One of the great things about SUs is that they hold up well over the years. Their operation doesn't depend on a bunch of small jets, rubber diaphrams, or gaskets, so there is little that can deteriorate simply due to age. Stromburg carbs are a similar design to SUs but do depend on a flexible rubber bellow, and so are not as reliable.
General rebuild kits can be obtained, though probably not for your specific
carburetor. If you know you have 1600 Roadster carbs, then you can order
a kit from Napa and the like. Z SUs and 2000 Roadster SUs also have specific
rebuild kits available. Depending on the maker of the rebuild kit, it might
include a float bowl gasket, various spacers and washers, a fuel needle
and seat assembly, and possibly needles. Most of these parts are available
individually from Nissan, and similar parts should be available from a good
British/foreign car parts store.
If you are unable to locate a rebuild kit or find it terribly expensive (~$25 per kit, two needed), chances are that you can still get the SUs working well without a kit. Use gasket paper to make new gaskets for the float bowls, insulator spacers, and air filters. Clean and check the operation of the fuel float and seat. Make sure the brass floats are not cracked and leaking (there should be no fuel in them!). If you have float related problems, replace them with 1600 Roadster parts. These are now a new and improved design.
Check the throttle shafts for wear, they shouldn't leak air. If they are
worn, have a machine shop re-bush them (~$100 per pair). Read Z Therapy's
take on rebushing SUs so you know what to ask for at your machinist.
Dissasemble, soak, and clean the carb body and slide piston. Check that
the needles are not bent by holding the piston and slide chamber upside
down and spinning the piston and needle, the needle shouldnt wobble. If
you plan to re-use the needles I'd suggest not removing them from the pistons
because they get stuck and bend when you try to pull them out. The needle
and jet can wear from sliding but you can compensate for this by raising
the jet adjustment as needed. New needles and jets are a definite fix but
in my experience; new needles alone may fix jetting wear problems. SU needle
part numbers can be found here.
When the piston slides are reassembled to the carb, you should be able to
raise the piston up and release it and hear a distinct metalic clunk as
the piston hits the carb body. If the slide gets hung-up, check alignment
of the slide chamber on the carb body. The later flat-top SU slides are
self aligning because they have little rubber points that align the slide
to the carb body as it is assembled. The earlier round tops must have the
slide chamber carefully positioned. Slightly loosen the 4 screws that hold
the slide chamber and wiggle it untill perfect alignment is obtained (when
the dropped piston makes the clunk) and then carefully tighten the screws.
Bent needles will also cause the piston to bind.
The Haynes weber/stromburg/SU tech book is a good reference if you have
never dissasembled a SU before. ($12 at pep-boys). Has good info for rebuilding,
synching, tuning the carbs but does not list part numbers specific to the
Hitachi built carbs.
If you are very serious about returning your SUs to Better-Than-New condition, you can go to the Z Therapy web page at www.ztherapy.com. They have links to their SU Rebuilding and other SU related services and products.
The folks at Z Therapy offer various stages of remanufactured SU carburetors. You can get a rebuild kit, get your current SUs rebuilt with new shafts, or get the whole kit-and-kaboodle done, down to authentic zinc plating of parts. Yes, it does cost, but what you get back looks to be a pretty good product. They also offer a pretty good warranty as well. If you have the means, check them out.
So, say you have your SUs all torn apart, but you can't figure out how to put them back together. Or maybe you bought a set of SUs but didn't realize there would be some assembly required and all you have is a box of funny looking bits and pieces. Well, you're in luck. The fellows at Z Therapy have created a few pages to help you out. This link to their site gives you a parts breakdown and basically a step-by-step assembly order of the parts. This isn't an assembly guide, but it's darn close.
Moss Motors, Ltd., America's oldest and largest British sports car specialists. They obviously know a thing or two about SU carburetors. Check out this Adobe.pdf page from their MGB catalog (You'll need Adobe Acrobrat Reader for this). It shows a good-resolution, zoomable exploded view of an SU HS4 carb, with all the parts labeled for ordering. You can use this to help your assembly woes. In addition, quite a few of the British SU parts will fit Hitachi SUs, helpful if you can't find the Hitachi-SU parts. Contact Moss Motors Ltd. if you have questions about their parts.