Performance Carb Setups - Part 1
By Ryan Ballou
Continuing the bolt-on theme I started last month we move on to the next upgrade most worth the time, effort, and money; Carburetors. A common saying among auto enthusiasts is "you can't get it in if you can't get it out". This usually refers to the exhaust side of the equation, meaning all the heads, carbs and cam in the world don't mean a thing if you are still running a restrictive exhaust system. In this case, however, it's the opposite, assuming you've followed the trend of adding exhaust as one of your first upgrades. You now need to find a way to 'get it in' and a larger, less restrictive form of carburetion is the first step.
While there isn't really a large selection of carbs out there, there are quite a few configurations in which you can run them. First there is the center mount configuration. In this category you have the stock carb, the Bugspray carb, the Weber Progressive, and the center mount configuration of Weber IDFs and Dellorto DRLAs. Center mounts tend to be more popular among the off-road crowd for two reasons; float configuration is good for harsh cornering, and the central location protects them from debris. The downside is that the long intake runners needed to get fuel to the heads tends to cause fuel to drop out of the mixture and puddle in the runners. This problem becomes compounded by manifold icing when proper manifold heat is not present. These problems are most frequently recognized by low speed bucking and a stumble off idle. The most common fix is to just run the idle circuit richer, increase fuel delivery from the accelerator pump, and/or run emulsion tubes that bring the mains 'on' sooner. While these fixes work, they all harm fuel economy by causing you to run unnecessarily rich.
The next route is to take one of a few dual carb configurations. The low end of this setup is the dual 1bbl configuration, frequently seen as dual Weber ICTs, or dual Kadrons (actually Solex carbs). While both of these are a huge improvement over a single carb setup, they still have their restrictions. First and foremost, they are small. ICTs have 34mm throats, and Kadrons have 40mm throats. One advantage that Kads have over ICTs however is that there are larger aftermarket venturies available for them to increase their flow capacity. The second down side is that being single barrel carbs, they are forced to feed two cylinders each. Even though only one cylinder pulls air at a time, you still have to consider that they are feeding two cylinders in progression of the firing order. Add a little reversion to the equation and one barrel will always suffer at lower rpms, commonly seen as an uneven idle when using larger than stock cams.
Finally, the most popular configuration is the dual 2bbls setup. In my opinion, this is what you should be thinking about when it comes time to upgrade. The three options you have here are Weber IDFs, Dellorto DRLAs, and the monstrous Weber IDAs. IDFs are available in 40mm, 44mm, and 48mm throat sizes. DRLAs come in 40mm, 45mm, and 48mm sizes. IDAs in stock form come only in 48mm throat size, though through the aftermarket they can be bored and modified to 51.5mm, or in spin off versions they can be found in 58 and 62mm throat sizes.
There are four things to consider when deciding the size and make: engine size; intended usage; budget; and if you frequent the drag strip, what fuel type, as IDAs are the only real choice for running alcohol. The rule of thumb that I would follow regarding engine size is 40mm for 1600s (1641, 1680), 44/45s for 1776 up to the two-liter range, and 48s for anything larger. As always there are exceptions, but this will provide good performance without the need to go out and buy costly venturi upgrades right away. Following this rule of thumb will also provide satisfactory performance in these displacement ranges for up to about 6500 to 7k rpms provided the rest of your engine combo is up to the task. If you wish to turn more rpms for a given displacement, then the next size larger carb should be considered. Likewise if you want to run a larger engine but plan on limiting your rpm range, smaller carbs will work.
Expect to spend about $800 for a set of either 40 or 44 IDFs, and $1000 for 48 IDFs. This price will get you manifolds and linkage as well. Dellortos are no longer available in new form so the price for these can vary greatly depending upon the condition of a used set. IDAs will run you over $1000 just for the carbs. For the sake of this article, my main focus will be the Weber IDF series as these are still available, and a much better choice for street use.
To get the best performance from your dual carb setup, it really helps to have at least a basic understanding of the working theory behind them. The easiest way to visualize it is to follow the path the fuel takes through the carb starting with the inlet valve and float bowl. The fuel enters the carb by passing through the inlet valve, also known as the needle and seat. The inlet valve is the first of the metering devices used to regulate fuel flow. It is a calibrated size and denoted by its size in thousandths of an inch. Hence an inlet valve with an opening size of .175" is known as a 175, a .200" is called a 200 and so on. There's no easy formula for calculating the necessary inlet valve size. It's not as simple as calculating the sums of the opening areas of the idle and main jets as these act under the influence not only varying, but a greater overall pressure than the inlet valve sees from fuel pressure. The best bet is to stick to what came with your carb setup and change it only if you find a fuel starvation condition exists at wide open throttle (WOT) during high speed.
Ideally you want to use the smallest size inlet valve you can safely get away with because as the size increases it also becomes more difficult for the float to control the fuel flow through the valve. In most naturally aspirated situations you'll find a fuel pressure between 1.5 psi and 3.5 psi to be best; experimentation is the key here. Which brings us to the float. The purpose of the float is to actuate the inlet valve as dictated by fuel level in the float bowl. A higher fuel level in the bowl will tend to make the mains come in sooner as well richening the entire fuel curve. A lower fuel level does the opposite. This is because the level of fuel in the bowl affects pressure on the backside (inlet side) of the main jet stack. It's the balance of positive pressure here and negative pressure in the carb throat that determines fuel flow through the carb.
The main jet is the next destination for the fuel. The fuel enters the bottom of the float bowl and passes through what's known as the jet stack. This consists of the main jet seated in the bottom of the emulsion tube, and the air jet seated on top. The main jet meters the fuel to match the requirements of the engine. The main jet mostly affects the fuel metering from about 3k rpms and up. Its size is also denoted by thousandths of an inch. Mains are available in increments of 5 and each change of 5 is commonly referred to as one size difference.
Next in line is the emulsion tube - a long cylindrical object that has a series of holes in it. The physical size of the tube is what determines the size of your fuel well. This fuel well is what prevents a lean condition under certain acceleration conditions. The larger the well, the more fuel that is delivered at specific points in the rpm range. The size and locations of the holes play a role in determining what rpm range the fuel well will affect enrichment. The word "emulsion" in emulsion tube refers to the fact that this is where air from the air correction is emulsified (mixed) with the fuel from the main jet. The numbering system for emulsion tubes is pretty nonsensical. With Webers the tubes come with F designations, such as F2, F7, and F11. The numbers really only mean when in the production series of the carburetors the tube was developed. So an F2 tube came before an F7 tube and so on. Don't forget these carbs were used in many different production cars over the years and not specifically designed for any one use. For reference though, the F2 tube is generally used when running alcohol, the F7 and F11 tubes are what most end up using in a VW application.
At the top of the emulsion tube you will find the air correction jet. The purpose of this jet is to add air to the mixture, generally from about 3500 rpm and up. Without the air correction jet, the main jet stops metering in a linear fashion in this range and an over-rich condition occurs. Air jets are also measured in thousandths of an inch. However, when speaking of air jets, one size is a change of 15. So if you were running a 175 air jet, going up one size would yield a 190.
Now that the fuel mixture has been metered, it's finally ready to enter the carb throat and be taken to the combustion chamber. The exit point for the mixture from the idle jet is through the progression holes near the throttle plate. These are a series of holes in a vertical fashion in the throat of the carb. The lowest hole should be just covered by the edge of the throttle plate. When you start to just crack the throttle this first hole becomes exposed which starts the fuel flowing. The holes above this first one continue flowing fuel together as the progression circuit. This circuit is active throughout the entire rpm range and overlaps the main circuit. This is also the circuit that is most responsible for the mixture under cruising conditions and thus plays a major role in fuel economy.
It should be noted that the idle mixture screws play no role with the progression/idle circuit per say. The mixture screws are only considered important at a pure idle with the throttle completely closed. The size of the idle jet will play a part in determining how far in or out the screws are adjusted, but they should never be used to correct an improper mixture condition at any point other than zero throttle.
The man circuit plays it's part from about 2500 rpm and up and becomes more and more pronounced as the throttle is opened up. So at WOT the main circuit plays a much larger role than the idle circuit. Fuel from the main circuit exits through the opening in the auxiliary venturi, the funny looking bulls eye part with a stack that is found on top of the main venturi. In Carbs that have been upgraded with a horizontal discharge tube (available from CB Performance) the main and Aux Vents are replaced by one large venturi/sleeve with a brass bar passing across the middle. The brass bar has a series of holes on its underside and this is where the fuel would exit the main circuit. The biggest advantage of a spray bar (horizontal discharge tube) is that it removes restrictions caused by the Aux vent.
Finally, the last part we need to be concerned with is the accelerator pump circuit. This is the part responsible for giving that shot of fuel when you step on the pedal. Without it your engine would bog every time your stomped your gas pedal. Air has mass just like everything else and thus inertia. As you open the throttle, it takes little time for it to start moving and this creates a short lean condition. This lean condition is remedied by the accelerator pump.
With this basic understanding of the operation of your dual carbs, next month we'll start into the installation process.