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PostPosted: Tue Mar 06, 2018 9:20 pm 
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Joined: Tue Mar 12, 2013 2:12 pm
Posts: 30
Location: milford, ct
I'm the 2nd owner of an early 74 260z that I've owned since 1975. It hasn't been on the road or run for over 20 years. Started working on it 2 years ago, but other priorities & life issues soon intervened. Having second hip surgery soon & plan to get back on project after that. Engine has the original flat top carbs which haven't seen use in over 20 years. In the years I drove the car the flat tops never gave me a problem, but I'm aware of their bad juju. I'll have to spend time & money on refurbishing these less than desirable units. Question is would this effort be better spent on a set of round tops? Know someone who has a set from a 73 which I probably can get for under $200 and will also probably need to be rebuilt. Should I go this route & what's involved in switching to the round tops.
Thanks for any help/info you may send my way. - Harry


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PostPosted: Tue Mar 06, 2018 10:00 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 12897
Location: CT
Hi Harry, good to hear from you again. Here's my two cents:

I think any carbs which haven't been used in 20 yrs need to be rebuilt before use. If you're going to have to rebuild a set anyway, you could actually have a performance advantage with the newer flattops.

The issue with the 1973-74 flattop carb systems was vapor lock, which the factory combatted by wrapping the fuel lines with insulation.

The flattops were intended to be a limited-time stop-gap measure btwn the good performing roundtop SUs and the upcoming fuel injection unit available on the 260 & 280. Several other Datsun vehicles used them singularly on smaller engines. They actually had the advantage over the roundtops of an accelerator flap which opened an auxiliary fuel circuit when you stomped the pedal, similar to an accelerator pump plunger on an American carb. The roundtops never had this feature. As issued from the factory the flattops suffered from the percolation and vapor lock issues, but Z racers today prefer the flattops due to this acceleration flap advantage and the fact that today's rebuilding procedures overcome the perc/vap problems. A set of triple Webers beats either combo, anyway.

If you send your flattops to a professional SU specialist to be rebuilt, you will not have the vapor lock issues, and you'll actually have better-performing carbs. The swap to a roundtop setup is not actually straightforward; there are different hoses involved, the manifold and carb throats of some carbs are square and others round, and some of the attaching hardware from one will not fit on the other. Also, the flattops and roundtops tune differently. No huge issues and all can be overcome if you're intent on installing the roundtops, but just little issues you should be aware of and prepared to face going in.

BTW, Z-Therapy contradicts themselves concerning the flattops. On the internet you can read them call the flattops "boat anchors" and refuse to accept them as cores for rebuilt units. However, on their 2-disc SU Carb DVDs they praise the flattops if they are prepared and tuned properly and acknowledge their performance advantage over the roundtops. I think the roundtops look better, so they would be my choice, but that's just aesthetics.

http://www.ztherapy.com/technical_stuff ... potter.htm

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1970 240Z


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PostPosted: Sun Mar 11, 2018 9:12 am 
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Joined: Fri Jun 23, 2017 5:34 am
Posts: 54
Getting flat tops to run right can be tough because of all the different air and fuel passages. Private message me your email. I have a what looks like a dealer document that explains the 5 types of flat tops there are, paint markings to determine what kind you have, dealer modifications, and different tuning procedure from that of the chiltons and autobooks procedure.

I have flat tops also that run awesome on hot days when the car is fully warm. I’m planning on doing a extremely slow disassembley, marking and measuring everything. The power valve is a sandwich of plates with passages and there were changes to it over the years. Probably going to have score lines on everything and ultrasonic clean mine. My car sat from 1991 until last fall.

Start with, valve adjustment, ignition timing and verify fuel pressure. My rear fuel pump was froze after sitting.

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock


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PostPosted: Sun Mar 11, 2018 9:48 am 
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Joined: Fri Jun 23, 2017 5:34 am
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And I have the official Datsun carb adjustment procedure for the 260z.

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock


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PostPosted: Sun Mar 11, 2018 10:11 am 
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Joined: Wed Apr 09, 2008 5:53 pm
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Location: CT
Bruce, are your flat tops the round throat or the "square" throat types?

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1970 240Z


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PostPosted: Sun Mar 11, 2018 10:28 am 
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Joined: Fri Jun 23, 2017 5:34 am
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Certainly square at the airfilter housing. I thought all flat tops are square on the airfilter housing hence the round top airfilter housing is not interchangeable.

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock


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PostPosted: Sun Mar 11, 2018 4:09 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 12897
Location: CT
Some are "square", some are round, and there are actually "D" shaped throats, too. Datsun used the flat tops on several different vehicles so you could encounter individual units in any shape. In the Z application they had both square and round throats, so the Round-throat / Round-top carbs & manifolds can be a direct swap-in for many 260s.

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1970 240Z


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PostPosted: Tue Mar 13, 2018 11:34 am 
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Joined: Fri Jun 23, 2017 5:34 am
Posts: 54
Sent over both manuals I have. Thought I should post the small manual I do have for everyone.


Attachments:
260Z Carb Adjustment Manual.PDF [774.29 KiB]
Downloaded 47 times

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock
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PostPosted: Fri Mar 23, 2018 7:17 am 
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Joined: Wed Jul 23, 2008 2:38 pm
Posts: 3098
Location: rhode island
I have a set of flat tops that might be new, or just were never used. Very nice condition with air cleaner.


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PostPosted: Sat Jun 09, 2018 7:39 am 
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Joined: Fri Jun 23, 2017 5:34 am
Posts: 54
https://youtu.be/l3q0ZHMf7qA

Good video of a flat top tune up.

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock


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