The Standard Nikki Carburetor

The standard Nikki carburetor is simply too small for an application where maximum engine output and efficiency is desired.  The Nikki can be modified to improved its performance, but compared to the cost of a second hand Weber, modifying the Nikki through a carby workshop is expensive.  

Putting aside its physical dimensional inadequacies, the Nikki is still a reasonably sophisticated carburetor with several features that even the replacement Weber does not have.  The Weber however, has a better functioning fuel control circuit as discussed on the New Setup page.

The Nikki's list of features include a fuel shutoff solenoid, commonalty called a dieseling control solenoid, as with a combination of a certain amount of ignition timing and some design carburetors, engine run on after the ignition is switched off can occur if enough fuel is allowed to be drawn into the engine during idle conditions.  This solenoid stops fuel flow to the idle circuit.

The Nikki also features a diaphragm operated vacuum secondary throttle butterfly.  This system will open the secondary throttle butterfly of the Nikki when a certain amount of engine vacuum is created when a certain amount of throttle is applied through the accelerator pedal.  Therefore, only the amount of air required by the engine will be supplied.  This is a very good system for fuel efficiency, but as a certain level of vacuum is required in the inlet manifold for the secondary butterfly to commence opening, the engine can not achieve its maximum amount of volumetric efficiency at that rpm.  The Weber on the other hand has a mechanically operated secondary butterfly which begins to open after about 3/4 of throttle applied.  The mechanically operated secondary system is a very noticeable improvement to throttle response, at the expense of fuel economy.

The Nikki also utilises an electric choke system that operates automatically using the temperature rise of a bimetallic strip to represent the temperature rise of the engine coolant.  The Weber DGV has a cable operated mechanical choke, whereas the DGAV variant has an automatic choke, with both electric and water control types being available.  When installing a Weber downdraught DGV or DGAV onto the standard Gemini inlet manifold on TC-TG models, the choke housing hits the brake fluid reservoir on top of the maser cylinder, so the choke mechanism must be removed.  On TX models, the fluid reservoir is remotely mounted, so this may not be an issue.  This is further explained within the New Setup section.

The coasting enricher solenoid is another feature that I am led to believe leans the fuel mixture at part throttle in top gear in a manual gearbox equipped Gemini.  This is an economy feature.

To remove the carburetor, you must first remove the air cleaner, then disconnect the fuel lines and accelerator cable. You can now best reach the nuts that secure the base of the carburetor to the studs within the inlet manifold.  After removing the nuts, the carburetor will slide off the studs.  The insulating spacer plate can now be removed from the manifold.  It should be noted that a gasket resides on both sides of the insulating plate.  The plenum (inlet volume for the individual runners) of the manifold should look like this:

If you are after the best from your new carburetor installation, you should heavily modify the plenum area of the inlet manifold for better air flow capability, and better mixture distribution.  For details on how to achieve this, go to the Modified Manifold page.

Check out the New Setup page for installation of the Weber DGV carburetor.