Curious about my 40 IDF's

[retnavdvr]

Hi,

I bought a pair of 40 IDF's and a waffle manifold 6 or 7 years ago off Ebay from a seller in Italy. I cleaned them up with a couple rebuild kits and installed them on my 1608. When I received them they were set up with all the right jets and components as originally spec'd by Fiat. The carbs have worked great on my car for a few years now with no jet changes at this point.

My setup however utilizes a 40 IDF 10 as the primary or forward carburetor and a 40 IDF 12 as the secondary. As far as I can tell, these are identical to the well known 13/15 series pair that other owners have typically used. This question which has always nagged me, and one I've talked with various vendors and Weber dealers about over the years, is what the original application and /or history of this variation might be. Any Weber gurus out there that may be able to shed a little light on this for me?

Mike

[SteinOnkel]

Due to manifold and combustion chamber design, it can be possible for the inner and outer cylinders to run at slightly different air/fuel ratios on an inline 4. Usually the manufacturer just ignores this. The more elegant solution is of course to supply each cylinder with its required ratio.

Perhaps this is why you have different sized secondaries on your setup.

On my individual throttle bodies, this is evidenced by the fact that the velocity stacks are staggered. See here:

https://www.ebay.com/itm/12-16-Honda-Cb ... 2237131018

And that's on a modern engine, designed with CAD and machined with CNC.

[retnavdvr]

I'm not sure what you mean when you talk about secondaries. I may have confused you by referring to the rearmost carburetor as the secondary. I simply have the two carb setup common to our twin cams, each carb being a Weber 40IDF. So one venturi per cylinder, all four throats the same size. The only difference I can see from the typical 13/15 Webers is the fact mine are identified on the bodies as 40IDF-10 and 40IDF-12.

[Nut124]

What matters is what choke and jets are installed.

Pierce Manifolds might know what the p/n means and whether it matters.