Mechanical Secondaries
The standard vacuum secondary system is removed and secondaries are converted to mechanical operation. Secondaries will now open at around half throttle. This gives the driver more control over mid-range acceleration, along with a performance boost if used properly.Accelerator Pump
Modified to increase displacement, so it shoots out fuel over a longer stroke. This gives a much smoother transition when using mechanical secondaries. Accelerator pump linkages are also repaired, as these are metal parts that suffer a lot of wear and are not included in a rebuild kit.Fuel and Air Jets
Primary main fuel jets are enlarged to prevent the primaries leaning out too early in the RPM range if the secondaries are not opened manually. Primary main air jets are then enlarged to maintain a good AFR after the fuel jet enlargement. Secondary main fuel jets will be enlarged for bridge port setups, otherwise left standard. Secondary main air jets come in several sizes, I generally use the smaller ones to stay on the rich side and allow them to be drilled larger after dyno testing.Booster Venturis
The booster venturi centering arms are removed to reduce flow impediments through the carburettor. The booster intakes are also polished to remove any minor damage.OMP Nozzles
The OMP Nozzles are shortened to reduce flow impediments through the primary barrels.Main Venturis
The main venturis are cut for more optimal inlet angles, but are not bored out any larger. My reasoning for not boring them out (a common modification) is taken from Sterling Metal Works, who have done extensive research and development on the Nikki:...flow bench testing done by my associate Carl Perez has shown the stock Nikki venturi shape to be so inefficient and far from optimal that flow gains comparable to boring the venturis several mm can be achieved solely by optimizing shape. The ultimate difference is that an optimized venturi shape allows air flow to produce a stronger signal to the main fuel circuit, whereas simply boring out the venturis to attain the equivalent air flow will weaken the signal at low to mid throttle so much that the carburetor will have little power below 3500 RPM.
The primary main venturis are cut as part of the street mods. This results in better acceleration and throttle response at low RPM. The secondary main venturis are cut as part of the heavy mods, but only on 12A carburettors, as the 13B Hitachi secondary venturis are cast in place and cannot be removed. This results in better peak performance on ported engines where the standard carburettor cannot flow enough air to meet the engine's needs.
Throttle shafts
The primary throttle shafts are thinned by around 50%, and secondary shafts are thinned by around 70%. This results in roughly 20% more cross-sectional area at full throttle, providing greater air flow and volumetric efficiency at high RPM. Button head screws are used on the throttle plates to reduce flow impediments.
