Carby Mods

The page has been written to detail the installation of a Weber DGV downdraught carburetor onto the standard inlet manifold of a Holden Gemini, models TX-TG.  There are many different carbys that have been bolted to gem engines over the years, but this has to be one of the easiest conversions around.

The standard Nikki carburetor is insufficient in air flow capabilities to allow maximum torque to made made on even a standard G161 1574cc engine.  The Nikki's chokes (or venturis) are slightly smaller in size to the Weber's, as are the butterflies.  This is calculated on total cross sectional area of the chokes. 

This leads to an air restriction as air flow increases.  The small cross sectional area that the air must pass through increases air velocity to a point where frictional losses through the carburetor become unacceptable.  These friction losses effectively limit the amount of air that can pass through the carburetor at a given pressure drop.  

The Nikki has a 30mm primary butterfly, with a 22mm diameter primary choke.  The secondary system consists of a 34mm butterfly, with a 29mm choke.  The Weber chokes are slightly larger overall, being 26mm and 27mm for the primary and secondary circuit respectively.  The Weber's butterflies, at 32mm and 36mm for the primary and secondary throttles, are also larger than the standard Nikki.  This allows for lower air velocities at the same magnitude of airflow, so frictional losses are not as great.

One of the greatest benefits from the use of the Weber carburetor over the Nikki is the greater throttle response offered by the Weber's mechanically operated secondary system, whereas the Nikki's secondary system relies on a certain level of vacuum measured at a point below the carburetor.  This means that the Nikki has to wait for a rise in manifold vacuum before the secondary throttle can open, whereas the Weber's secondary throttle plate is opened as soon as the driver wants.  Because of these differences, the Weber tends to give better throttle response if tuned correctly, whereas the Nikki should be a bit better on fuel usage when flooring the throttle.

It should be noted that there are many Weber and some Holley carburetors that share the same stud pattern and that can be installed on the standard inlet manifold with a suitable adaptor plate.  On one of my engines, I am using a Weber 38 DGMS that is very similar to the 32/36 DGV, except that the butterflies are both 38mm diameter, and they open synchronously, i.e. both at the same time.  Other examples include a 32/32mm downdraught as found factory equipped to Alfa Romeo Suds, and the Weber 34 ADM as found factory fitted to 3.3 and 4.1 litre Ford Falcons, from the XD to XF model.

The results from this carburetor swap are obviously an increase in airflow capability and more noticeably throttle response over the standard Nikki, so more power, acceleration and efficiency can be had from the same engine.  

The installation of a larger carburetor is usually associated with a loss of throttle response due to the decrease in air velocity at lower airflows (lower engine speed), giving a boggy feel when flooring the accelerator.  From experience with this carby on a mildly worked 1600, I can tell you that the Nikki carby had no where near as much throttle response as what the Weber delivers.  This would suggest that the Weber is not too large for the engine.  The Weber also offers much more in the area of tunability than the Nikki, as new parts are still available, although expensive.  Most of the tuning components can be modified very cheaply though.  This means the Weber is a better alternative if further modifications to your engine are desired.

This overall is a fairly simple carburetor swap, with the adaptor plates available off the shelf at stores such as AutoPro, Bursons, Autobarn, etc.  The reason for needing an adaptor plate is obvious after examining the picture of the two base gaskets below, the Weber being on the left and the Nikki gasket to the right.

As can be seen, the mounting holes for Weber are spaced further apart on the longitudinal axis than the Nikki holes, so an adaptor plate is needed to make up the difference.  The adaptor plate bolts to the manifold where the Nikki would normally bolt onto, whilst the Weber bolts onto the adaptor plate.

The only problems encountered with this carby swap is the tendency for the Weber DGV to be jetted slightly too rich for a standard 1600.  This is easily rectified, but it must be noted that a car equipped with an incorrectly jetted Weber will still be very drivable, however fuel economy will be worse.