SU Carburetor Differences
There were 3 different stock SU carburetor designs installed on US made 240Z's from late 1969 through 1973.
- Late 69 production date up to the 71 models had a "bottle" or "bell" top piston chamber that was secured to the main body unit with 4 screws. Both front and rear carburetors used identical rebuild parts. These carburetors did not have water warm-up circulation chambers in the rear base and hence, used a square, 4 hole carburetor to manifold spacer/vibration dampener. There have been some reports of modest external visual differences in the carburetors of these years, mostly in the lower hardware, but essentially they are all the same. These units are most often used to replace the troublesome '73 240Z & 74 260Z "flat top" Hitachi SU's when they go bad.
- 72 carburetors were almost identical in looks, but differed in several refined areas. The "bell" tops were secured with 3 screws rather than 4 and the rebuild parts differ between the front and rear carburetors, primarily in the inlet jet assemblies. The 72 units also had their base casting altered to accommodate 2 warm water re-circulation inlet passages to facilitate quicker warm ups from cold starts and for other subtle technical improvements. These 2 inlet passages match up to a unique intake manifold designed with matching water holes that must be bolted up to each other using a proper manifold spacer/vibration dampener part with corresponding holes in order for the warm-up feature to function.
- 73 Hitachi SU units are technically quite different and visually distinct from the "bell" top types described above. These are usually referred to as "flat tops" because, instead of a gentle, curved upper piston chamber design, Hitachi made the piston chamber larger in diameter and machined off the upper shoulder as flat, creating a "Flat Top" look on the shoulder with just the small oil damper stick hole in the center.
These carburetors were Nissan's response to the stricter smog laws being rapidly implemented in the US back in the early 70's and, in hindsight, they appear to many Z owners to be nothing but poorly designed, troublesome "dogs", especially after they had about 10 years wear on them. Unlike the early SU's, these carburetors were very expensive to rebuild and usually had to be totally removed from the car for any rebuild parts to be installed. Internally they were more complex and were the first Z car SU carburetors to have a "power valve" added on the side of the body, somewhat the equivalent to an acceleration pump on a conventional down draft carburetor, but mechanically, not the same. It is these little power valve units that can be altered, rotated and replaced to often make "bad" carburetors good again.
More 73 SU facts: The 73 flat tops came matched to a totally new intake manifold, balance tube with the infamous EGR valve unit added, mounting spacer, water warm-up system made up of numerous hoses (very messy and complex and prone to plugging up from rust and other water born crud) and it's own unique center linkage. Persons swapping out their 73 SU's for the early SU's soon discover that some of the linkage between the 73 manifold and the early SU's will not match up. Contact me at Z PARTS if you need any of this small stuff. I'll try to help you if I have any extras in stock.
When these 73 carburetors start to wear out one of the most observable problems is that they start to run very rich. Owners usually discover that most repair shops and average mechanics are notable to adjust these worn carburetors to pass smog and hence, they are often chucked in the garbage and replaced by either early SU's or Weber DGV's (You Tri-carburetor guys zip your lip on this one. We're talking "legal" at this point).
What is not commonly known about the "flat tops" is:
- There is a trick or two that can be used to make them stop running so rich and to get them through smog without a rebuild.
- There are reportedly shops in the US and racers in Europe from the distant past, that have long known ways to make these "bastard" flat tops into very credible performance carburetors. I am not well informed on the facts, but have met some old guard carburetor builders that will swear to that little known fact being true, but long forgotten. If you see a "FS" number inked on the top of any of your flat tops, send me the number and I will try to match it to the scant research I have here.
- 1974 260Z cars in the US also came with the "flat top" carburetors. They were virtually the same as what came on the 73 240Z's, 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 SU's 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 in case any of you wish to buy one for your own projects.
Sources used on this page:
- (Authored by Eric Neyerlin of Z Parts)