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Thread: are all round tops the same?

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    Registered User clutchdust's Avatar
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    Default are all round tops the same?

    forgive me if this sounds like a rookie question, since i am, after all, a rookie.
    my experience with the Z consists of a little work i did on a friends '72 earlier this year. well, the good news is i'm picking up a nearly identical '73. apparently, the hitachis have been replaced with SU's. i am slightly confused though because although they look very much like the SU's on the '72, i noticed these have 4 bolts holding the 'dome' on where the other ones had 3. at least i think the other ones had 3. am i imagining things? were there two (or more) different types of SU's? anything i need to know about these carbs?

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    There are indeed two types of Round Top SU's. The earliest ones have the 4 screw domes that you mention. They are supposed to be more desirable to racers, though I don't know the reason that they are more desirable. It could be that they do not have provision for the engine coolant to preheat them like the 3 screw one do. The 3 screw Round Top SU's are from later 240Z's, like in 1972 (prior to the Flat -top Hitachi's being used in 1973) You may be able to find more info on www.ztherapy.com. They are reputed to be the best rebuilder of SU's in the country. Good Luck!
    Last edited by Bambikiller240; 11-04-2003 at 12:30 AM.

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    The main reason the 4 screw are more desirable is that they use 4 screws to mount the dome, which keeps the dome alignment in the proper postion relative to the carb body, which lessens the chance of piston binding. It just makes it less likely the dome can be installed off center enough that the piston could possibly bind in its travel.
    "If it weren't for fools, the rest of us could not succeed." Mark Twain.

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    Registered User clutchdust's Avatar
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    thanks guys. sounds like either way, i'm golden! right?

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    Hi guys,
    Bruce Palmer here. I'm the voice on the phone a ZTherapy and wanted to say hello and indicate to everyone we are going to try and set aside the time to get more involved here. We have even invited Mike to come to the shop to see what we do. You might want to take a miute and read the owners bio page to get a feel for who Steve and Pam are all about.

    To further highlight this discussion, Hitachi built the Datsun SU under license from SU in the UK and between you and me, we think they did a better job than SU did. Having been involved with ZTherapy since Steve and Pam bought the biz, I can also testify to the fact that ZTherapy is putting out a better product today than was available previously. That's all I'll say on that subject.......

    I personally have put about 400,000 miles on SUs so have a fair country knowledge of the product so run your questions by here any time and if I don't have the answer, Steve will.

    Cheers
    Bruce Palmer
    sales@ztherapy.com
    Bruce Palmer
    Salem Or
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    www.ztherapy.com
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    philo "Z" opher Zedrally's Avatar
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    I believe that there is also a smaller diameter SU that was used in the triple ''S' es. From memory they have a smaller suction chamber and throttle body, and can be confused with 'zed SU's.
    Anyone with further info?
    Mike of the Mire

    73 240Z Rally
    77 260Z Touring

    Bogged but not beaten

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    Default It's true....

    ... there are 38mm (1 1/2") round & flat top Hitachis that were found on the 1600 Roadsters, 1600sss 510 and the 1800sss 610. All very good carbs but easy to tell from the 44mm Z & 2 liter roadster variety because they only have a 2 bolt mounting flange to the manifold. There was also a setup made for the Datsun 1200.....
    Bruce Palmer
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    Unhappy 3 & 4 screw SU's.

    I was under the impression that the eariler ones were 3 screw and later ones were 4 screw?

    Also ClutchDust if your carbs were replaced by the British SU's then they are a slightly different carb as Bruce stated and from what I've read from the Ztherapy website they were not as good quality.

    Personally I'd love to order a set of SU's from Ztherapy but after converting to AUD$ it's not worthwhile I'm better off buying the video only and say the SM needles then trying to build my own Ztherapy set in Australia.

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    Default Now I'm getting...

    ... confused. Gavin, are you saying you have HS-6 1 3/4" SUs on a Z? Not saying it can't be done just never heard of it being done. Does the fuel line from the float bowl to the bottom of the body have a wire spring wrapped around the fuel line?
    Is there a little brass tag on one of the float bowl lid screws with an AUD # stamped into it? Just curious. After we do the machine work to install the roller bearings, one of the major quality differences between the Hitachi and the SU is pretty much negated so after having been "ZTherapied" they are both killer carbs.
    RE: 3 screw or 4, outside of 1 less screw and the water port on the later 3 screw carbs there isn't $.13 worth of difference between 'em.
    Bruce Palmer
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    Originally posted by Bruce Palmer
    Hi guys,
    Bruce Palmer here........................... Having been involved with ZTherapy since Steve and Pam bought the biz, I can also testify to the fact that ZTherapy is putting out a better product today than was available previously. That's all I'll say on that subject.......
    Hi Bruce:

    Without getting into a BASHING of the previous ZTherapy owner or his product; can you enlighten us on what changes have been made to the SU's currently being sold by your company to make them better than before?


    BTW, Were you at the MSA show in 2002? If so, I may have met you on the bus tour to the Nissan Museum.

    Regards, Carl

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    Talking Will go into further detail later.

    At the moment I think a picture is a 1000 words as im in a hurry and will disect what you just wrote later bruce.
    Here is a shot of my engine don't know if that help much.

    I had a set of British SU carbs on one of my z's I will take a photo of them to show you I don't know much about em' just they were not as good for tuning and seemed to not stay in good tune.
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    Thumbs up And another.

    Not sure if these answer your question I will try take a photo of the british ones not on this car or any car at the moment for you later.
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    Default Hey, Carl, Happy to oblige....

    .... and You are right this is not intended as a bash, only a statement about the process being improved through the talents of another craftsman. Gotta be real careful here......

    Steve was able to, in about the first 3 days they had the company, redesign the machining involved in the installation of the double sealed roller bearings allowing tighter tolerances in assembly and tighter seal around the throttle shaft and butterfly.

    We were in Orange last year and this, but because of my other duties to Zero Rust, I fly down for the show and come home after. Steve and Pam were most definitely on the tour last year though..... Good excuse for a road trip for them. Don't know about next years scheduling with MSA and the national.
    Bruce Palmer
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    wow, quite the busy little thread we have going here!
    since i know next to nothing about these carburators, bruce can you explain to me what the biggest advantages and disadvantages are to them in stock form. and then tell me what you do to fix that (and an idea of how much that costs).
    at this point, i just want to get it running well enough and focus on the suspension and brakes. then, once that stuff is done and i have a few bucks to spare, i'm finding a chevy 4.3 v-6!
    love my chevy power!

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    Default What about 30 something year old SUs?

    And I'm using "SU" interchangeably with Hitachi.......

    These carbs came new with bushings on the throttle shafts and that fit, leaked some air the day they were new. In the ensuing 30+ years they have not gotten any tighter, so this is the Achilles heel of these carbs to this day. Once you start leaking massive quants of air past the shafts, the carbs will run rich or lean and back and forth just based on the air leakage taken place past where the air and fuel are supposed to mix.

    Steve machines the carb bodies (after polishing) to accept double sealed roller bearings and hand fits these along with the shaft and butterfly. It is really interesting to watch this process that takes what was a fairly crude casting design from the manufacturer and turn it into a trouble free trouble free carb.

    Okay, the complete verbiage is on the web site that covers all the steps we go through on the carbs, but what I'd like to do is lay out for anyone who is looking at their carbs wondering what to do next with yours.

    For most new customers we get, I suggest they buy our JUST SUs video (the 2 tape version for $20 is better quality) and use the video as a teaching tool to go through your carbs, clean them up, check out the few components that are there, put them back together, run 'em and see what you've really got. Cleaning alone can solve a lot of running issues. If they run well, you just saved $600, if the shafts and bushings are worn out and the general condition is too far gone, then you know you are spending the money because it needs to be spent.

    Here we offer several ways to go too. The complete reman deal gets carbs ready to bolt on (w/3 yr warr), or you can buy just the machined bodies and swap your stuff over or add the Tune up kit that essentially has all the consummable "wear parts" that need to be renewed in a real rebuild. Our tune up kit should not be confused with the "rebuild kits" from the parts store. Kit contents are spelled out on the web site so I won't go into that here.

    This is the way we approach helping anybody get started with squaring away a set of 30 something year old SUs. If, by reading through this and gaining the feeling that it is maybe not an undaunting task, you can take the next step, then we've helped.

    SUs are not rocket science. They are really very simple in design and function and once the 3 or 4 things on them that need renewing and adjusting are right, they are hard to beat. I am a life time subscriber to the "KISS Thereom".......

    Again, the JUST SUs video that Scott did is a real valuable training aid. It's not fancy from a production standpoint, but he drilled the content.

    Sorry this has run on so long but I go through this over and over and thought if by spelling it out here in writing, it will shorten anyone's learning curve, then it needed to be done.
    Bruce Palmer
    Salem Or
    Sales@ztherapy.com
    www.ztherapy.com
    503-587-9800

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