From: hejl@ssd.dl.nec.com (Mike Hejl)
Newsgroups: rec.autos.tech
Subject: Re: Re: What kind of oil to put in 260-240Z carbs?
Date: 4 Feb 92 17:05:13 GMT

In article <3970026@hpsad.sad.hp.com> ggg@hpsad.sad.hp.com (Gary
Goodlund) writes:

>...another question for the masses:
>   How often should the Hitachi(S.U.clone) carbs need this oil replenished?

I'n not real shure about the Hitachi SU clones but my old SUs rarely needed
"topping off".  However, the Strombergs were notorious for this:  they had a
small o-ring seal at the bottom of the bore that fit around the jet holder.
This o-ring was essentially non-replaceable: the jet holder was held at the
bottom of the bore with one of those one-way spring steel clips (what the
hell do you call those?).  Consequently, I've never seen a Stromberg that
would maintain the dashpot oil for any length of time:  the oil is sucked,
under manifold vacuum, past the o-ring in only a short time.  This is also
the main reason that the diaphrams in the Strombergs go bad (SUs and the
Hitachis don't have diaphrams). The SUs, on the other hand, have a simple
blind hole for the bore with no "out" for the fluid except at the top.


>My '72 240-Z has about 135K miles on it so I'm sure the wear on the throttle
>shafts and carb bodies is causing increases over the normal consumption.

This won't cause the symptom you're observing.  It will cause other problems,
however.
-- 
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=+
| Mike Hejl    NEC America, Inc.  Switching Systems Division  Irving, Texas |
| hejl@ssd.dl.nec.com    I don't even speak for myself, much less NEC...    |
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=+

From: lusky@hal.gnu.ai.mit.edu (Jonathan R. Lusky)
Newsgroups: rec.autos.tech
Subject: Re: What kind of oil?
Date: 4 Feb 92 06:51:31 GMT

In article <1992Jan31.182637.24937@hubcap.clemson.edu>
lharris@hubcap.clemson.edu (Larry Harris) writes:

>quang@YOSEMITE.CSUFRESNO.EDU (Quang Ngo) writes:
>
>
>>I have a datsun 260Z year 74.  It has 2 round carburators (I think
>>that's what they are called).  Each has a cap on it which I can be turned
>>to open/close.  Oil is supposed to be in there.  I notice the oil
>>is almost empty.  What kind of oil do I put in these things?  I stopped
>>Kragen's Auto Parts store over here where I live and asked the guys and they
>>had no idea.
>
>These are oil bath air filters.  My friend has these on his '71 240, but
>I forget what oil he uses in them (It seemed like a heavy weight if I
>remember correctly), but he found out by asking at a Nissan dealer.

Wrong!  The oil is for the purpose of dampening the open of the slides when the
throttle is opened quickly.  This effectively richens the mixture.  It's
the constant velocity carb equivalent of an accelerator pump.  I'm not
sure what weight oil belongs in them, to some degree its a matter of tuning.
One thing to note, tho, is that just because the car is a 74 260Z doesn't
mean it has 74 model (Hitachi) carbs.  Most of the 73 & 74 Z's I have seen
have early model (1972) SU Carbs.

From: spidell@hpcc01.corp.hp.com (Bob Spidell)
Newsgroups: rec.autos.tech
Subject: Re: Re: What kind of oil? SU carb question, trivia
Date: 4 Feb 92 02:08:44 GMT

(David Basiji) writes:

>lharris@hubcap.clemson.edu (Larry Harris) writes:
>
>>quang@YOSEMITE.CSUFRESNO.EDU (Quang Ngo) writes:
>
>
>>>I have a datsun 260Z year 74.  It has 2 round carburators (I think
>>>that's what they are called).  Each has a cap on it which I can be turned
>>>to open/close.  Oil is supposed to be in there.  I notice the oil
>>>is almost empty.  What kind of oil do I put in these things?  I stopped
>>>Kragen's Auto Parts store over here where I live and asked the guys and they
>>>had no idea.
>
>>These are oil bath air filters.  My friend has these on his '71 240, but
>>I forget what oil he uses in them (It seemed like a heavy weight if I
>>remember correctly), but he found out by asking at a Nissan dealer.
>
>I don't think so.  As I recall, those carbs use a variable venturi system,
>increasing venturi size with increased manifold vacuum.  The oil acts to
>damp venturi movement during rapid changes in manifold vacuum and counteracts
>the momentum of the venturi piston to keep the engine from bogging down
>during fast stomps on the accelerator.  I'd call a Datsun racing conversion
>shop and ask them what they recommend.  

Dave is correct.  Early 'Z-cars' used SU carburetters which are variable
venturi type, as he accurately described.  Later models use SU copies
made by Hitachi.  Most manuals recommend 20W-50 oil, but some British
car mechanics (many older British cars use SU carbs) swear by ATF
(automatic transmission fluid).  I've tried 20w-50, straight 30W,
straight 50W and ATF and couldn't tell the difference.  If the cap
on the dashpot has a small hole then fill 1/2 inch above the inner
piston guide, if there is no hole then fill to 1/2 inch below the
inner piston guide (unscrew the cap, and you'll see what appears to
be a hollow tube inside another hollow tube; the inner tube is the
piston guide).   

Regards,
bs

From: pharvey@mipos3.intel.com (Paul Harvey)
Newsgroups: rec.autos.tech
Subject: SU/Hitachi variable-venturi carbs theory of operation
Date: 12 Feb 92 19:28:41 GMT

These carbs are very simple and very reliable relative to multiple-
jet/air-bleed type fixed venturi carbs. They have only one jet!
So how do they work?

Through a sliding piston arrangement, they maintain the same air velocity
through the venturi, independent of air flow, by varying the size of the
venturi. So as the air flow increases, the piston moves up and the size
of the venturi increases and the air velocity remains the same!

Now, if the velocity through the venturi is constant, that means that the
vacume on the fuel bowl is constant, (Bornoulli's law), and that means
you can exactly meter the correct amount of fuel into the air. A sliding
needle is attached to the piston and it is machined with a slight taper
so that as the piston moves up, more fuel is metered out of of the jet
port so that the air-fuel ratio is kept constant, independent of air flow.
The port is threaded, and a dial turns it so it can be moved up or down
relative to the needle so the idle mixture can be adjusted. Note this
also affects the mixture at all speeds.

An engine needs fuel enrichment during acceleration, so this is done
by a one-way oil damper on the piston. The oil damper delays the piston
on its up-stroke, so in this case the velocity through the venturi is
higher than normal and more fuel is sucked out of the jet port than normal,
which gives you enrichment! The thickness of the oil changes the duration
of the enrichment period as well as the ratio of enrighment.

That's it! 

Compare this to a modern Weber-type carb sometime. I'll let someone else do that!

Sure, there are a few negatives!

This type of carb is not easy to modify relative to a Weber-type which
can me modified in ten-thousand different ways. The only things that can be
varied are the jet and needle valve, the idle mixture, and the damper oil!
Of course, this means if you modify your engine, you probably will not
need to modify your carb to compensate.

The machining of the piston needs to be precise, so some British manufactured
carbs are a problem. Of course, Weber-type carbs need more precise machining!

You do not get the instant throttle response of a good Weber setup, but
acceleration is limited by the speed at which the engine can push the car, but
most people like fast throttle response anyway!

Air leaks around the throttle bushings cause them to perform even worse than
Weber-types with the same problem.

And this carb never really made it through the emissions era, there
are some emissions versions, (73-74 Datsun Z), that mostly are junk and 
cannot be maintained.

But there is nothing like the woosh sound of these carbs, and just imagine
how much fun you can have asking people what type of oil to put in them!

From: bcollins@axion.bt.co.uk (Bill Collins)
Newsgroups: rec.autos.tech
Subject: Re: SU/Hitachi variable-venturi carbs theory of operation
Date: 13 Feb 92 10:19:23 GMT

In article <9012@inews.intel.com>, pharvey@mipos3.intel.com (Paul Harvey) writes:

[ ... good decription of operation deleted ...]
 
|> This type of carb is not easy to modify relative to a Weber-type which
|> can me modified in ten-thousand different ways. The only things that can be
|> varied are the jet and needle valve, the idle mixture, and the damper oil!
|> Of course, this means if you modify your engine, you probably will not
|> need to modify your carb to compensate.
|> 


Don't forget the spring against which the moving piston pushes! That, too,
is changeable. I stick to standard engine oil for the dashpot (20/50 or
whatever): the only annoyance is the regular topping-up required.

I agree with your last sentence above, and would use that as a plus point
for the carb. Due to its simple nature, and the hundreds of needles 
developed for this carb, it is easy to modify; although you'll still need
a rolling road to do it justice, and you may not get a perfect metering
profile across the range (so, who's perfect?).

Anyone contemplating using an SU on a modified engine would do well to 
read the Haynes manual on SU's, or Vizard's book on the "A"-series engines
(for Mini's, etc).


|> You do not get the instant throttle response of a good Weber setup ...


I've never used a Weber, but with a balanced & lightened engine I had
immediate pickup using a single 1.75 inch SU. I couldn't squeeze a Weber
into the engine bay without using a hammer, so I stuck to the existing SU
which only needed a needle change.


As an aside, does anyone know what happened to the Reece/Fish carburettor?



Regards,
	Bill Collins

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