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Thread: Flattop to Roundtop carb conversion. Help with hoses, intake manifold, etc please!

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    Default Flattop to Roundtop carb conversion. Help with hoses, intake manifold, etc please!

    Hi, so I am in the process of converting my car from flattop to roundtop carbs. I have the intake manifold, balance tube, and carbs for both setups. The engine is an L28 that came with flattop carbs on it and a mess of hoses and wires. I need help identifying hoses and figuring out what I don't need, etc. I have removed the flattops and their balance tube, then bolted on the balance tube for the roundtops to notice that there's no spots for some of the hoses that are there. I have pictures attached. The next thing I noticed is the roundtops won't fit on the manifold and need the larger bolts of the manifold that came with them, so does this mean they won't work on the n33 manifold? Next, there's no spot for the coolant hose, does this mean the roundtops don't need coolant, and if so, how would I remove this hose and block it? As you can tell I am pretty clueless but thanks for all the help!
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    Dangit!! There was a drop dead killer thread on here within the last week showing step by step photos with discussion on this very subject. Hateit when that happens. Bueller? Anyone?

    Why isn't this ibn the carb section?
    Bruce Palmer
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    I searched the forums but couldn't find anything, as for being in the carb section, I didn't realize there was one sorry. Any help is appreciated, the flattops were poorly installed so I am almost positive there are extra hoses than on a normal flattop to roundtop conversion. It is a 240z that has an l28 with flattops which I am now converting to roundtops so its technically gone through two intake systems before this (fuel injected and flattops).
    Last edited by yoshi_w; 02-02-2012 at 09:20 PM.

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    Here's the thread that Bruce was talking about. It's not that detailed as far as exactly what to do with the hoses, but it does give a good mechanical overview.

    http://www.classiczcars.com/forums/s...apy-round-tops

    I'll get to the details of your hoses in a second. I have to open up your pictures first...

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    Do you have copies of the service manual(s)?

    You need two copies... One from 74 and one from 72. The 74 will help identifying the hoses from the flat-tops, and the 72 will help identify what you need once you're done. The manuals aren't perfect and there will still be questions, but they will help a lot.

    OK, Here's your first pic. The two hoses that come off the original flat top balance tube... The larger one should be for your brake power booster, and the smaller one should be the vacuum source for your heater system. If you want power brakes and a heater that works, you're going to need to figure out a way to get vacuum to both of those hoses.


    Your second pic... A little blurry and tough to figure out what you're identifying, but I'll give it a shot. Going clockwise starting in the upper right.
    a) Brake booster - You need this.
    b) Coolant out of rear flat top returning to water pump - Pull the hose and cap it off near the firewall.
    c) Coolant supplied from thermostat housing into front flat top - Pull the hose and cap it off at the thermostat housing.
    d) Bundle of snakes identified with star - Can't tell. Need a better pic.
    e) Two fittings on front intake manifold - F = Throttle opener system, R = anti-backfire valve system.
    f) The smaller starred circle - Can't tell. Again, need a better pic.


    The red and yellow fittings you have capped off in this pic are for the throttle opener system. It's function is to reduce emissions by opening the throttle a little under very high manifold vacuum. There are some good pictures of that system in the FSM. Look there first and after that, if you still have questions, come back and ask.


    On your last pic, the tube in question is coolant supply to the balance tube. Coolant is supplied to the tube you have identified, flows through the balance tube under the EGR valve, and then returns back to the engine through the cut off tube sticking out the bottom in your pic.


    Study your service manuals.

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    Pictures worth 1000 words?








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    Is this a great forum or what?
    Final analysis. Our Euro balance tube has provision for PCV and brake booster and one cartb has a tap for vacuum to the distributor. All the rest of that is just supefluous..... Flat top manifold, early balance tube, long studs, thick insulators, round tops, heatshield of your choice and you should be good to go. Oh, and the early air cleaner. Been done thousands of times.
    I am still surprised about how many cars are just now getting around to making the swap as NISSAN did the swap for new owners when the flat top cars first hit the streets, to keep the new owners from mutinying.
    Bruce Palmer
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    You guys are both amazing help, especially captain obvious hahah. You seriously are not only helping me work on my car, but its also motivation at the same time, cause when you don't know what to do you just lose the spark to work on the car. Its also amazing how you labelled the function of each hose, very helpful and its nice to look in the engine bay and know what everything is there for. Thank you soo much I cannot express in words how helpful this is! Ill try to nab a shot of the unidentified hoses by tomorrow. Thanks again!

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    I managed to work on the car and bolted up the intake manifold that came with the roundtops since it has the longer studs. I also managed to get some pictures of the hoses which I am attaching to this, the numbers correspond to show what they are attached to. Also, after everything is hooked up, is there any specific starting procedure? or should I crank it over until it starts? Thanks for all the help again!
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    I think those two tubes you show in the last two pics connect to your flow guide valve. It's an early crude evaporative emissions control system who's purpose is to capturing and store the fuel vapors that evaporate out of the gas tank when the car is not running. It's the predecessor to the carbon canister containment systems that are on newer vehicles. Plenty of details in the fatory service manuals.

    By the way, I know you've got an L28, and I know you are pulling off flat tops and replacing them with round tops, but what year is the car?? With flat tops and a flow guide valve... I'm guessing it's a 73?

    If it's a 73, then you can find info on the flow guide valve on page EF-20 in the Engine Fuel section.

    As for starting it... When you get to that point, you could just crank it and (hopefully) go, but sometimes it can be a pain to get enough fuel to the carbs to get it to catch in the first place. I would recommend filling the carb float bowls with a funnel and short piece of tubing first, before you connect up the real fuel lines to the carbs. That way you shouldn't have to count on the fuel pump until after the car is running.

    But before you bother even putting the carbs on the car... Have you checked your round top float levels?

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    It would have been easier to screw the longer studs out and put them in the other manifold than it was to swap manifold. A small pair of vice grips should start'em and finger 'em out from there. I know, too late now. Hope you install seals uo alright and you have no leaks.

    Oh, and I'm one of those who surmise the flat top manifold "might" flow a little better than the early ones. That's a totally undocumented "nothing behind it" surmisings based on just looking in the intakes side by side......
    Bruce Palmer
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    Quote Originally Posted by Bruce Palmer View Post
    It would have been easier to screw the longer studs out and put them in the other manifold than it was to swap manifold. A small pair of vice grips should start'em and finger 'em out from there. I know, too late now. Hope you install seals uo alright and you have no leaks.

    Oh, and I'm one of those who surmise the flat top manifold "might" flow a little better than the early ones. That's a totally undocumented "nothing behind it" surmisings based on just looking in the intakes side by side......
    FWIW, never use vise grips on an exposed stud. Instead, lock together two nuts by spinning them into each other and then proceed to loosen. I've learned this lesson more than once...

    Good luck Yoshi, hope you get it started without trouble! Filling the floats as Captain suggests is a good idea if you want to get it started without cranking forever.
    2/74 260Z

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    Needle nosed Vice Grips in that un threaded section in the middle works fine.
    Last edited by Bruce Palmer; 02-07-2012 at 10:20 AM.
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    My car is a 1973, and I haven't checked the float levels yet, but I'll be sure to do that. I appreciate all the help you guys have provided! And as for starting the car, are you saying I give the carb fuel, then hook up the fuel lines? And also, do I need those emissions devices? because Im trying to keep everything as simple as possible, thanks!

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    To prime my fuel system when fully dry from tank to carbs, was to remove the valve cover and manually work the fuel pump arm until the bowls filled. This way you can see if the fuel pump works without starter / engine noise, you can see the fuel go into the fuel filter and it will help spot any small leaks in the system before firing up.

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    Yes, I'm saying that you use a short piece of tubing to connect a small funnel directly to the input nipple on the carb float bowl, and then slowly pour gas into the funnel until the carb won't take any more. Then after the bowl is as full as it's gonna get, do the same thing to the other carb.

    After you're done with both, set the funnel and short tube aside and connect up the fuel lines from the fuel rail and then try to start the car. If everything is working properly, the fuel in the carbs bowls should be enough to run the engine for at least thirty seconds or more before they need more fuel, and by that time, hopefully the fuel pump is on-line and pumping it's little heart out.

    Your question about "do I need those emissions devices?" is a little tougher to answer. It all comes down to what you mean by "need".

    All of the emissions devices on your car were put there for a reason. That reason is to make the world a cleaner, greener, better place for all those who walk this earth after you. Your car would probably run smoother and perform stronger without any of them in place, so if you look at it that way, you don't really need any of them. However, if you care about doing your own little part to preserve what's left of the environment, then you should try your best to comply.

    Some systems on the car do not cost anything in the way of performance yet do have environmental benefits:
    The PCV system
    The evaporative emissions system
    The throttle opener system


    And then there are some systems that do potentially cost performance:
    The EGR system
    The AIR (Air Injection Reaction) system (the air pump)


    All depends on where you stand and that part is up to you...

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    Quote Originally Posted by yoshi_w View Post
    My car is a 1973, and I haven't checked the float levels yet, but I'll be sure to do that. I appreciate all the help you guys have provided! And as for starting the car, are you saying I give the carb fuel, then hook up the fuel lines? And also, do I need those emissions devices? because Im trying to keep everything as simple as possible, thanks!
    Yoshi, this is Leon, the guy that sold you the carbs. I ran these on my first 240Z with the emissions stuff plugged off. The emissions hoses were cracked and old, and there was no AIR pump to be found on that car, so I decided to plug them off as to alleviate some vacuum leaks.

    The PCV valve should still be in the balance tube, all you have to do is connect your block breather tube, located underneath the distributor to the PCV valve. The flow guide valve hoses should be easy to hook up as well, reference the manual. [EDIT: I think one hose goes to the block breather and the other to the air cleaner, but I'm not 100% on that.] The throttle opener is currently not hooked up, but you could if you wanted to. I rarely drove the car and didn't see a need to make it work, but you could if you wanted to, again the manual will show how it goes (remember to reference '72 and older manuals). If you don't have an AIR pump, and your AIR gallery is not functional or present, then you obviously will not be using the AIR system. There are no provisions for EGR with these carbs, although I suppose you can make that work if you wanted to by using the flat-top balance tube.

    Good luck and let me know if you run into any problems!
    Last edited by LeonV; 02-09-2012 at 09:29 AM.
    2/74 260Z

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    Alright, I shall try this on the next sunny day! Thanks all for the help, can't wait to get the z running again finally, and yup leon, I shall definitely ask you for help, took me way too long to finally muster up the courage to change the carbs hahah. Hopefully all these suggestions work out, otherwise I'll be back on here lol. Thanks again everyone!

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    Quote Originally Posted by yoshi_w View Post
    Alright, I shall try this on the next sunny day! Thanks all for the help, can't wait to get the z running again finally, and yup leon, I shall definitely ask you for help, took me way too long to finally muster up the courage to change the carbs hahah. Hopefully all these suggestions work out, otherwise I'll be back on here lol. Thanks again everyone!
    Not a problem. I'm pretty busy these days, but I could probably manage to stop by sometime if you're having trouble. Feel free to PM me, or give me a call if you still have my number.
    2/74 260Z

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    So I am in a bit of a trouble spot. I am getting kicked out of the garage I have been working in by March 31st, but the car is not running, I need to look for another garage, but I need to find one that lets me work on the car as I'll probably have to bring over on a tow truck, otherwise I could just have a shop finish what I've started. It has been slow working on the carb swap due to missing some stuff and having midterms and lots of studying. I have the carbs and the linkages on but noticed in pictures that I need a spring hooked onto what looks like a heat shield? Is this vital, and Leon, do you possibly have this part? Or alternatively, where could I get this? Or is there anything I can salvage from the flattop carbs? Thanks for all the help!
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    Yes, you need the springs for the throttle return. I've seen many different types of springs used on them, just enough strength to close sharply, but not too hard to push open. The heat shield is for that purpose, there are several variations, the one above has the cut out for a header tube. I have one with out the cut out.

    One of the more interesting spring uses I've seen, was 2 screen door return springs cut down to attach to the front brake line. It worked!

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    We have heat shields and new return springs....
    Bruce Palmer
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    Quote Originally Posted by yoshi_w View Post
    So I am in a bit of a trouble spot. I am getting kicked out of the garage I have been working in by March 31st, but the car is not running, I need to look for another garage, but I need to find one that lets me work on the car as I'll probably have to bring over on a tow truck, otherwise I could just have a shop finish what I've started. It has been slow working on the carb swap due to missing some stuff and having midterms and lots of studying. I have the carbs and the linkages on but noticed in pictures that I need a spring hooked onto what looks like a heat shield? Is this vital, and Leon, do you possibly have this part? Or alternatively, where could I get this? Or is there anything I can salvage from the flattop carbs? Thanks for all the help!
    Yoshi, I'll have to take a look and see if I can dig up a heat shield. I can't guarantee it though, my garage is a mess!

    I can come by and have your car running the same day, we can improvise on a throttle return if you just need the car moving. I commute to Petaluma so I drive through SF everyday. I can stop by and give a few pointers or an extra hand sometime, just let me know if/when and we'll try to schedule it.
    2/74 260Z

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    Leon, that would be massively helpful! I'm on spring break this week, so if you are available any time this week I would greatly appreciate it!

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