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Thread: Flattop carbs vs. round top carbs?

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    Default Flattop carbs vs. round top carbs?

    i own a 73 240z that has flat top carbs. i also bought another 73 240z for parts the carbs on that has a rounded top. which are better or is ther no difference.

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    Flat Top SU Carbs = Boat Achors

    Round Top SU's = Good Carburetors

    www.ZTherapy.com

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    Registered User seerex's Avatar
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    ditto sell the the flat tops on ebay )
    72 240z Yellow
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    Nickname "Rusty"
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    71 240z Red
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    Getting Done

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    Biafra for President e_racer1999's Avatar
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    Quote Originally Posted by seerex
    ditto sell the the flat tops on ebay )
    you'd probably get more $$ if you melted them down or sold them as boat anchors
    Jason King
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    In 73 the emission laws changed and affected the 73 Z. The flat top carbs were designed to meet those emission requirements. You will notice that the flat tops are almost impossible to work on. It's even harder to find parts.

    Like everyone has told you the round top carbs are a much more desirable carburator. They are easy to work on and are very good.

    If your flat tops are still working keep them on your car for now. Rebuild the round tops or get them replaced by Ztherapy like Carl suggested. You can't beat there work, anywhere. Once you get the round tops done you can throw those flat tops in the trash.
    The 1973 240Z 165540 Slightly modified. L28, 5sp, cam, headers, MSD 6AL, power windows, power door locks, leather seats, custom this and custom that.

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    Quote Originally Posted by Ed
    If your flat tops are still working keep them on your car for now. Rebuild the round tops or get them replaced by Ztherapy like Carl suggested. You can't beat there work, anywhere. Once you get the round tops done you can throw those flat tops in the trash.
    Very sound advice from Ed. Bare with your Flattops only as long as it takes to have the early 3 or 4 screw set reworked. It's too bad Ztherapy won't take the flats as cores, I guess they are not in the aluminum recycling buisness!
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    will the round top increase the horse power? or does it just make the car run better.

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    philo "Z" opher Zedrally's Avatar
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    Disreguard all those comments the wowsers wrote.
    Perform a search using flat tops...and reach your own conclusion...

    Reinterating what has previously written, flat tops when overhauled [like round tops] perform just as well. Simply put, very few people have the patience and skill to perform a succesful overhaul on these units, hence they are looked down on in preference to round tops.

    I run both, and believe me the flat tops are not to be sneized at.

    The choice is totally yours...

    MOM
    Mike of the Mire

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    77 260Z Touring

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    Quote Originally Posted by Zedrally
    The choice is totally yours...

    Of course it is your choice. However, the consensus of the vast majority of people who have had Flat Tops on their own car is that they are JUNK when compared to Round Tops.

    MOM's Flat Tops may be manufactured to perform differently than what you have in your USA delivered Z. (since he lives in another country far, far away from US Emissions standards)

    Make your own decision, but don't ignore the opinions of the many, due to a few lone discenting voices.

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    I know a couple of guys with flat top carbed cars. One a 240 and the other a 260. They both run fine and I think they both kept the stock carbs for originality. I was glad to see Zedrally's post because both of these guys work on their own cars, do their own carb work, etc. They both learned these carbs because nobody really knows anything about them. All everybody seems to know is what they've heard. My opinion is that the flat top/boat anchor thing is just like anything else. If you say it often enough it becomes the truth or at least the accepted truth. Flat tops are no better or worse than anything else on these cars. 'Patience and skill' is what MOM said. After working with Nissan/Bosch type f.i. for years, I've learned to troubleshoot it myself because it seems to be such a mystery to so many so called skilled mechanics. I could have also thrown away the f.i. and put 'Z Therapy' s.u.'s on my car when I had problems with it but what would that really have accomplished. Any thing is 'JUNK' if it doesn't work.

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    The fact remains that they were not designed to provide power or performance. They were solely used to satisfy the EPA requirements for emissions. They were used here for 2 years and dropped like a hot rock by the people (Nissan) who chose to use them on their product.

    Did the HP of the L24 drop when Flat Tops (and the late E88 head) were added? Absolutely.

    Can they be made to operate satisfactorily? Probably.

    Can "Patience and skill" overcome the inherent design of the Flat Top carbs? I strongly doubt it.

    Do Round Top carbs out perform them? Regularly.

    When was the last time any of you heard of a Z with Flat Tops running 12 sec quarter mile times like Norm? N.E.V.E.R.

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    philo "Z" opher Zedrally's Avatar
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    Each to their own here.

    There is no "right" answer, but I'm surprised at the ferocity of the response?

    My reasons for leaving the 260 with "flat tops" was purely originality. Hence, my surprise after I overhauled them. Prior to this performance was poor, sluggish, you name it.

    After the Overhaul they performed as good if not better than the 240. Go figure?

    As I wrote above it is purely a Originality issue here, else I would go for FI.
    Mike of the Mire

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    Mike,

    I think my response is being seen with more "ferocity" than I intended. I was just pointing out the reasons / facts as I see them. Indeed to each his own.

    The only ferosity I have in this area is shadetree mechanics (I don't recall any of us being ASE certified, or professional auto mechanics and yet disagreeing with the wisdom of the manufacturer, and professional mechanics who serviced these cars. Who had many problems with these things, even to the point where they issued service buletins, and dealers illegally replaced the Flats with Rounds when their "professional" mechanics could not resolve the customer complaints. It's not just a bunch of "shadetree" mechanics who have low opinions of these carbs. The manufacturer didn't like them either.

    That's it, that's all I was trying to get across. If they work good for you (or anyone else), great. My first Z was a 73 and had Flats on it. I didn't have any reliability issues, and performance was fine (but I had nothing to compare against). When I rebuilt my 71 Z (using the 73 Z motor) I switched to Rounds, and I noticed a marked difference in the seat of my pants, and the smile on my helmeted face.

    Originality is something I understand and value, but that wasn't what Shane4golf seemed interested in. F.I. (properly set up) certainly should out perform either carb.

    Peace?

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    Former frequent poster sblake01's Avatar
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    Regardless of who doesn't like them, they do work. Evidentally, pretty well if you know what you are doing. This opinion comes from one of my friends who IS an ASE certified mechanic and has an auto repair shop that specializes in Datsun/Nissan/Infiniti and has been in business for over 30 years. He can make flats perform as well as rounds. I have discussed this with him many times. That is why I don't share the common opinion on round vs flat. According to Elvan, with the technology that is available to properly rebuild a carbureator today vs 30 years ago, the is no appreciable difference between a round top and a flat top, performance wise. I know that's hard to accept considering the general consensus, but I've seen, rode in, and driven examples that would seem to verify that statement.
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    Quote Originally Posted by sblake01
    Regardless of who doesn't like them, they do work. Evidentally, pretty well if you know what you are doing. This opinion comes from one of my friends who IS an ASE certified mechanic and has an auto repair shop that specializes in Datsun/Nissan/Infiniti and has been in business for over 30 years. He can make flats perform as well as rounds. I have discussed this with him many times. That is why I don't share the common opinion on round vs flat. According to Elvan, with the technology that is available to properly rebuild a carbureator today vs 30 years ago, the is no appreciable difference between a round top and a flat top, performance wise. I know that's hard to accept considering the general consensus, but I've seen, rode in, and driven examples that would seem to verify that statement.
    I must agree with Stephen. I started out with the flat tops on my 73. Over time they slowly deteriorated (cold starts were a pain). I worked on them trying to improve there performance but the parts were expensive and hard to obtain (compared to the round tops). But they did work. Once I replaced them with my rebuilt round tops I can honestly say there was no performance gains.

    Which do I like better? The round tops. Why, because they are easier to work on. Which is why they are more popular. Try adjusting the float on the flat top.
    The 1973 240Z 165540 Slightly modified. L28, 5sp, cam, headers, MSD 6AL, power windows, power door locks, leather seats, custom this and custom that.

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