Boring Weber IDF Chokes (Venturi)

[mack124]

Hello Guy,

a while since my last update on my 124, work has been a bit full on but I'm on end of year break now so I can get back to the Fiat.

Just to refresh the engine spec: It is a standard 2.0 ltr 132 bottom end with a ported head, running standard cams and a pair of 40 IDFs on a Waffle manifold.

The carburettors have 145 mains jets (for now), 180 Air Correction jets and 34 mm chokes.

Now it is the chokes I need to ask about. They were 32 mm chokes that have been bored out to 34 mm. They were bored in 4 stages.
1st, I bored them out to 34 mm (Parallel)
2nd, I bored the taper out to just meet the new 34 mm bore
3rd & 4th, the radii were blended in.

That was the plan anyway. What happened was, when I was taking out the tapered bore I didn't take it out to meet the new 34 mm bore but instead I stop short leaving an 8mm wide parallel section. I was worried that the small radius at the bottom would disappear and be to thin. I then thought I would change the angle of the bore so that I could blend from the existing small radius to the new 34 mm bore.

What do I do?

1: Leave it the way it is with a small parallel section?
2: Keep the taper at the original angle and take it out to meet the new bore size?
3: Alter the angle so as not to interfere with the small radius?

I've been led to believe it is also very important at which point the top radius blends with the bore. What distance should that be from the top of the choke?

I have attached a diagram to help explain but if you need more info just let me know.

Keep up the great work on a fantastic site

Kindest Regards
Gill

[Guy Croft]

Mack, hi

thanks for letting me know this post was not urgent.

First note below, the narrowest section of your chokes at 34mm dia should be placed exactly where the exit of the secondary (auxiliary) venturi (shown as black rectangle) stops.
The leading edge will be elliptical and the trailing edge a straight taper finished with a rad and maybe a small locating face too. I need to pull out an IDF from store and strip, measure, draw up. I imagine that the included angle of the taper will need to fall inside 15 deg to prevent separation and this will to an extent dictate the trailing edge radius.

I'll get back to you on the other profiles asap,

GC

[mack124]

Thanks Guy,

I know where I'm headed now. I'll check to ensure that the 34mm section is in the correct position and I may take a tiny bit more out of the tapered section.

I suppose the best thing about chokes, is they are relatively easy to remake if I miscalculate along the way.

I’ll let you know the results.

Thanks again

Gill

[BasilBadger]

Hello Gill,

How did you fare, with this modification?

Wifey and i have been reading about it with interest, since our 124 Coupe will probably end up with 40 IDFs.
She has been doing a couple of test runs on the CNC mill and will probably make a first set of chokes sometime this week.

All the best,
Henrik