High-Performance Weber Upgrade Part 3

The last things to be removed are the venturis and the 37mm chokes.

Simply and straightforward, the bare body is placed in a lathe and bored out to, in this case, 51mm. Jack sets the lathe to do this in several passes, as not to damage the metal and his bits; plus, he's able to better monitor the progress.

The only thing limiting the 48 IDAs to being bored out 51mm or 52mm or 85mm for that matter is this little pinhole, the working end of the mixture screw. Since building the IDA is a machining process, the location of this hole can vary from carb to carb--the farther back in the body, the better--but 99 percent of all IDAs can be bored out to 51.5mm with no problem. This one will stay at 51mm due to the size of the engine, a mere 2500cc.

Next in the process is something you don't have to worry too much about, the making of the butterfly valves. Since this is a custom job, custom butterflies have to be made to precise specifications, as most problems occur here.

Here is a comparison between the original stock 48mm butterflies to the new 51mm units. Also note the 12-degree turn on the edge of the butterfly to fit flush against the barrel wall. At the point they met, you don't want any leaks, so the clearance must be no more than 0.0015mm smaller than the bore.

After new screws have been set in the throttle shaft, each screw has to be shaved off as well as a section of the shaft to fit in the bore. Make sure that only one side of the shaft is profiled, so if your engine ever backfires, it won't break.

This is a before-and-after shot of two throttle shafts. The top one is ready to be installed. Note the screws are countersunk into the shaft. Though slight, it allows for better air flow.

The heart of the conversion comes with the milling of the carburetor body to allow for more fuel in the float bowl. To do this, a half-inch in-mill cuts away a wall that separates the float bowl with an unused chamber.

This is where Jack's experience comes to play as he knows exactly how much to remove so there's no damage to the carburetor. At this point, it is easy to break through an outside wall and render that carb useless.

A side-by-side comparison of two carburetors. Note the removed wall of the left carb. This allows for 120cc of fuel to be stored in the newly expanded float chamber...a 20.8 percent increase over the stock 95cc.