S30 Carburetor Information

Carburetor Differences

1. Late 1969–1971: “Bottle” or “bell” top piston chamber, secured with 4 screws. Both carburetors used identical rebuild parts. No water warm-up circulation chambers in the rear base, used a square 4-hole spacer/vibration dampener. Often used to replace '73 240Z & '74 260Z "flat top" Hitachi SU's. Externally, some minor visual differences exist, mostly in lower hardware.
2. 1972: Very similar to earlier models, but with a “bell” top secured by 3 screws, and different rebuild parts for front/rear (mainly inlet jet assemblies). The base casting has 2 warm water recirculation inlet passages for faster cold starts, matching a unique intake manifold and corresponding spacer.
3. 1973 “Flat Top” Hitachi SUs: Technically and visually different from “bell top” types. The piston chamber is larger and flat on top (hence the name), with only a small oil damper hole in the center. Designed to comply with US smog laws, but generally regarded as problematic and expensive to rebuild. Key features:
   • More complex internally, with a “power valve” (like an accelerator pump, but mechanically different).
   • Matched to a new intake manifold, EGR-equipped balance tube, unique center linkage, and a complex water warm-up system prone to clogging.
   • Running rich is a common problem as they wear out; most mechanics can't get them to pass smog, so many are replaced with early SUs or Webers.

   Additional “Flat Top” facts:

   1. Tricks exist to help them pass smog and stop running rich without a rebuild.
   2. Some shops and European racers have ways to make these perform well; if you find an “FS” number inked on the top, contact Z PARTS for research.