Boring Weber 40's to 46mm?

[EarlySport]

Hey guys,

In the past I have heard stories of Weber 40's being bored into 46mm carbs. Can anyone shed any light on this procedure? After the main bores are opened up into 46mm diameter, what else besides the throttle plates and main venturi's need to be replaced with 46mm parts?

Thanks,

[1QuickS]

I offer this service. I have made several 42mm conversions and am ready to perform the task to convert to 46mm. There are a few items to convert for this task: the venturis are the same outside diameter as used on the 40mm throttle bodies but the bottom edge must be machined to a knife edge which then requires the venturi set screw and the venturi be modified to positively lock the venturi into the bore; the throttle shafts need to be modified (elongate the slits) to allow the larger butterflies to be installed and the transition ports need to be reviewed (modified) to provide good fuel supply during the progression phase. I have the data from the four hole progression circuit as used on the later 46mm Webers and believe the conversion is best performed on IDTP carbs rather than IDA or IDAP carbs. Also, IDTP's are not as attractive as the others for performance usage so they are lower cost options as a core value. Another item of interest is that due to the larger throttle plates the bearings in the throttle bodies have less support of the throttle shafts. This, combined with larger loads on the throttle shafts due to the larger butterfly area cause the bearings to wear more rapidly than on the 40mm bodies. For this reason I would strongly recommend rebushing the bodies to assure well supported shafts. Don't forget that the manifolds need to match the 46mm interface and blend to that of your intake port dimeter.

Cheers,

Paul Abbott
www.PerformanceOriented.com

[ajwans]

Hi Paul, did the IDA46 use the longer secondary (inner) venturi? I have seen a parts
diagram with both the 40mm & 46mm secondary venturi.

andy

[1QuickS]

The secondary venturis are interchangeable; routinely the tall ones were used in the 46mm Webers as they provided a better fuel delivery at midrange which was important for the 2.0 liter engines they were first used on. Also, the early 46mm Webers had a meager transition circuit system that was later modified for street usage so the tall secondaries were important for race usage; a recitation of info I have read over the years.

[ajwans]

Does anybody know how tall the tall and short secondary venturies were?
I have a set here on my desk which measure 50mm. I think they are the
short variety.

andy