Desmogging Hitachi Flat Tops

[Weasel73240Z]

I've got a 73 240Z with the original Hitachi flat tops, and very low miles. I'm going to remove the smog control stuff, since I don't need it in Massachusetts anymore. The car has sat for a long time, and hasn't run in a year or so. I took apart the carbs and found a stuck float needle. I haven't started the car yet, I'm waiting until I have cleaned all of the fuel lines and the fuel filters are all replaced, but I'm sure that was the problem.

A friend who has worked on older Z's before suggests I get the car running properly with the smog stuff on, then remove it and adjust. It is my understanding that these carbs work much better without the smog stuff, so why not get the carbs running and tuned with the stuff gone already? What parts of the smog equipment (i.e. shared vaccuum lines, etc.) directly affect carb function? Any advice and/or tips would be appreciated.

[justin260z]

just replace them with early round tops and save yourself the trouble.

[Weasel73240Z]

I hear a lot of differing views on this board about whether to keep the Hitachis or trash them for older SU's. The car only has about 26,000 miles on it, and other than desmogging, I want to keep it original.

[BRE-240Z]

I'd want to get it running as is, first, and not to complicate the equation too much. If you give in to the temptation to pull the smog stuff off of the car and then can't get the car started, you won't have a baseline to work from.

Flush the fuel and go through the fuel system, get the car running and tuned halfway reasonably, and then pull off all of the anti-pollution stuff. Less potential trouble that way, and people will be able to help you more easily if you get jammed up somewhere.

[BRE-240Z]

If you ever decide to go back to the round tops, just store the flat tops somewhere. That way, in the event that you ever decide to sell the car, you can say that it's low mileage, and has the 'better' carbs, but can easily be put back to original condition.

[Z train]

Get a set of SUs complete.Remove Hitachis(manifold & all).Store the Hitachis and use the SUs.
I have a SU manifold minus everything except the x-over tube.

[Exdamyankee]

The flattops were a poor attempt to meet emissions requirements until the Fuel Injection system was ready for market. If you want a museum piece, keep the flattops on the car. If you want to drive it and enjoy it, swap them out for a fresh set of round-tops and put the flatties away where they will stay in #1 condition.

Go with the roundtops if you are desmogging the car.

If you want to do it right, go to www.ZTherapy.com and look at their roller-bearing shaft rebuilt roundtops. Pricey, but a permanent solution to the carb problem.

BTW, you'll also need wide spacers for the round-tops if you swap, and longer manifold studs.

[Tony D]

I'll gladly pay shipping for any 73 Flat Tops anybody wants to discard my way, maybe even pay a couple of bucks too... Literally 'a couple'... Shipping costs more...
PM Me for Address and shipping information.

[hls30.com]

Best answer-clean and box up the complete Flattop assy(heatshield, manifolds, Balance tube and carbs) and get a complete assy of three screws from ZTherapy(Bruce has a twisted sense of humor-run with it!) bolt 'em on, tune them as directed, and find what you have no clue was missing in your car! I had a '73 with flat tops that I thought was well tuned until I swapped to three screws...night and day difference, never would have thought if I hadn't been persuaded. Best thing to do to a '73 with the original drivetrain.

Here is most of what it looks like!

[ta240]

I just swapped good running flat tops off my 'new to me' '73 for my Z therapy carbs that were on my old '73 and it was a great improvement. With all the hoses and junk on the flat tops I'd be afraid I'd mess up the way they ran trying to remove any of it. I can't even bring myself to put the flat tops on my old Z and it is just going to be sold when I'm done swapping parts off of it.

[EScanlon]

Ditto with the general consensus.

Swap out the "flat-tops" with a good set of Round Tops.

Save the Flat tops AND any other of the smog items you remove (air pump, vacuum solenoids etc.) and put them in zip-loc baggies or just in boxes until that day years from now when you sell the car or give it to your kids.

While that thought may be untenable at this point in time, when you're looking to move into a nursing home they'll be an added plus to the next owner. He may not put them on, but they're an original piece of the car.

Or he may bless you for it when the Republic of Californicate's concept of imposing non-existent and ludicrous smog standards on 40+ year old cars rolls around to your state and he's required to put on the original smog equipment .... whether it was functional or not .... or pay a fine.

Don't laugh. They're looking to "decrease pollution" by the 1% of all cars represented by older cars that are still around, while ignoring that those 35+ year old cars are better maintained and less driven than the average 5 and 10 year old cars.

Wait till you have to find a 35 year old air pump that you can install on your pristine 72 Z just to placate some DEQ weenie.

2¢
E

[Tony D]

With all you guys taking off functional 73 Carbs, and my standing offer to pay for shipping to relieve you of them...I'm offended nobody has taken me up on it.
I paypal right away so you can ship quickly!
P.M. me if you're not interested in those old boat anchors.

Seriously guys, I'll take them off your hands!

[jayhawk]

Like TonyD, I'll be happy to help you eliminate the pollution in your garage by taking the flat tops off your hands. (I really need the linkage actually..)

But Weasel, you might want to look a little deeper into the whole flat top story than the oft repeated "boat anchor" or "emission carb" claims by posters who believe that round tops are "SUs" and flat tops are not. Or who believe that only the flat tops were made by Hitachi.

I'm going to try and scan into PDF format a copy of the "mythical" and "impossible to find" 1975 Nissan 240Z and 260Z Fuel Modification Plus manual that turns out not to be very difficult to find (2 have sold on eBay in just the last couple of months). It has all of the service bulletins, fixes, and tuning information on the flat tops in one manual (that should have been in the FSM, but wasn't....)

Conventional Wisdom may be conventional, but it is not always wisdom.

--Bob

Original Owner of 1973 240Z HLS30-125678, who drank the Conventional Z Wisdom kool aid and paid a lot of money to replace the flat tops with round tops. And now regrets that....

[beandip]

Weasel73, First off the intake on the engine right now is a vary good intake. Leave it on and just remove the flat tops . Like all before me here save the old emissions parts. If ever you want to bring the car back to original you will need the stuff. As Escanlon said just zip lock the stuff and put it away. Find a good set of SUs , either 4 screws or 3 screws. Either will work the same. At the same time pick up a early air cleaner and heat shield. '70-'72 vintage. You will also need the thicker insulators that go between the SUs and the intake also the linkage that goes between the SUs. the flat top linkage is a different length. Gary

[Weasel73240Z]

Thanks to everybody for the input. I think what I'll do is get the car running with the flats on it. The lady I bought it from said it ran fine when it was parked, so I plan to get the motor running before changing anything. Then I think I'll probably store the flats, and install a set of earlier rounds. I'll store everything from the flat tops (and the smog stuff), but in the end, I want to drive this car, not show it.

I want the most reliable carbs I can use, and I want the car to perform the way a 240Z should. And it seems to be the concensus that rounds tops are the best way to get there.

[sblake01]

You might want to at least give some consideration to jayhawk's post. (#13). I hope he doesn't mind me quoting him in my signature.

[Weasel73240Z]

Since I'm going to get the car running with the flats on, I guess there is always a chance I could keep them. If the car runs well with them, I suppose I wouldn't necessarily change them. If I do decide to keep the flats, will removing all the smog stuff affect them much?

[sblake01]

My friend and I used to 'de-smog' them back in the 70s-80s when I had carbed cars. I haven't worked on a set of them (flat or round) in years but they did run better without the smog equipment.

[Z train]

Like TonyD, I'll be happy to help you eliminate the pollution in your garage by taking the flat tops off your hands. (I really need the linkage actually..)

But Weasel, you might want to look a little deeper into the whole flat top story than the oft repeated "boat anchor" or "emission carb" claims by posters who believe that round tops are "SUs" and flat tops are not. Or who believe that only the flat tops were made by Hitachi.

I'm going to try and scan into PDF format a copy of the "mythical" and "impossible to find" 1975 Nissan 240Z and 260Z Fuel Modification Plus manual that turns out not to be very difficult to find (2 have sold on eBay in just the last couple of months). It has all of the service bulletins, fixes, and tuning information on the flat tops in one manual (that should have been in the FSM, but wasn't....)

Conventional Wisdom may be conventional, but it is not always wisdom.

--Bob

Original Owner of 1973 240Z HLS30-125678, who drank the Conventional Z Wisdom kool aid and paid a lot of money to replace the flat tops with round tops. And now regrets that....


While the performance of "flattops/Hitachis/whatever"can be dramatically improved,they are still inferior(completely) to a PROPERLY set up pair of "roundtops/SUs/Three screws/whatever".

[sblake01]

Performance. Well the JECS system used on the 280Z/ZX/810/RWD Maxima etc. is inferior to an EFI system with a MAF sensor and modern engine management, but that doesn't mean it can't be made to perform adequately. I.E. decent fuel mileage and drivabality, etc. Flattops can be setup to do that. If we're talking about building race cars that would be another story though some of the hi performance carbs available through Datsun Comp/Nissan Motorsports, though different from the emission carbs, were flat tops. The only thing that makes round tops more desirable is that no one has commercially put any effort into the maintainence, refurbishing, or improvement of flat tops. I have run satisfactorily performing flat tops in the past so I refuse to jump on the 'boat anchor' bandwagon. And PROPERLY set up is the key. I've seen a few cars with badly performing round tops.

[jayhawk]

Stephen, I'm honored you like my turn of phrase. While I did just think of that while composing that post, I would not be surprised if someone else had not thought of that earlier. Seems sort of obvious to me...:-)

[jayhawk]

Z train, you said "While the performance of "flattops/Hitachis/whatever"can be dramatically improved,they are still inferior(completely) to a PROPERLY set up pair of "roundtops/SUs/Three screws/whatever".

OK, why?

ie, what specific design features of the flat tops make them inferior? It seems to me that "properly set up" the flat tops ought to be better than round tops, since we know they have some nifty features that the round tops do not (separate idle circuit, power valve).

I don't know that they are better, but I cannot find any posts by any knowledgeable posters that point out the actual features or faults other than very generalized and incorrect claims that they are "emission" carbs and somehow cause all kinds of problems that they do not. (like the higher underhood temps of the '73s and '74s)

I look forward to your information.

Thanks --Bob

[Tony D]

I have to ask the exact same question: Why, what makes the earlier carbs "Superior"? The fact that their full throttle and idle miztures are inexorably linked, so that you can't adjust one from the other without custom retapering of the needle? The fact that the earlier carbs have no provisions for extra enrichment under load, so you have to adjust them for worst case scenario and then live with sub-optimal miztures the rest of the time?

Or is it simply the fact that you can screw the knob on the bottom of the carburettor to make a jet adjustment? How often are you doing that...really?

A main jet adjustment is not something that normally requires resetting at every turn if it's close to the correct range...

So what, exactly makes them 'inferior (completely)' compared to the earlier carbs? Have you ever even worked on a set of Flat-Tops?

Are you aware that the HIF-6 Flat Top is a preferred carburettor to the round tops when available for the application? Separate circuits for idle and main jet are the primary reason there, and jet adjustment on those is via screwdriver instead of pin spanner. The hitachi Flat Tops are a Hybrid HIF6 Carburettor. But again, I'm awaiting exactly why you claim the things you are claiming. Give us examples. We have given you two good reasons as to why they are not inferior.

[Z train]

O.K.maybe i shoulda left out the "completely".And ,sorry to dis-appoint but i can't give you any technical info only personal experience and some info passed on from Bob Sharp racing.

And the obvious question-if the Flats are so bitchin' why does "everybody" ditch when they are able to?

[sblake01]

Because 'everyone' say so. For years now. Especially Z Therapy and you can understand why they say so. They even suggest you ditch the EFI in favor of their round tops.

[ph240]

Flat tops bad, early round tops good.

flat tops will vapor lock in outside temps above 85. Datsun recalled the 73 and 74 z and installed heat sheilding and an electric fuel pump but it didn't fix it. Here in the south I remeber many a 73 that wouldn't restart afetr they heat soaked for a few minutes.

Keep the oe parts, install the early carbs and enjoy driving the Z

[sblake01]

I'll just repeat what I 'typoed' last year when I thought this thread was done as to why flat tops are bad. Because 'everyone' say so.

[conedodger]

Stephen, I'm honored you like my turn of phrase. While I did just think of that while composing that post, I would not be surprised if someone else had not thought of that earlier. Seems sort of obvious to me...:-)

It matches up well with a phrase I am always telling my students. Common sense is not all that common anymore...

[full house]

Common sense is not all that common anymore...

How did you say that?Common sense mostly help us to solve some problems.

[sblake01]

I rest my case.

[Sailor Bob]

JayHawk you could be creating a market for the flatops. Got me thinking I need a set - I am a fan of original setups and this is interesting.

[RIP260Z]

All I have seen for years is boat anchor this, boat anchor that, regarding flat top carbs. Fed up with it. I am another fan of flat tops. I see a lot threads on specific items on the early 240z and how people look to have these. Fair enough...you want to keep the car "stock/factory". Why not these carbs, which are a larger stock item than some metal emblem (not knocking these)? They are part of the heritage of the S30. And I thought this site was all about that?

Sure, they take a bit more set up, but you are only metering two components, fuel and air (and a little bit of vaccum). A problem with worn round tops also occurs on the flat tops, worn throttle spindles. Once you understand how the flat top works, you understand correct set up, and how they work, there is no reason why they should not work. Some cars in Japan are still running '73 flat tops and have no problem, and they don't have a different carb.

Stick round top carbs on if you want an easy life, but please consider the boat anchor comments for flat top carbs from the un-informed.