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Thread: 260z to 240z flat top to round top su conversion

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    Default 260z to 240z flat top to round top su conversion

    So I have a stock 73 240z with all the original frankenstein'd pieces in the car. I recently purchased ztherapy round top carbs to replace my old flat tops. The car can start and idle but I'm having issues with the old balance tube. It's an n33 balance tube and n36 intake manifold. It still has all the old EGR equipment but it can't be used since theres no where to mount return wires on the new round top carb. Can anyone help me figure out what parts I can plug up/shouldn't mess with? It's beautifully overwhelming haha. Thanks guys!

    I took off two ports as seen in the 5th pic, I'm not quite sure what they did. Wasn't the greatest idea to copy a picture I found on the forums, but I'm going to plug them up. Or should I? Anyone have a picture diagram/write up I could follow?
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    Start by plugging the vac port on the front manifold behind the isolator. Next hook the crank case vent to the PCV (balance tube center). put the air cleaner back on the car, these carbs need a filter to increase the manifold vac to run if they aren't properly tuned. Cap EVERYTHING that isn't being used on the manifold, and hook up the hot water circuit to the manifold, if you let everything get up to temp it will run better..

    My 73 wouldn't run off idle when I did the conversion at first untill I put the air filters on and dialed the carbs in from there. Once I got them really close I was able to take the filters back off and fine tune the carbs.

    Edit; check the bamper oil as well.
    Last edited by 5thhorsemann; 10-14-2012 at 09:07 AM.

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    I've not seen a write-up on "How To Switch From Flat Tops To Round Tops" anywhere. Note that I haven't actually searched the forum(s) for such info, but I've not seen a complete write up anywhere.

    Do you have a copy of the service manual? If not, download a copy.

    Looking at your pic (3468), I see a whole bunch of disconnected stuff, but I'm only going to talk about the ones that would cause you big problems:

    1) Vacuum port on the front manifold just behind the carb - Used to go to your vapor reclamation system. If I were you, I would reconnect your flow guide valve, but if you choose not to, just cap that port.

    2) Holes in the top middle of your balance tube - The one on the left used to go to your anti-backfire valve. If I were you, I would reinstall an anti-backfire valve system and use the port as part of that system. Look for pictures of an earlier round top car for how to connect the anti-backfire system.

    3) Holes in the top middle of your balance tube - The one on the right used to go to your idle air control system from your flat tops. You don't get an idle air control with your round tops, so you can just plug that port.

    4) Black hose coming out of the balance tube below the two ports mentioned above - That used to go to your temperature controlled idle mixture compensation valve. You don't get that privilege with the round tops, so pull that hose off the balance tube and cap that port.

    5) Short nipple sticking out of the balance tube below the two ports mentioned above - That's your PCV valve and it should be plumbed to the crankcase under the distributor near the front timing cover. There's a black braided hose sticking down towards your exhaust manifold... That may be your PCV connection hanging down there.

    6) Your EGR stuff is still intact, but it's not doing anything. It is currently disabled and will have no effect.

    There might be more going on than I see with the above, but look into the above and see what happens. If there's any in particular item you want more info on, just ask.

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    That was really descriptive, really appreciate it. Ill plug what needs to be plugged when I get home from class.

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    Registered User Powderkeg's Avatar
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    I'd call Bruce at Z Therapy. Didn't you also get a copy of the "Just SU's " DVD? If I remember correctly it shows a 260 to 240 conversion.

    Good luck.

    Dave Ruiz
    76 280Z

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    I got the video but the resolution isn't that great. I can't clearly see which ports are/aren't plugged up. I know how to tune it now, but theres no clarification of what certain ports do/what damage the previous owner has done...

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    do you guys happen to know what size the idle air control valve is? I can't find a plug size at autozone and home depot. I hope its not special order.

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    It's a metric thread, just a little smaller than the MIP thread. I don't remember the exact size and pitch but you wont find these in many hardware stores, I got some at an automotive shop that deals with japanese performance motors. Might try the Nissan Dealer.

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    Do you remember if it was an m13 or higher? all they had at autozone was m12 and homedepot m10. I'm trying to find it in the forums but no one says the plug size, just that they plugged it haha

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    just found out theres no such thing as m13, it's funny because this doesnt fit m12 or m14...oi this car.

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    Quote Originally Posted by tmsaws View Post
    just found out theres no such thing as m13, it's funny because this doesnt fit m12 or m14...oi this car.
    All of those designations you are using (M10, M12) are for straight threads. The hole you are trying to plug is probably a tapered thread (not a straight thread) and if that's the case, then none of those "M" size designations are the right ones.

    That aside... My suggestion for any of the holes you are trying to disable would be to screw the original nipple back into the balance tube and then use a rubber cap for the nipple. Might not be the prettiest solution, but get the bugs worked out first, and make it pretty later if you still have any money left.

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    1/8" NPT works OK for the vac holes but it's not exact so use some teflon tape. BPT (British Pipe Thread?) is the real deal - how do I know? The Search feature on the home page. My 73 is slightly different I think. You have an automatic I'm guessing from that steel vac tube on the back. Most people get rid of all that junk from the cars I've seen. Some of those holes aren't vacs, leave them open or put a stainless metric bolt for looks. That cast iron thing has to stay unless you do something else to support the fast idle screw, I don't like it - got the rest shined up. The flying saucer thing on top of it can go. Maybe that junk is worth some money?
    Those carbs look sweet but now the balance tube sticks out like a sore thumb. Fortunately the balance tube comes right off (seal the gaskets when you put it back). A half hour work with a soft wire brush, then a bunch of aluminum polish and buffing ....

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    I read a science fiction story when I was a kid, maybe it was by Asimov. Anyway it was a future world when robots did everything. Somehow, the robots started getting cranky and sick. A smart robot mechanic noticed the robots had a lot of unnecessary parts. So he removed those unecessary part and suddenly the robots were much happier and did their work better.

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    That's pretty much what I'm doing with this car, a lot of unnecessary parts. Unfortunately I've made the mistake of getting rid of too many things before so thats why I'm here. The car is actually a 4-speed, I'm pretty sure it's original, but beats me.

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    Any of those holes can be chased with standard pipe taps so you can use standard plugs...... Don't tell anyone.....
    Last edited by Bruce Palmer; 10-16-2012 at 06:57 AM.
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    Quote Originally Posted by Bruce Palmer View Post
    Any of those holes can be chased with standard pipe taps so you can use standard plugs...... Don't tell anyone.....
    I was going to suggest that very thing, you can sometimes get away with running the plug through the MIP die backwards. You will loose the taper on the plug threads, but a good sealent like locktite thread sealer will do the trick on a low pressure or vac circuit. I wouldn't use this fix on a fuel system though.

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    Registered User Stanley's Avatar
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    Right, no AT - should have worn my glasses. The AT vac line if you had it would be Tee'd off the master vac connection.
    BTW make sure there's no vac or exaust leaks. My air galley tubes that go into the exaust manifold were rusted out and made fumes til I had them cut off inside and welded shut.

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    Guys, i'm having issue tuning the carbs. I hooked up the antibackfire(green), plugged misc. holes(red), and connected the pcv(blue). The carbs won't run well without running rich or run at all. The mixture adjusting nut has to run really rich, dont know what to do. Help guys?
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    *also running high rpm the car backfires through the carbs, especially the front.

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    I suspect you've got a vacuum leak somewhere and you are running way lean. Are you sure you didn't miss a vacuum port somewhere?

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    How does it run with the air filter installed? If the tuning is too far out the air filter(s) make a difference.

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    Sounds like either a vacuum leak (likely), or timing is way out (possible).
    2/74 260Z

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    Here is my two cents worth, for all its worth. If you need a manual for the carbs try downloading the fsm on http://www.xenons30.com/reference.html
    You might be frustrated now, but you made the right move. The round tops are the better units. Flat tops are commonly known as emmision carbs and therefore less performance.
    Goodluck with it.
    Chas

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    okay so I'm getting a vacuum leak somewhere. I know theres a slight leak in the throttle butterfly but would that really cause this much of a problem? idling at 5-7 turns of the mixture nut. Ill post a video when I work on it again tomorrow. also, I noticed the linkage bar/balance screw that shuts the car off smoothly is too short, is that a common problem? I'll try to take a pic tomorrow as well.

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    When you say you have a leak in your throttle butterfly, I'm assuming you mean you have a leak around your throttle shaft where it meets the carb body? Your throttle butterfly is supposed to leak. That's how the engine gets it's air and fuel. But assuming you meant a leaky throttle shaft... I doubt you have enough of a leak to require 5-7 turns down on the mixture nut. If that truly is the case, then your carbs are all wallowed out and need to be repaired or replaced.

    I can't figure out what you are talking about when you say "the linkage bar/balance screw that shuts the car off smoothly". I have no idea what that means.

    Might I humbly suggest that you look through the factory service manual and use the same terms for parts that are in the manual. That standardization of terms would make troubleshooting a lot easier.

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    lol I'm figuring it out as I go along.....one problem after another. yes, throttle shaft is correct though. I sprayed a little carb cleaner around the throttle shaft while the car was idling and the idle speed dropped. it's just weird because these are brand new carbs, I don't know how this couldve happened.

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    Learning as you go is great. Take some time and at least read through the tune up and engine fuel sections of the manual. It will not only add to the learning experience, but it will also help you communicate here.

    Not sure how to interpret your results with carb cleaner. Usually the typical reaction to a carb cleaner spray around leaks is an increase in the idle speed, not a decrease. Especially if you are running lean.

    What do you mean by "new" carbs? None of these carbs are new at this point, right?

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