S30/carbs php

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S30 Carburetor Information

There were 3 different stock SU carburetor designs installed on US made 240Z's from late 1969 through 1973.

Carburetor Differences

• 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.
• 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.

- 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:

• Tricks exist to help them pass smog and stop running rich without a rebuild.
• 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.

Note:

1974 260Z cars in the US also came with the "flat top" carburetors. They were virtually the same as what came on the '73 240Zs, but with a few modest smog differences added—notably different smog specs and a slightly different balance tube and EGR system.

BTW: Both the '73 and '74 balance tubes are often coveted for their performance potential when used on a performance L6 engine running performance-enhanced SUs with re-profiled needles. I supply a lot of these balance tubes to L6 performance engine builders around the country, but usually have a few in stock if you wish to buy one for your own project.

Sources Used

• (Authored by Eric Neyerlin of Z Parts)