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We're bringin' back the Flat Tops!

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Has been great watching this progress. Seems like a great outcome for all involved.  Congrats and well done. 

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I don’t think I’d want to compete in the class (“Stock”) that this car will be in at Zcon this year. You may want to throw on a set of after market wheels and go “Street Modified”. Congrats on a perfect Z Rich. How does it feel to be “The Best in the World” in something?


Edited by Diseazd

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12 hours ago, Diseazd said:

I don’t think I’d want to compete in the class (“Stock”) that this car will be in at Zcon this year. You may want to throw on a set of after market wheels and go “Street Modified”. Congrats on a perfect Z Rich. How does it feel to be “The Best in the World” in something?


I tried to get Motorman7 to install a carbon fiber cup holder for just that purpose, but he just laughed... 🙂

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No Jayhawk, I was talking about the other Z’s thinking about entering Stock Class......they are the ones that need to slap a set of Panasports or Watanabees on and go for Street Modified. I think this car is pretty well a “shoe in” for that big piece of shiny wood and brass......good luck.

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On 7/12/2019 at 9:13 AM, Diseazd said:

I don’t think I’d want to compete in the class (“Stock”) that this car will be in at Zcon this year. You may want to throw on a set of after market wheels and go “Street Modified”. Congrats on a perfect Z Rich. How does it feel to be “The Best in the World” in something?


Hi Guy, thanks for the compliments. Will you be out in Branson?

The car is not perfect, but it's pretty doggone nice, the colors are awesome.  I wish we had another week or two to finish up some small details and do some final tuning,  but for the most part she's about 99% complete.

We chose in several cases to go with the original parts versus buying new or aftermarket replacements.  The windshield would be a good example.  It is the original 46 year old glass, complete with small rock chips and wiper scratches.  In cases like these, the original part trumps a perfect aftermarket windshield for the build. The small defects are not super obvious in the pics,  but I am sure the judges will ding us for such things.  That is fine and understandable, it's just very cool to see the original part  on the car where it's reasonable. 

Anyway, I am thrilled that the car will be at the show.  I am looking forward to seeing all those awesome Z's!.

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No Rich.....I won’t make this ZCON, but hope to meet you sooner or later. We just missed each other in Atlanta last year. Best of luck this year to you and Jayhawk....If you don’t win the big one, I’ll call foul!



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On 7/9/2019 at 7:25 PM, 240260280 said:

Some flat top tuning tips  I hoovered a long time ago. Just noticed on my HD. Enjoy!


Flat top tid-bits









On the 260 Flat-Tops, jet adjustment is done through float bowl cover removal, and adjustment of the jet height, OR easier, the adjustment of the needle in the suction piston. Along with the set jetting for set jetting for the idle circuit. So there are drillings that are required for idle mixture adjustments. Basically you just make one "drill adjustment" to richen that circuit, and then go with the screws on the external parts for fine tuning from there.
The 74 does have a VERY small float bowl internal volume, so if you are planning on running hard, or adding something like a cam MAKE SURE BOTH YOUR FUEL PUMPS ARE FUNCTIONING PROPERLY!
The volume in the late carbs are decreased by a measure of probably 10 over early round tops, and by probably 2 or 3 over the 73 carbs. This was to keep fuel flowing through them quickly, so as not to have any chance to heat up and cause problems. So if you need more fuel, make sure you have good flow to keep you from sucking the floats dry.






See those little pin spanner holes in the brass round "thingy" in the center of the float chamber?

Turn it counter clockwise and you richen the mixture throughout the main circuit operation....






Funny, if I don't get to them quick enough seems all the ROUND TOPPERS pull the suction domes to REMOVE THE SMOG NEEDLES... Wonder why?
Could it be that the SMOG NEEDLES give a BETTER enrichment curve? Ironic isn't it, that people will tear a carb all apart for ONE component, rather than drill a few small orifices and just install the complete carb and have the BEST of both worlds: better emissions, better performance, a REAL choke system for cold start, AND visual emissions compliance?



Hi Al,
Well,  the car is setup for SCCA ITS class racing.  So basically we have to keep the engine fairly close to stock.  We are allowed to modify the carbs as far as jetting and removal of chokes, & smog equip.  My carbs have been COMPLETELY de-smogged.  Pretty much in the same way as described at another site I saw referenced in a post here.  All of the choke stuff has also been removed.  All the remaining holes and smog passage ways have been epoxied(sp?) shut.  
The idle adjustment stuff has been scraped.  The fuel line goes directly into the carb body (from an adjustable press. regulator).  The inlet valve has been replaced with a Gross-Jet inlet needle valve. Everything else is the same as stock. Oh...I use SM needles also.

The power jet is tuned by changing the metering assembly that bolts onto the side of the carb (it is next to the float bowl site glass).  It is a round thing with six screws in it.  I don't know of any way to test them other than trial and error.

I am building a new engine for my car this summer.  Just got the block back from the machine shop yesterday.  It was bored 1mm over to match the pistons (Nissan 1mm oversize).
If I can remember too, I will "borrow" the digital cam from work and take some pictures of the carbs so that I can post/send them if you are interested.









What I'm thinking is that the oil you have in the carbs is too thick and creates a flat spot in the response of the carbs because the slider won’t move quickly enough. 
One thing that was mentioned to me from Tony is the fact that you need to have the butterfly plate closed off all the way at idle, if you are trying to get a faster idle out of the engine, the set screw in the center of the shaft is not the place to increase the idle.
Also, you may want to try adjusting the needles in the slider up a little and that will provide more fuel to the engine quicker. Those carbs have  a lot of adjustment between the needle and the jet that is screwed into the bottom of the carb housing. 
Anyway, those are some observations I have made on the carbs, and I think I can get them running well on my car.






I use 10WT in mine, or ATF. Some people use marvel mystery oil. Oil will affect the response rate of the carbs, as will the spring tension inside the suction dome. Take a look at some of the Jaguar Tuning Sites, and you get reams of data on tuning SU's. There is more to doing it than just turning the screw!
The flat tops used in the US differ from the ones used elsewhere. The unitized float bowl, and idle circuitry is the key points. You can get just as much fuel out of them for top end performance. It's the tuning of the IDLE and TRANSITION circuits that takes a little tinkering. Once you unlock the idle-to-main transition (tip: make it richer, just like everybody does with the OLD ROUND TOPS) you will find they work just as well as round tops!









Move the needle up EQUALLY in both carbs MAYBE 1mm.
What you are doing is running at idle on the needle.
Like he stated, these carbs were designed to idle with the plates CLOSED, and run off the idle circuit in the front carb. Unless you want to open it up and drill passages, your easiest fix is to fatten up the main circuit, and keep it running on the main system at idle.

Not the correct solution, but functionally operable.
BTW, the Marvel Mystery Oil should be about 5wt. This allows easier lifting of the suction dome, allowing a fatter mixture across the power band. By running a higher viscosity (10wt or ATF) you LEAN out them mixture under load. 

The SU is a basic carburettor, but respect is due to those who tune it, because simple things are amazingly complex to get RIGHT. You are finding this out right now.
Good Luck!






Thin oil, with a needle that's very blunt. So when you slam open the throttle plates, the suction dome rises, and you go to full jet available gas dumping.
Zolorin is correct about the cross sectional area, while a slower opening carb will have a higher vacuum, and suck some gas out of the jet, the needle is covering the jet, restricting the flow. If you expose more of the jet, you get disproportionately more fuel than you do when you simply increase vacuum.
The oil DAMPENS the action, and allows you to trim tip-in response. To actually CHANGE the rate at which the piston rises, you must install another spring, and then work out the oil you need for tip-in.
For throttle response, at WOT, you either run another taper, or run the needle higher in the piston(also running a higher fuel level in the float bowl helps), and run lighter oil, to allow the piston to be less dampened, rise quicker and fall quicker, letting FULL POWER settings of the carb to be reached quicker.
Now, this all assumes that the car is being DRIVEN correctly, and you aren't expecting something akin to a V-8 sort of launch from 1500rpms. That just isn’t going to happen.
For proper tuning of the SU's, you should be concentrating for throttle response above 2500 at least, as this is where the engine actually starts making useable power.









Not vibrating, but oscillating. What would happen if you hit a hard bump and no damping in the carbs? The piston could slam down and kill the engine from the pistons' inertia! It also slows the ascent and descent somewhat for a gradual and smooth tip in transition...









Generally the power valve is the culprit. Have you broken off the needle stops on the idle mixture screws to dial it down? The throttle plates are supposed to be CLOSED, ALL the air comes through the starter idle circuit on the front carb in essence. If the plates are cracked, the main jet contributes to the mixture (from the taper you have noticed never plugging---this means very quick main tip-in in driving to keep from stumbling LIKE THE ROUND-TOPS ALL DO!)

If those plates are open AT ALL---close them. This gives all vacuum to the idle circuit, and draws in proper emulsion air to lean the mix... if the air bleeds don't have the proper vacuum; they will 'go rich' as well.

The idle issue is the IDLE circuit. The two SHOULD NOT be on at idle. ONE only. As SOON as you touch the throttle the mains start to work. This is the problem most people can't get through their heads working on these carbs. They are designed for TOTAL closure of the throttle plates at idle. TOTAL CLOSURE just like EFI! They have a separate air fuel bypass circuit to fuel the engine at idle.

This allows it to be tuned totally and separately from any other portion of the engine. Station indicators and running around with the hood off or on a rolling road anybody?









Seems like stiffer springs would have the effect of "delaying" the maximum rise of the piston/needle. This might possibly be good for smooth driveability- has sort of same effect as using a smaller carburetor. I found that the piston/needle was fully raised on my L20B around 4000 RPM (w/ stock Hitachi springs). I don’t think it is necessary to delay the piston max rise all the way to peak HP PRM or redline. Maybe match the full piston rise closer to the torque peak RPM? The SUs just go into "non constant-velocity mode" and function more like a traditional e-tube carb once the piston fully rises. Using stiffer springs will have the effect of raising the venture velocity (and vacuum) through the rest of the low/mid RPM range before the piston reaches full lift. Stronger vacuum is produced to counter the stiffer spring down force and increased vacuum will also work to richen the low/midrange RPM air/fuel mixture (just like thicker dampener oil richen the mixture under acceleration). So- sounds like your choice of the stiffer red springs may in part be contributing to your rich-at-idle/midrange problem. Before customizing you needle profile to work with the stiffer springs, try using the soft Hitachi springs again and see if your mixture balance at low/high RPM is improved.









A 260Z will run smooth and clean to 7000rpms with a power peak around 6500 with EITHER the SU's or the Flat Tops. Frankly, the FLAT TOPS will run stronger on the top end because they are possessing the correct taper on the needle for the larger engine compared to the L24 Round Top Transplants.



I've started a rebuild on a 73 with flat tops. Can you recommend a good rebuild kit?

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