weber jet change: what to conclude?

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Juha Vane
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Re: weber jet change: what to conclude?

#16 Post by Juha Vane »

Checked the float levels, one is 10,0mm, the other one is 10,1mm. There is no groove in the needle valves,
but I change them. I measured the distance from the float to cover when float was hanging down. Top cover
vertical, is this correct way to measure it? There is picture in Passini book like this.
Existing needle valves are 2.00, my kit has 1.75, hope they are big enough.

The accelerator pump seem to deliver equal amount of fuel, just visual check, I have not measured it.
The stroke of acc. pump can be adjusted.

I'm somewhat lost with the jetting, been collecting the various jetting people tell here and there are quite a
variation, how can I know what is correct? Trying them all? Very few reveal complete jetting. I have a good
selection of main and idle jets, also air correction jets. Have 32 and 28 venturis, and F7 and F11 emulsion
tubes. I don't fully understand what role the emulsion tubes play here as isn't the fuel metered with the main jets?

Feel really stupid as I have nothing solid to grasp here, have not been able to understand how these jets & emulsion
tubes have an influence to the whole picture. Idle circuit is quite simple, or so I think now, but the main circuit is
not clear.
Papers with my jetting are upstairs, but my wife are there already sleeping and I don't want to wake her up.
Start to understand why fuel injection is so popular...
KTF,

Juha Vane
Finland

'59 308
'63 356
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Martin Benade
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Re: weber jet change: what to conclude?

#17 Post by Martin Benade »

So are factory carburetors, as the jetting has already been perfected.
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Wes Bender
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Re: weber jet change: what to conclude?

#18 Post by Wes Bender »

+1 Martin. If Webers were better, Porsche would have specified them. I know I sound like a broken record, but Webers are best used by racers who know how to set them up. They like them because you can dial them in to better handle changes in track altitudes. On the other hand, well tuned Zeniths and Solexes won't provide as much power, but will cover a wider change in altitude and make life easier for the bulk of us. Of course your mileage (no pun intended) may vary. If you are into competitive driving or want more power, you get to experiment to your heart's content with Weber jetting.
Some days it's just not worth chewing through the restraints.....

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C J Murray
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Re: weber jet change: what to conclude?

#19 Post by C J Murray »

Today I had my race car on the chassis dyno which also measures air/fuel ratio. The engine is 1622cc and makes peak power at 7200rpm. It has Weber 44IDF carbs with 36mm venturis. The camshaft is mild for a race engine and has a 110* lobe separation which is also more street oriented than most race cams. That cam probably needs less fuel than the typical race cam.

With F11, 140 mains, and 200 airs the fuel curve was very flat and very close to perfect at 13afr start to finish. 140 is only 3.9 times the venturi size. Redline/Weber claims that the standard jetting in their brand new carbs will work acceptably on any street car. The 44IDF comes from Weber with F11/135/175 which seems odd. I tried that jetting on my 2133cc street 356 and it wasn't even close to correct. Maybe the 40IDF jetting is better out of the box?

If you are getting frustrated with your Webers you should try to run your car on a Dynojet dyno equipped with a weather station and A/F sensor. It will tell you a lot, not everything but a lot. Some drivability problems will only show up when driving on the road. Take a jet selection with you to the dyno if possible. If not you can at least know which direction you need to go. The mixture on the dyno will usually change through the rpm range and that will tell you which direction to go with the main and air jets. It can save you some time.

Sorry for babbling aimlessly.
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Re: weber jet change: what to conclude?

#20 Post by C J Murray »

Wes Bender wrote: Fri Apr 19, 2024 6:23 pm +1 Martin. If Webers were better, Porsche would have specified them. I know I sound like a broken record, but Webers are best used by racers who know how to set them up. They like them because you can dial them in to better handle changes in track altitudes. On the other hand, well tuned Zeniths and Solexes won't provide as much power, but will cover a wider change in altitude and make life easier for the bulk of us. Of course your mileage (no pun intended) may vary. If you are into competitive driving or want more power, you get to experiment to your heart's content with Weber jetting.
Not exactly, well not 100% correct.

Solex 40s with 42mm throttles and 36mm venturis(modified for racing) flow better than 44IDFs with 36mm venturis. Tuning parts are easier to get for the Webers and jet changes are easier but Webers don't make more power. IDA Webers were designed as racing carbs and probably make more peak power but they don't work well on the street. They fixed the street issues for the 911 IDAs.

Porsche used Weber carbs often and not just for racing. Remember the 911?

Zeniths work well on the street because they are smaller and better for lower rpm use.

There is no reason to change from the Solexes to the Webers except for cost. I don't use Chinese Webers or Solexes but that is the cheapest solution I guess. On a valuable car where originality is admired then spending a little extra for first rate rebuilt original carbs is justified. Personal choice.

For an outlaw or a race car Webers can be an acceptable period modification.

That is my convoluted logic.

Let's move on to "which oil is best". :D
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Wes Bender
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Re: weber jet change: what to conclude?

#21 Post by Wes Bender »

Thanks for the clarification CJ. 👍
Some days it's just not worth chewing through the restraints.....

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Re: weber jet change: what to conclude?

#22 Post by C J Murray »

Wes Bender wrote: Sat Apr 20, 2024 9:37 am Thanks for the clarification CJ. 👍
Hi Wes. So much of what is said on the forum is not wrong but is rather a different opinion, a personal choice. There is more than one way to skin a cat. Apologies to the cat lovers.
'57 Speedster
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