The flat-top SUs were factory carburetors on non-US market L18SSS engines. This engine was found mainly in what we here in the States know as the 610 and 710. Because Datsun/Nissan never imported that motor to the States in any car, parts for these carbs are not available in the U.S. These carburetors made their way to the US bolted to L18SSS motors that were heavily imported in the mid '80's (among other decades) and sold through a variety of import-engine resellers. However, if you're about to pitch your newly acquired setup into the trash, hold on for a minute...
Many, many 510 owners run these carbs on their motors with no problems. I, myself, have a set of flat-top carbs on my motor. When properly desmogged and set up, they are functionally identical to the dome-top carbs. Despariging comments about flat-top SUs only apply to the Z flat-top SU carb, a horrible carb even when new.
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In the Identification section I said that parts for these carbs are not available in the US. Nissan Motorsports has part numbers but they can't get the parts. Any Nissan parts guy will generally give you a blank stare after you say the words "Flat-Top SUs, please." So what are you to do? Take a deep breath, then figure out what you need. Chances are many of the dome-top parts will fit fine and work great.
In most cases, you won't need to do a full rebuild/rebushing of your SU carbs. If you plan on using these carbs for a long time and want to get the best performance and economy possible, it would be wise to check them over and replace what needs replacing:
Refer to the SU Rebuilding page for information regarding the rebuilding process. As you disassemble your carbs, keep a careful track of what parts go where and keep the parts for each carburetor separate! I managed to take apart my set, rebuild them, and put them back together in working order with decent mechanical skills and zero knowledge of SUs. Just be patient.
If you find a need to replace parts in your flat-top SUs, don't despair. For the best access to Nissan factory parts the very first thing you need is to purchase the latest Nissan Motorsports catalog (#99996-CAT98, $5) and Nissan Motorsports Schematic catalog (#99996-M8015, ~$5). The Motorsports catalog will give you a list of available Hitachi-SU parts, even entire new SU carbs! The Schematic catalog will give you exploded diagrams of dome-top SU's. See the References page for details. These documents are also available on CD-ROM.
To replace unobtainable parts with those you can order from Nissan in the future, you'll need to go one of two routes:
1- Order the following bits from Motorsports (order parts for the Roadster dome-tops):
After installation of the above parts, you will have the ability to go to Nissan and if you should ever need to replace those parts, you know where to get them.
2- Rebuild the
"unavalable" fuel inlet valve with the avalable neoprene
tips from 240Z inlet valves. Much cheaper to purchase a set of
240Z inlet valves than the entire "conversion" to 240Z
floatbowl top described above. The needle portion of the valve
(the neoprene tip) is the only portion that is likely to wear,
not the metal seat. While the entire valve/seat assembly for the
tuna can SUs isnt available in USA, the neoprene tiped needle
from 240Z inlet valve is identical to the tuna can needle. The
rest of the 240Z seat isnt interchangeable but you can buy the
entire 240Z replacement seat/needle and just use the needle to
"rebuild" your tuna can
valve, simpler and cheaper than buying entire new floatbowl tops. Might also be possible to replace the entire fuel inlet valve with aftermarket "gross-jet" valve, these come in lots of different threading and length configurations, it should be able to select a suitable replacement valve from their catalog.
3- Go to the junk yard and pull the float bowl covers, floats, and needle valves from a Z-car dome-top SU setup. The Z-car guys will hate you, but this is probably the cheapest method. You may still have to replace worn needle valves with new ones, but at least you can easily order parts now.
Click for a list of parts that are available from Nissan.
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As I said at the top of this page, the flat-top SUs were imported carbs bolted to imported L18SSS motors. If your set of flat-tops have ever been run in the States, they probably have had the smog equipment fittings removed already. If this is the case you'll find a small, triangular aluminum plate screwed in place just behind each dome/tuna can. If so, then your flat-top SUs are desmogged and are the functional equivalent of dome-top carbs. They're just a little more art-deco in style.
However, if you find a triangular plate with hose connections/pipes pointing upward, then your carbs have not yet been desmogged. (a third alternative is that you have nothing covering the holes in the SU bases, in which case just follow along) A third piece of smog equipment is the hose connection/nipple screwed into the balancing tube in the middle of the manifold. Refer to the Nissan Motorsports Schematic diagrams of the SSS SUs to visually see what needs to go.
The only hose connection you need for the carbs to perform properly is the distributor vaccum advance that is pipped off the front carb. (Look on the float-bowl side of the front carb and there will be a small vacuum port. Connect this to your distributor when you bolt the carbs onto the motor.) The hose connections were connected to various devices in order to control emissions. Remember when you had to run hoses and backfire valves around your engine compartment every smog check? Well, same sort of deal.
NOTE: You'll want to leave the PCV valve hooked up to your crankcase and manifold. It doesn't cost you any power to run this and it does help with emissions. In addition the needles are calibrated to run with the extra air from the PCV valve, so if you remove it you might have a bit of a mixture imbalance.
Good luck, and go at it.
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