Tuning the Weber DGV for the L-Series 510 motor
Article by Dennis Hale
The Weber-DGV series carburetor is a popular aftermarket replacement for the 510. It is a progressive two-barrel design which incorporates great adjustability and precision. The original applications for this carburetor were various European cars, usually Fords or Renaults, with displacements from 1300cc to 2000cc, and power levels from 75 to 125hp. Perhaps fifty variations of the DGV exist. The Weber naming system is a little odd, but the DG seems to indicate a two-barrel design, the V a power-valve system, and if you have a DGAV, the A means an automatic choke. The 32/36 indicates the sizes of the throttle bores. If you still have the tag with the numbers after the DGV part, this indicates the application, in numerical order, for which your carburetor was factory tuned. The application will be similar enough to work on a Datsun, but custom tuning will improve performance. The good news is all the parts are available and all this tuning magic is possible. You can tune this carburetor so that the motor will use less fuel and produce fewer emissions than with the stock Hitachi. The bad news is you'll likely make no more power than you did before you began, unless the jetting was really off.
On a motor that is sized (displacement and cam design) properly for this carburetor, 85% to 90% of available power can be delivered by this production carburetor as compared to more exotic special-application carburetors. You may find carburetors in this family labeled DGV, the manual-choke design, or DGAV, the electric-choke design, or DFV, a reversed-base design, or DFAV. The F in the name may stand for French, as these seem to be found on Renaults. The reversed design is not a good fit because the primary and secondary bores are backwards and the accelerator linkage is a problem. The base of the Weber is a little larger than that of the Hitachi, and usually, an adapter plate is used to fit it. Tall plates work the best but can be too tall for L20B applications. Welded and remachined stock manifolds seem the best combinations for fit.
There is a little theory which will help your appreciation of carburetors. In many ways, fuel-injection systems are simpler devices than carburetors. (You could see carburetors as hydraulic computers while fuel injection is electronic.) The gasoline in the carburetor is not sprayed into the airstream by the fuel pump (that is fuel injection), it is pushed into the low-pressure area created by air flow through the venturis. The fuel pump merely fills a reservoir in the body of the carburetor through the float valve, which stops the flow when the reservoir is full. The trick is to get the fuel into the air reliably, in correct proportions, and in fine liquid droplets like a fog.
An interesting thing is, as air flow slowly increases, as when the engine speed increases and the pistons suck more air through the carburetor, the fuel is willing to flow faster than the air, while during quick changes the air speeds up easier and will flow more than the fuel. The air always needs to flow more, between 11 and 15 times as much air as gas, by weight, is needed for combustion. We have to be clever to get the air and gas mixture just right, by adjusting the flows correctely. Generally, an 11:1 A/F ratio for power and 15:1 for economy are used. Finally, we can make all of these adjustments with accurately-produced pieces with holes cast and machined to flow as need. As expected, large holes will flow more then smaller ones, but moving them around or changing their edges will also have a large effect on their flow. This last cahracteristic is what makes the replacement of parts so special with the Weber. All of the Weber parts are labeled for flow, not just hole size. This is also why drilling jets is such a lousy way to tune a carburetor.
The Weber design, like the Hitachis and other downdraft carburetors, breaks the problem of matching air/fuel requirements of an engine into steps, most of which are adjustable. Low flow conditions are handled by the primary (32mm bore/26mm venturi), while high flows are handled by this primary carburetor plus a secondary (36mm bore/27mm venturi) carburetor operating at the same time. This is very similar to the original Hitachi DAF carburetor except the Weber is larger and activates its second half with a mechanical linkage rather than a vacuum mechanism. The Weber offers a larger variety of tuning parts and is easier to service than the DAF. For the most part, the DAF is simply an old and worn rather than a badly designed or tuned unit. Within each of the twin sections of the carburetor, the fuel and air mixtures are handled by overlapping systems. The idle circuits handle the very low flows which bypass the throttle plate when your foot is entirely off the pedal. As the throttle is opened, the primary enters into play, and the main jet will dominate the tuning effects. When the throttle is fully opened, the air jet will be fine tuning the effects, then the secondary system will come into play in the same fashion. The main fuel jets and air jets both flow into a common tube with a series of mysterious little holes that are called emulsion tubes, or e-tubes, which help turn the "raindrops" of gas into a fog.
The accelerator pump circuit squirts in a little gas when the throttle is quickly opened, to compensate for the air responding quicker than the fuel. The air jet compensates for the increased fuel flow when things settle down. There is also a power-valve jet which drops the 15:1 ratio to 11:1 at WOT to produce full power, and a choke plate for cold starting. Closing off the top of the carburetor increases the suction on the fuel jets and richens the mixture for cold starts. All of this works rather like a band playing music: everything does its job and you hear music. The tuning concept is very much the same as with a band: get each part working in tune with every other part.
