion systems of ALL types – you must always run air filters! Lack of filtration is a common source of piston ring wear, since the dirt is abrasive and will abrade your rings and cylinder walls. This wear also affects the jets in your carb! It’s not uncommon for jet sizes to “grow” from the wear associated with non-filtered air and fuel. In addition to wear, dirt WILL clog the jets on your carburetors! Carburetors meter both fuel and air, and many of the passages and jets are very small — it doesn’t take much to clog one up. When it’s clogged, your car will run erratically, and then you have to chase down the problem. Save yourself some grief and run filters, and just avoid these complications altogether. Filters do not limit performance, and don’t let anyone tell you otherwise.
Venturi size refers to the narrowest channel in the carburetor throat. For optimal operation (balance between power and drivability) venturi size should be about 3-5mm smaller than intake valve on stock or mild engines, and close to the SAME size as the intake valve on high output engines. You should also note that heavy vehicles must be more conservative with venturi sizing than lighter cars. All 2BBL Dellorto and Weber carbs have changeable venturis, so you DO have some tunability and re-sizing. However, venturis are expensive, so it’s best to get close from the outset! Ask what venturi sizes are in the kit you are considering for your car. You want to get in the ballpark from the beginning since re-jetting a set of dual 2BBLs can cost from $100-150, and take a fair amount of time.
WHICH SYSTEM IS BEST FOR YOU?
Here are Aircooled.Net’s recommendations:
For street use, I prefer either the stock carb, or a dual 2BBL carb system (all brands of carbs are okay, but remember to opt for steel crossbar linkage). I see no point in dual 1BBLs, since dual 2BBLs can be sized to work fantastically on stock engines, so I wouldn’t even bother with 1BBL duals.
Off roaders, should opt for a centermount progressive for smaller engines, and the centermount IDF/DRLA for larger. Choose the 40 IDF for <2000 cc, and the 44 IDF for 2000 cc and larger.
So – there you have it. All the ins and outs of carburetor and carb system options. Are you asking yourself why you’ve never gotten this kind of straight, down to earth, necessary information from the average VW parts supplier? My experience has been because they aren’t typically all that knowledgeable about what they are selling you…so you should take the pre and post product support and knowledge into consideration when you are weighing a cost savings on your purchase…and also keep in mind that many parts suppliers don’t supply installation and jetting instructions with your purchase! If you have a problem with installation and dialing in, you sure don’t want to be on your own! Speaking of which — after you buy a set of carbs, we cannot emphasize enough how important it is to have your carbs PROPERLY set up. Most people don’t realize that carbs as they are supplied from the factory are assembled, but NOT set up. Aircooled.Net offers Carb Setup as an option when you buy your carbs from us, but we also provide a basic “how to” article for doing a VW Carb Set Up Procedure yourself.
|Other VW Carburetor Tech Articles|
Hopefully this extensive run-down on VW carburetion has helped you make your carburetion system decision, and we’ll be hearing from you soon! Aircooled.Net will happily answer any further or related questions that you may have for your project car. Keep Fweemin’!
by John Connolly, Aircooled.Net VW Parts
originally written: February 6,2000
last updated: 02/25/2012