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PostPosted: Sat Apr 23, 2011 8:18 am 
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Posts: 331
Location: Finland
I have a set of triple Weber DCOE 40s for my -74 260z. The engine block is standard so displacement per cylinder is 427cc. Head is mn47 resulting in 10.5:1 CR. MSA header and 2,5" exhaust. Electric Dizzy. I do not know max rpm but would estimate slightly conservatively 6500.

I'm trying to establish initial jet and venturi sizes.
Using the formula from the "How to modify your Nissan&Datsun ohc engine" -book, I firstly realize bore recommendation would be 45 instead of 40 but 40 is what I will have to stick to.

using the calculation given in the book, I get 32.9-34.2 as main Venturi size.
Therefore initial main jet would be (according to the book) 1.32-1.36mm and
air correction jet 1.52-1.56mm.
Auxiliary venturi is recommended at 4.5mm.

However the book states that MOST L-series require 1.4-2.1 main jet, so it kind of contradicts what I get from the calculation.

What do you guys think from your experience, should I start from 34mm main venturi, main jet around 1.35mm, air correction around 1.55mm and 4.5 aux venturi?
Or would you start from larger venturis and jets altogether? Any recommendations?

Given the higher than standard CR and improved exhaust do you think these would have any impact on the size of venturis and jets I can use?

Any advice will be appreciated.

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PostPosted: Sat Apr 23, 2011 8:23 am 
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Location: Finland
forgot to ask about emulsion tube, any recommendations for that?

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 Post subject: re:
PostPosted: Mon Apr 25, 2011 9:45 am 
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Joined: Fri May 23, 2008 9:34 pm
Posts: 362
Location: Hamden, CT
Weber recommends a 40DCOE baseline of 28mm chokes,115 mains, F11 emulsion, 170 air, 50F9 idle jet for the 240Z/260Z.

My new 40DCOEs came preset with the values above but with 30mm chokes. Since you're throwing the MN47 into the mix the best bet is to schedule some dyno time to have them check the 02 ratio so you can determine what changes need to be made.

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Datsunzgarage.com

1970 240Z - enhanced F54 L28 w/P90, Weber DCOEs, 4:11 R200, Nissan T5 5-speed


Last edited by Datsun Z Garage on Tue May 24, 2011 2:57 pm, edited 1 time in total.

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PostPosted: Mon Apr 25, 2011 2:31 pm 
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Joined: Fri Apr 02, 2010 2:56 pm
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Location: Finland
Thanks Bryan, that's a great info! Will check what my current values are as soon as I get the carbs back from the manifold welder..

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PostPosted: Tue May 24, 2011 8:34 am 
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Posts: 331
Location: Finland
I got my carbies back - finally.
they currently have
idle 50 f9
emulsion f11
main 120
air 170
choke 28

the difference to the recommended setup is air (recommended 200 mine 170); main (rec 115 mine 120) and idle (rec 45 f9 mine 50 f9)
what do you guys think, as the air is smaller than recommended and main and idle are bigger, what's the impact of that? should I get the recommended jets before installation or do you think these are close enough to give it a go?

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PostPosted: Tue May 24, 2011 9:55 am 
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Location: CT
My understanding of the Weber DCOE series carbs is that they are favored because they are so easy to make quick changes to, while on the car. Everything falls right to hand as soon as you open the top cover.

Racers around the world carry boxes of jets with the car. When they get to a new track they can quickly swap the jets to the best combination needed for that event.

I think you shouldn't consider your jets as a "permanent" thing. If you drive along the seacoast, then up into the mountains, you might favor a jet change. Try what you have for now. Make notes about how well (or poorly) that combination performs. Make changes as needed. Right now you should be looking for a general, all-around combination which gives you happy mid-range performance at different temperatures, altitudes, and humidities.

If you decide to only drive your Z in one condition (fixed altitude, temps and humidity), you might never have to change jets again. If it sputters a little when you charge thru the mountains, you can either live with that or change some jets for those conditions. Once you know which jets make your car run best under certain conditions, it only takes a few minutes to unscrew the covers and swap them out.

That was the beauty of the SUs. They sort of self-adjusted to the needs of the engine under all conditions. Last year Thor drove his 240 thru some of Europe's most-challenging mountain passes, with extreme changes in elevation. He reported his engine ran perfectly in every condition.

The Webers might make you work a little harder than the SUs did, but they will give you sparkling performance in exchange for it! :wink:

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PostPosted: Tue May 24, 2011 11:20 am 
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Joined: Thu May 28, 2009 3:44 pm
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Location: Massive Sangwich
This is why I chose EFI.

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-Chris


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PostPosted: Tue May 24, 2011 11:24 am 
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Location: Naugatuck CT
PhrogZ46 wrote:
This is why I chose EFI.


:D

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1978 Datsun 280Z - HLS30-438694


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PostPosted: Tue May 24, 2011 11:24 am 
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Location: Finland
my holley didn't handle over 2000m altitude, freezing temp and wet rain very well.. :D lower altitudes it works fine on the mountains less well and when it gets cold and humid my z felt like suzuki swift (without the swift..)

yes i know those webers are great for easy tuning, was just double checking with experts that they're close enough to even try. would be nice to have a big selection of different size jets available and while they're not expensive when you have 2 of each in one carb and 6 of each in the setup it quickly adds up.. anyway I just ordered the recommended sizes to have some to try and service kits for all 3 carbs - will give the whole setup a good clean and new gaskets before installing them.

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PostPosted: Tue May 24, 2011 11:28 am 
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Location: Finland
theZman wrote:
PhrogZ46 wrote:
This is why I chose EFI.


:D



yeah I was going for efi too but changed my mind. it's like choosing between a real woman and a barbie.

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PostPosted: Tue May 24, 2011 11:32 am 
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Location: Naugatuck CT
carburetors are like women on any given day you never know how they are gonna act..... :lol: :lol:

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1978 Datsun 280Z - HLS30-438694


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PostPosted: Tue May 24, 2011 1:26 pm 
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kingofapes wrote:
theZman wrote:
PhrogZ46 wrote:
This is why I chose EFI.


:D



yeah I was going for efi too but changed my mind. it's like choosing between a real woman and a barbie.


Well in my case you throw a turbocharger into the equation and things get really complicated...

If my car was NA I might think differently.

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PostPosted: Tue May 24, 2011 1:43 pm 
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Location: Finland
that's understandable. and to be totally honest I'm not against EFI at all. I really was going to have one because it was going to be cheaper and easier and in a way more flexible. but before finding an EFI I came across a set of those triple webers for a good price and I just had to have them. They are awesome and great for a Z. If I will ever have a second s30 it'll probably have EFI, MS, Turbo etc. this one I'm happy to keep carburated and NA.

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PostPosted: Tue May 24, 2011 1:54 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
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Location: CT
King, you're not (not) running Champion spark plugs, are you?

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PostPosted: Tue May 24, 2011 1:59 pm 
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Posts: 331
Location: Finland
Frank T wrote:
King, you're not (not) running Champion spark plugs, are you?


TBTH Frank I have no idea.. I've meant to check them generally to see the condition but haven't gotten around to do that. (I'm in the process of moving nearly 2000 miles north so that's keeping me pretty busy..)

why is that? Champions pose a problem? What do you recommend?

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