Wednesday, January 23, 2013

Weber Carburetors, Part I

I've always loved Weber carburetors.  I think they make a V8 look just that much better.  Lately, I've been working on a few ideas.  I'm planning to use 2V style heads since there are so many manifold choices these days.  However, up until recently, there hasn't been a Weber intake for the 2V Cleveland that is readily available.  There is a manifold called the "Cain" manifold for a 2V Cleveland that was not produced in numbers and is pretty much non-existent.  However, recently a version was made in Australia by Aussiespeed that is the same thing:

Aussiespeed Manifold

The intake manifold is built for four Weber carbs on 2V heads but will seal against 4V iron heads.  I understand that the manifold really can't be ported to the full 4V iron port size.  There is a central bellcrank, ideally suited for the Pantera.  The catch is that the manifold is made for IDF carburetors, not the common IDA carburetors.  Great, right?  Why would someone want IDF carburetors over the IDA carburetors?  IDF carbs are better suited for the street, plain and simple.

Here's a set of four IDF's on an Aussiespeed manifold, courtesy of Brian Halamar:


Brian sent me these pictures of his mocked up set.  He is working on linkage and fuel supply.

The IDF is 4" tall from bottom to top of carb, not including the main jet extension.  The carb consists of a main zinc-alloy body, a matching zinc-alloy top, throttle shaft on ball bearings with brass butterfly plates, individual main & idle jets for each barrel, emulsion tubes for each barrel, interchangeable venturis (also known as chokes), floats,  a needle & seat, and various other parts.  Velocity stacks are specific to IDF's.  40 and 44 IDF's share the same velocity stack while the 48 IDF is different.

IDF carbs were made to readily accept air cleaners due to the flat top plate.  The IDF takes the same main jets, emulsion tubes, and idle jets as the IDA.  The IDF has four progression holes for transition from the idle jet to the main jet at part-throttle operation.  On the front of the carburetor, there are provisions for ported vacuum for vacuum advance distributors.  Webers are marketed as having no rubber parts, but the IDF carb has a rubber diaphragm-type accelerator pump.

Differences between IDA and IDF Carburetors

IDA's need velocity stacks to help the venturi work properly. IDF carbs do not have the same venturi design and should not need velocity stacks.  IDF's, again, were made to take air cleaners.  To get an IDA to accept an air filter, you need to use a velocity stack extension and modify it since the velocity stack serves as the upper part of the venturi.  IDA carbs only have two progression holes standard for part-throttle operation, but can be modified by drilling a third hole.  Even with the third hole drilled, it still doesn't match the IDF carb's four progression holes.  IDA carbs have a brass piston accelerator pump whereas the IDF carbs have a rubber diaphragm-type accelerator pump.

Are Velocity Stacks Necessary?

As stated above, the velocity stack or some form of it is necessary in the IDA for operation, but not in the IDF.  Yes, the velocity stacks create a nice transition for the air to go into the carb to reduce loss of flow, but I would be surprised to see it make a big difference for the average joe with an air cleaner instead of the velocity stack. I would take an air cleaner & better engine longevity any day over a small percentage of power lost.

Linkage, Linkage, Linkage

Linkage is utterly important on Weber carburetors as I understand it.  Slop in any linkage element is frowned upon.  Most builders use some form of bellcrank in the Pantera, choosing to mount the throttle cable in the valley between banks of carburetors.    Here's an example of a Ford 302 with Weber IDF carburetors and associated linkage:

Racetep IDF Carburetor Linkage on a Ford 302

Notice that the carb throttles are linked from one side to the other with a long rod link, and then linked to a bellcrank by a shorter rod link.  Several people have said that this isn't the ideal setup, while others like it very much.  It has the advantage of being able to adjust all four carburetor idle speed using the short link.

Other people like a center bellcrank,  noting the difficulty of getting the carburetors synchronized.  I'm not sure how the center pull helps.  There are still four rod ends on the center pull setup, so slop isn't reduced in that manner.  I am learning about the process and don't know everything, so I'm open to new ideas.

More on the linkage later as I learn more and Brian has a chance to get things going on his manifold.

Experience Counts

It turns out there is a local VW shop near where I live that rebuilds Weber carbs. I talked at length with the guy who rebuilds and tunes Webers, Zeniths, Delortos, Solex, etc. The rebuilder/tuner seemed to like the Delorto carb that has the same mounting pattern as the IDF, but said that they are no longer being made and scrounging up four 48mm versions all the same would be much more expensive than four brand-new IDF's.  When asked as to which carb would be best for the street, there was no hesitation in telling me that the IDF was the cat's meow of Weber-type street performance carburetors.

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