Carburetor Differences
 

SU Carb Differences

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

1. 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 carbs used identical rebuild parts. These carbs did not have water warm-up circulation chambers in the rear base and hence, used a square, 4 hole carb to manifold spacer/vibration dampener. There have been some reports of modest external visual differences in the carbs 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.

2. 72 carbs 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 carbs, 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.

3. 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 carbs 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 carbs 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 carbs to have a "power valve" added on the side of the body, somewhat the equivalent to an acceleration pump on a conventional down draft carb, but mechanically, not the same. It is these little power valve units that can be altered, rotated and replaced to often make "bad" carbs 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 carbs 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 not able to adjust these worn carbs 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-carb guys zip your lip on this one. We're talking "legal" at this point).

What is not commonly known about the "flat tops" is:
3a. 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.

3b. 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 carbs. I am not well informed on the facts, but have met some old guard carb 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.

4. 1974 260Z cars in the US also came with the "flat top" carbs. 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.

(Authored by Eric Neyerlin of Z Parts)

Dang wish I would have read this before I bought my 240Z (I wasn't a ZDriver member yet).....I woulda bought an earlier 240 or a 280Z. As per the article I was having probs with the car runnin real rich...plus sputtering and dying alot...thought it might just need a tune-up....but according to the shop I took her to, there are carb issues....probably needs a rebuild...and according to the article, these ain't cheap. Damn '73 carburetors!!!!!!

Yeah, the troublesome flat-tops on my 73 240Z caught on fire, so I trashed them and bought a used set of bell-tops and had those rebuilt.

Total cost for used bell-top carbs, rebuild kit, rebuild and fine-tuning was $225. :)

You didn't have any problems matching the older carbs with the '73 manifold?

If not I need to check into this.....price sounds reasonable.....plus I don't really feel like catching on fire.....

No. But, I forgot to mention that the guy I bought the used carbs from also gave me the linkage I needed to install them. So, yes, you will need the linkage for the conversion too.

Well, I thought of a possible solution for the carb problem.....drop a 280ZX turbo motor in her! (or a Buick Gran National...)

I don't know....but I'm just looking into all my options

Check out ZTherapy
 

These guys will help you with a carb solution. I rebuilt my 4 bolt bottle tops using one of their kits and an instruction video. The website has some good info on it also. I believe the later carb manifold (73 and 74) is a preferred option to use with the older style carbs!!!

http://www.ztherapy.com/

I've got the 3 screw roundtops on my '74 manifold.

And I've also heard a lot of good things about ZTherapy and will be using their services this winter to get some rebuilt carb bodies.

what su carbs are the best for performance 3 or 4 screw?

Ok heres a question, how difficult would it be to switch my 280Z from FI to Carbs?

Anyone that could give me a write-up on that process would earn a nice present.

Thanks.

Not difficult at all. Pick up a 240 intake and SUs with the air cleaner. The linkage is needed as well. You will also need the fuel rail that supplies the carbs. Either buy a pressure regulator or change the fuel pump. Injection pumps put out way to much pressure. I am running SUs on a 82ZX motor and she runs strong. I installed a RX-7 fuel pump I pulled from a '80 RX, non injected,$15.00 at the u-pull-it. This setup has been in my Z for 7 yrs.
Gary