The Datsun 240z came with a single carburetor or twin carburetors. The single carb was the DAF342 on the G130 model and also on L20A engines. The twin carburetors were designated as the Hitachi HJG46W. Many people call these SU, Skinners Union, carburetors. They are SU type by design, but are manufactured by Hitachi. The 46 designated the diameter of the opening, 46mm. This ingenious design was perfect, a variable Venturi and variable Choke carburetor, probably the simplest carburetor ever made. Fuel available was dependent on the vacuum of the motor which raised the piston which had a needle attached to it, as it raised out of the nozzle the tapered needle allowed more fuel to be delivered. Changing the needles could change the entire profile of fuel available, a common upgrade is to run “SM” Needles. They provide richer fuel at around 2000rpm. The idle circuit was simple, the float level established a fuel level and the mixture adjustment screw allowed fine tuning of the depth of the nozzle. The butterfly was adjusted to allow a small amount of air flow through the Venturi. The choke was a simple cable that moved the Nozzle down, allowing more fuel for a cold start. Tuning these carbs is a very simple procedure. Setting them to idle with an idle screw and then tuning them at 4000rpm with a flow meter. Each carburetor fed 3 cylinders, balancing the airflow between the 2 intakes was necessary for proper combustion and power delivery for each cylinder fire. They incorporated balance tubes to help synchronize the air flow, but a flow meter is required to match the flow exactly.
The 1970 – 1971 Datsun 240z used carburetors scavenged from existing surplus from Roadsters. The domes had 4 screws and did not have water passages. The 1972 Datsun 240z came with a redesigned version of the Hitachi HJG46W. The redesign made it a 3 screw carb with an alignment tab to help align the needle with the Nozzles, with tabs to lock the mixture adjustment nut, and hot water passages to help heat the fuel so it better atomized. The only disadvantage from having the fuel introduced at the bottom of the Venturi at the nozzle, was fuel atomization. Most carbs release the fuel in the center of the Venturi orifice for better turbulent conditions to get the fuel to atomize better. The 1973 240z and the 1974 260z used another design, this carburetor is designated as the HMB46W. The domes were a flat top design and were very problematic. I am not sure if there were licensing or patent issues with Skinners Union, but the redesign was horrible. These carburetors were junk and are called Clunkers. The sought after carburetors are the Hitachi HJG46W that came on the 1972 Datsun 240z, 3 screws domes with an alignment tab and water passages to help atomize the fuel. The 1975 Datsun 280z and subsequent Z cars used an Electronic Fuel Injection System. An ingenious system, the fuel injector was basically a magnetic valve which released fuel precisely, eliminating power balance problems. The entire fuel system was redefined, a higher fuel pressure was required and many parameters were measured with a brain box to control the fuel delivery. The Cold Start Valve replaced the Choke and many other devices were incorporated into the system.
Below are images to help ID carbs and additional resources for schematics and EFI Factory Service Manual by Nissan Motor Corp.
The 1970-1971 Datsun 240z 4 Screw No water Jacket Hitachi HJG46W
1972 Datsun 240z Hitachi HJG46W 3 screw with Water jacket with different tabs
1973 Datsun 240z and 1974 Datsun 260z Hitachi HMB46W Carburetor
The Datsun 240z Hitachi HJG46W Carburetor Disassembly
Factory Service Manual Specs for Datsun 240z Twin Carburetors
Please note the different needles, springs, and nozzles
The Datsun 280z Electronic Fuel Injection System Components
The best article written about the SU design can be found at Zparts.com by Mal Land.
Excellent Schematics can be found in Nissan Motorsports Schematic Catalog for Mikuni / Solex and SU Carburetors.
The Factory Service Manual for the 280Z Electroic Fuel Injection System, Theory and Troubleshooting.