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Discussion Starter #1
I made this table and chart while working to tune my 1750 with DCOE 151s
I've been experimenting a bit again and resurrected it. It might be of interest to folks here.

Rather than just do what others have done I read up a lot on how the webers work, what circuits are used when and general rules. How do the jets mix fuel?

The idle jets are the hardest to compare (air and fuel together) so I focused on them in this chart.

I calculated the area of air and fuel inlets and determined the ratios. Of course venturi, emulsion tubes, pump bleeds, vaccum leaks and timing figure in as well but once you are trying to make idle jet changes to get the lower end sorted this helped me understand what I might try next. I use a AEM wideband to monitor changes and went to a dyno once I got close to really dial it in. As an example I can see a 50F9 is a bit richer than a 55F8 but less so than a 60F8.

I may try to make something similar to evaluate different ETs, air and main jet combinations - probably focus on F9,F11, and F16. Webers are remarkable carbs.

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A "lean" to "rich" chart for idle jets would be helpful, if such a thing is possible.
There doesn't seem to be a linear, logical naming convention with the way they are labeled.
There probably are some out there, but I've just not seen them.
 

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Discussion Starter #3
That is pretty much what's above. I might have missed some jets but these are common ones. 50F2 on the lean side to 65F9 on the richer side. The chart is sorted by AFR (purple) the combination of air and fuel. You can still see components red=fuel and blue = air
 

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Interesting!
I swapped back and forth between 55F9 and 60F8 and in my case I had a lean hesitation with 55F9 that disappeared with 60F8 but 60F8 was a bit too rich in other places. That seems to be at odds with your table. I made a set of 57F9's which are in the car now and seem to be the best choice.
Jim Steck told me that idle jets are like miniature emulsion tubes and the fuel orifice has more influence on the lower end of the progression and the air orifice has more influence on the higher end.
My perhaps incorrect analysis is that if a 55F9 ad 60F8 theoretically make the same mixture strength, the 60F8 will supply more emulsion which will richen the transition.

In Des Hammill's book he says that the fuel orifice size approximately varies with the engine capacity and that a 500 cc cylinder is likely to work best with a 55 fuel orifice.
 

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This is very helpful.
I see where the 50F11s that are in my duetto's 1600 are over at the left.
If I use ones going to the right, they get gradually richer.
The purple aggregate bar in each grouping indicates the cumulative strength of each jet,
which determines where you've placed them on this chart.
Thank you!
 

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The chart does not state at what RPM the AFR was measured. Perhaps more important would be a chart showing the AFRs at several RPMS, ie idle, 1,500, 2,000, 2,500, 3,000.

The data would be increasingly corrupted above about 1,500 as the main circuit starts to kick in, but we need to be able to correlate the fuel diameter and air diameter with where in the throttle opening does each most impact.
 

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Discussion Starter #7
The AFR is not a true AFR like "14" measured with a wide band but the ratio of fuel orifice area to air orifice area which is constant. It is really just a way to understand the relationship on the jet itself.
The AFR we measure with a wideband is affected by dozens of factors and I think would be impossible to describe in a chart. This is just a tool to help choose your next idle jet.
 

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Discussion Starter #8
Minor update. the AFR column was showing the ratio of the fuel and air but using the diameter not the areas mm^2. The progression is the same but the values changed a little.
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Discussion Starter #9
Interesting!
I swapped back and forth between 55F9 and 60F8 and in my case I had a lean hesitation with 55F9 that disappeared with 60F8 but 60F8 was a bit too rich in other places. That seems to be at odds with your table. I made a set of 57F9's which are in the car now and seem to be the best choice.
Jim Steck told me that idle jets are like miniature emulsion tubes and the fuel orifice has more influence on the lower end of the progression and the air orifice has more influence on the higher end.
My perhaps incorrect analysis is that if a 55F9 ad 60F8 theoretically make the same mixture strength, the 60F8 will supply more emulsion which will richen the transition.

In Des Hammill's book he says that the fuel orifice size approximately varies with the engine capacity and that a 500 cc cylinder is likely to work best with a 55 fuel orifice.
Also interesting. The air holes are on the sides of the jet while the fuel is at the bottom so indeed it is like a mini emulsion tube. If he is right a lean stumble I am getting with a 55F8 might be cured with a 50F9 but we'll see. I never thought to invent a new jet like you did. Eventually you get to a point where you are moving between compromises.
 

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Ah. My understanding (under constant revision) is that the fuel orifice and air holes do not really have a linear relationship to each other. So, comparing them in a linear formula tells us what?

In my own tuning I was taught that the fuel orifice has a fairly singular effect on idle, further adjusted by the idle “mixture screw”. The goal with the earlier, coarse thread screws is to pick a fuel orifice that results in about a 1 1/2 turn of the screw, more or less.

THEN, one starts choosing different F holes to obtain the right mixture up through the progression holes and into the transition zone. In general, one should be able to tune the idle circuit without the E-stack in place, ensuring a smooth throttle transition up through about 2,000 - 2,500 RPM.

THEN, you start tuning the main circuit.

My point being, I don’t think the idle jets are correctly understood by viewing the orifice number and F-hole diameter as being additive.
 

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Progression holes are another factor which work hand in hand with idle jets. Some Webers have two big progression holes and others have 4 or 5 small ones and that can affect idle jet choice.
I think that the chart is useful, especially if you are going to buy some more jets for testing. I add the square of the diameter of the air holes to get an effective Air number when selecting jets. But the proof of the pudding is in the eating - how do they drive!
 
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I used the awesome data above (and info from the Weber manual) to put these in order as an alternative way to look at it.
I have not confirmed these with AF actual measurements. Your results may vary.
I'm hoping some on the bb have done testing and can add their experiences.
Happy idling,
Dave

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I like that presentation. Having said that, I think that the fuel orifice has a greater impact than is shown in these calculations just as changes in main jet size usually have more influence than similar changes in air corrector.
For example, my experience, as noted earlier is that 60F8 is richer than 55F9.
 

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Am I understanding correctly then that the AFR in the chart above would be an AFR for the idle jet itself -- a measure of how rich the mixture is coming from the jet, but that the AFR that the cylinder sees is a ratio of the air in the idle jet mixture + the air passing by the throttle plates over the amount of fuel entrained in the idle jet mixture? If so, it helps me understand why the fuel orifice size is a driver for setting the correct mixture for idle, since we're getting air from two places (and can probably adjust a bit with the idle mixture screw) but we only get as much fuel as we can get through the fuel orifice. (I'm hoping that's right)
 

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Farleybob..

You’re close, but among other things, this thread is using technical terms differently than common practice, which is probably confusing things.

As the OP responded, his “AFR” is just the geometric difference between the fuel and air orifices. His first chart used diameters, the second the areas, to establish a ratio. My opinion (YMMV) is that this info is not likely to lead to a more scientific approach to tuning. It helps to make sense out of the hash Weber made of their nomenclature system, but beyond that, I don’t see how this changes one’s approach to tuning.

To take a further step onto the Wild Side...

Keith Franck is attempting to reestablish alchemy as a science. He occasionally achieves a sort of progress, but there’s a lot of white noise in between.

His more successful work is focused in two areas: refining what he calls the “tip-in” of the main circuit, and an alternative approach to idle jets. I think the latter are focused on engines with a low idle vacuum, ie race engines. For most of our modestly-cammed engines, idle vacuum is sufficient for standard idle jets.

Happy running with Weber’s is usually driven by how well the idle and main circuits overlap. The “F-holes” in the idle jets, and the dark art of E-tubes is where the magic lays.
 

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Discussion Starter #16
Don't disagree with anything said. Instead of AFR I probably should have said JOR (Jet Orifice Ratio) My main aim was to visualize differences in the idle jets which is hard from the weber scheme. I like the other chart too. The fuel orifice may figure in more, and if that is so, you can still use the progression to make decisions. Right now I have a 55F8 and am going to try a 50F9 then a 60F8 and see how they affect the critical transition spot I am trying to perfect. Farleybob you are following. I think it is true that you can get a weber setup to idle well with a wide range of idle jets as there is an adjustment for this with the idle screws. However once you are past idle the orifices on the jet matter. This is why there are certain rules that if you have to turn the idle screws out very far you probably have an oversize jet.
 

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Boston - what car are you working on?

With mixed feelings, I do recommend Keith Franck's VF tubes. He's now working on an "NF" tube, with the as-yet-proven idea that they'll replace all E-tubes on the planet. To add my thoughts to this...

The Idle jet fuel orifice will first be important to the idle. Yes, on a given car, perhaps a 50, 55, or 60 will make the idle happy.
Once the throttle is advanced, the F-orifices of the idle jet increasingly come into play. They, and the quantity, size, and location of the progression holes will determine the "curve" of the idle circuit fuel flow. This can be further refined by moving from a 50 to 55 to 60 fuel jet, but that is mainly trimming the curve determined by the interplay between the progression holes and F-holes in the idle jets.

There are other factors, but let's not complicate things now.

The E-Tubes (or in Keith's case, the VF tubes) determine when the main circuit starts, and it's curve. The main jets play a role here as well as the float bowl height, the size of the auxiliary venturi, and probably the strategic location of the plenum float bowl vents.

Generally, it is easier to get the idle circuit working well, with NO E-stack installed, before attempting to tune the main circuit. Keith's VF tubes appear to me to delay the onset of the main circuit, and make the "rise" more abrupt. Using my very early 45DCOE9s, the reduces the rich bog during the transition.

I think your chart is useful in this way...

I would usually start with a 55F9 idle jet and see if can get a sweet idle with that, including the right number of turns on the idle mixture. This would be done with no E-stacks installed.
Then, I would load in my E-Stack (with whatever you think is right) and see if I can get a good half and full throttle behavior. If that works, but the 2,000 - 3,000 RPM range is problematic, I would first try adjusting the idle jet, either more lean or rich depending up AFR during the progression, and using your chart to pick the leaner or richer jet.

I need to get back to my Touring 2300 at some point, and will have your chart to see how well it predicts the responses.
 

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Discussion Starter #18
Your process is exactly what I do. The stumble seems best addressed by messing with the idle. I may end up making my own jet like alphaparticle.
Very curious about Keith Franck's VF tubes.

I am working on a S1 GT 1750 Veloce with 40 DCOE 151s.
The engine is 1870cc, 10.1 compression and the head is ported with 11.1 lift (moderate) cams

I am running 34mm chokes, 55F8 idle, F11 ET, 125 Main, 190 Air. This showed 130HP (wheels) on the dyno at KTR in Ayer MA. Most of my session was nailing the main circuit so the under 3K can use further tuning. I may go back in the fall once I find something I like with my own wideband.
 

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Very curious about Keith Franck's VF tubes
Richard Jemison told me a story last week of a guy at Sebring who's race car was running like [email protected] It turned out that he was running those snake oil emulsion tubes. RJ told him to put in a set of F16's and it was transformed.
 
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Discussion Starter #20
I am very close on my jetting. I've tried F9 / F11 and F16 and the F11s seem to work best in these 40DOCE 151s and are the ones they come with. At first I started with stock DCOE jetting for a 1750 but the F9s didn't work as well. I also kept the 151 pump jets and bleeds which are different than stock DCOEs. The newer webers are subtly different.
 
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