Whats the difference between flat top and round top SU Hitachi carbs, and also what prices would I look at for rebuilding and are they good in regards to keeping them in tune?

The Hitachi's seem to be a well made carb, basrd on the Skinner Union [SU] design.
The square top carbs incorporate a few more passages for emissions control functions. I do not know if there is some significant reason for the change in shape of the tops.
Either type will be a sunstantial improvement over the single carb setup, & if you have the later types, it would be a good idea to learn what all the other fittings actually do before blocking things off.

I ran mine for many many years & tuning was a set & forget deal. I will be rebuilding them & placing them back in service. I love them.

ive got two sets, will try the flat top ones first as they are the same as my old set up on the a12gx motor. have to run the new motor in first on standard carb before playing around with su's though

Mine also needs a clean and a rebuild, I think.

When I pull away from standstill my car chug chugs asif someone turned it off and back on again.
It cannot be points as they were replaced and tuned a couple of kilometers ago (less than 5000km and it still did it after the new points). The carbs were cleaned really a long time ago.

My question to add to this forum.
What must one look at when cleaning them and what is involved in a rebuild of these carbs.
Any hints or tips will be appreciated.
I also am thinking of replacing my needle and seats with genuine nissan one's.
How do I get the float level 100% correct.
(Actually I know how, I need to know what is the correct float level)
I unfortunately do not have the factory tools to tune this part.
Thanks

Make sure that the dampers have oil in them. You undo the large plastic knobs on the tops & carefully withdraw the plungers. Most people use Auto trans fluid, but i was too cheap & used engine oil instead. It doesn't really matter too much if you overfill them a little as the surplus will find it's way into the carb, & eventually pass through the engine.
If the dampers have no oil, then you can have all sorts of problems, including something like you describe.

If the carbs are not flooding when the engine is idling, then the needles & seats are probably just fine.

Oops. I forgot to mention.
I did check the oil in the dampers.
They where on the dot.
I think I must rather remove and clean them up nicely again.
It was done about 8 to 9 years ago.
I think it is time to clean up again.

If you are referring to the SU's used on the early Z cars, all I can say is that this group refers to the flat tops as "boat anchors". That's about the only use they seem to get out of them. You may find this site helpful: Z Therapy SU Carb Spotter's Guide

Z Therapy rebuilds these carbs (round tops only!) to better than new condition. I've only heard good things about them. In fact I have a set of their carbs waiting to mount up to my Z if I ever get it finished. That's why for now I was looking around for a SU->A14 manifold. Anyone???
Michael

I can really only go on my experience with my 260Z when I had it. It had the flat tops as standard which gave me nothing but strife, tuning wise. I then got hold of a set of round tops from a 240 and they were so much more reliable, they were a set & forget.

If you have a choice, go with the round tops.

With regard to the oil that you use in them, go with the thinnest you can get, I used to run the Singer sewing machine oil in mine.

Everyone bar my old man has always told me
"get the lightest oil you can in the damper"

No one has ever told me why though.
Considering the function of the oil:
Basically it lets the whole shaft behave like a shock absorber.
The oil passes around the restrictor valve on the end of the screw in stem.
It does this to soften out the up down (open/close) action of the shaft (what's it's bloody name!?!).
So what happens if you vary the weight of the oil?
Heavier oil means it is less reactive to velocity and in turn vacuum changes in the manifold.
Vica versa for lighter oil.

So wouldn't lighter oil then be better?

Carrying it through to it's extreme you'd have no oil.

But then you'd end up with sudden weak mixture as the lighter air has a lower inertia than the heavier emulsified fuel.

Consider a sudden stamp on the throttle.
With a very light oil you'd end up with weak mixture until the fuel/air charge evened out with a steadier intake velocity.

With a heavy oil though you'd end up with a rich mixture.

Really it seems that there's no perfect oil, more a compromise oil weight.

If my engine was highly tuned I'd lean towards a richer mixture for safety reasons.

I have played around with different weights from ATF through to Gear oil and the difference was never really noticeable. The big qualification was that the needle and seat were never changed on a dyno and the carbs themselves may not have been ideally suited to the engine or been in the best shape.

I'd assume that choosing the right oil might be subjective to the engines state of tune, intake velocities at differing RPM and the closeness of the needle and seats fuel delivery curve to the needs of the engine.

I'd love some feed back on this one. Preferably backed up with a bit of scientific argument more than the "i did this and it worked" version (valid though it may be).

The more i delve into the instant by instant behavior of carbs the more i see why computer controlled fuel injection is so damn good!!

Been reading a lot about weber carbs lately. I can see why so many people have problems with them. They have a significantly more complicated system of fuel delivery before activation of the idle/main jets. Great if they are perfectly set up on a dyno (such as happened when a major manufacturer put them as O.E. on their cars) but it'd be a one in a million fluke if you wacked them on your car and they worked perfectly from day one.

But we were talking about S.U.s!

As I understand it, the oil helps slow or delay the rise of the piston when you step on the gas. This will cause more fuel to be sucked out of the fuel jet untill the piston rises to allow more air to come in. The delay is apparently very short.
I've heard this refered to as the Accelerator Pump effect.
So using really light oil should have the same effect as disconnecting the accelerator pump in a Weber.
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