Webers to replace Zeniths?

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Martin Benade
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Re: Webers to replace Zeniths?

#136 Post by Martin Benade »

Do you move the decimal point a few notches to the left? I can’t see a 128mm jet working very well. Or even existing
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John Clarke
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Re: Webers to replace Zeniths?

#137 Post by John Clarke »

Oh dear
Example 32mm Vents x 4 = 128 main as Main jets are sold 🤔
Yes Martin, We know it's 1.28mm !
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Martin Benade
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Re: Webers to replace Zeniths?

#138 Post by Martin Benade »

That should fit much better!
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Re: Webers to replace Zeniths?

#139 Post by Greg Bryan »

I'm running a 115 main in my 40mm Weber IDF carbs. It has 28mm chokes. 28x4=112
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Re: Webers to replace Zeniths?

#140 Post by John Clarke »

Perfect Greg
Just up on the richer side
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Brian R Adams
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Re: Webers to replace Zeniths?

#141 Post by Brian R Adams »

Greg Bryan wrote: Sat Dec 09, 2023 12:37 am I'm running a 115 main in my 40mm Weber IDF carbs. It has 28mm chokes. 28x4=112
Greg, the correct formula for nominal main size is:

(choke size) x 4 x 1.027

You're right on the money! ;)
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Martin Benade
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Re: Webers to replace Zeniths?

#142 Post by Martin Benade »

It’s really just a rule of thumb to get in the ballpark, right, since things like air correction jets and compression ratio affect what the correct size will really be.
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Re: Webers to replace Zeniths?

#143 Post by Alan Klingen 2 »

Nooooooo!!!!! webers are the worst choice for the 356 motor I know some people like to use them but the reason they do is that they don't know how to rebuilds ZENITHS Carbs Tuning a carb is fast becoming a lost art. Alan The Stable.

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Re: Webers to replace Zeniths?

#144 Post by Brian R Adams »

Martin Benade wrote: Sat Dec 09, 2023 9:55 pm It’s really just a rule of thumb to get in the ballpark, right, since things like air correction jets and compression ratio ...
... and density altitude, and fuel composition, ...
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Re: Webers to replace Zeniths?

#145 Post by Martin Benade »

I was actually surprised that a handful of examples matched up pretty well with the idea.
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Re: Webers to replace Zeniths?

#146 Post by Bruce Smith »

You guys realize that this is a coincidence, right? The areas of each are what impacts performance. Many other factors as well. The sun and the moon appear the same size from earth (thus eclipses). Another coincidence, unless you want to get spiritual about it.
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Re: Webers to replace Zeniths?

#147 Post by John Clarke »

Hi Bruce
The Areas of Each What ?
Just interested to Learn. It's amazing how accurate it seems ! 🤔
What a Coincidence
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Re: Webers to replace Zeniths?

#148 Post by Alan Hall »

Solex published a booklet, probably back in the 60's, called "Selection and Tuning of the Carburetor" which has a number of "rule of thumb" type formulas for setting up approximate size and jetting of a carburetor, Examples include: Approx size of carb (if it feeds 1 to 4 cylinders) = 0.82 x Sq. root of (capacity of a cyl in cc x peak rpm in 1000 rpms). If you plug in typical 356 numbers (400 cc per cylinder and 6000 peak rpm) you get 40.17 which is awfully close to a 40. They then say the choke tube (venturi) is approx equal to 0.8 times carb size (in our example 356: 40x .8 = 32. They then say main jet number is approx 5 times size of choke tube (In our 356 example 5x32= 160 and air correction jet is approx size of main jet + 60 (our 356 example: 160 + 60= 220). The booklet then recognizes that these are approximations and briefly talks about flat spots and the effect of emulsion tube selection etc.

These "rules of thumb" don't address whether the carb has dependent or independent idle circuit, but they appear to give a larger main jet size than what might be typical suggesting dependent idle (where idle circuit is downstream of main jet). and comparing to Porsche jetting for a super 90, which has independent idle and carb size of 40 (40-pii-4). venturi size of 32 but main jet of 115 and air correction of 180 seems to bear this out. (Total area of a 115 main jet and a 57.5 idle jet is approximately equal to the area of a 130 main jet.) Even with this, the Solex approximation "rule of thumb" main jet seems way to large. Perhaps the 4x venturi size is a closer approximation.
Last edited by Alan Hall on Sun Dec 10, 2023 12:24 pm, edited 2 times in total.

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Re: Webers to replace Zeniths?

#149 Post by David Jones »

Those theoretical numbers I found make a good start point when tuning a performance engine. I stick to factory specs on street engines. When tuning for max hp on F Vee engines on the dynamometer I found that each engine required custom jetting though idle jets stayed the same because the engines were run between 3 to 7k rpm so it was just a case of choosing air correction and main jet. It was surprising to find that two engines built to identical specs required different jets sizes to produce similar outputs. Also surprising was that some engines tolerated more ignition advance than others. I can only surmise that small variations in intake manifold and head porting were the cause.
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Re: Webers to replace Zeniths?

#150 Post by Al Zim »

1) we do not replace or use Solex 40 P11. 2) Certain years of Zenith carburetors are prone to problems which I have posted. We do NOT rebuild or service these "problem" carburetors. 3 ) There is a parts number change from 356B T-6 to the 356C UNFORTUNATELY there is not any indication what changes this part number change represents. 4) GASOLINE HAS CHANGED FROM 1965 TO 2024! Nearly 60 years. What does that mean for carburetion on 7.5 to 1 ( or 8.5 to 1) compression ratio.
SOMEONE WITH AN ADVANCED COLLEGE DEGREE IN CHEMISTRY NEEDS TO PURSUE AND REPORT ON THIS SITUATION. al zim
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