Thursday, January 24, 2013

Weber Carburetors, Part II

Anatomy of a Weber IDF

The carb in the following pictures is a junk 40 IDF that I picked up locally for nothing but I had to buy the top as it was still good.  I wanted the carb to serve as a mockup carburetor for my engine.  It's a great piece for exploring the inner workings of the carburetor.  In all, the carburetor isn't as complex as you might think.

Rear view.  Block-off plate is missing.

Fuel side view, showing the bare accelerator pump area and the early-style barbed fuel inlet.  Note the absence of a second fuel inlet on the other side.  Newer IDF carburetors have threaded M10x1.0 inlets on both sides, with one blocked from the factory with a threaded plug.  There are three holes at the base of each barrel; the outermost hole is for the idle air bypass screw, the innermost hole is a vacuum port, and the middle hole is for the idle mixture screw.   Idle jets are inserted in the carb body just to the inside of the fuel inlet.  

Top down without the lid, showing the fuel bowl area and main jet bosses.  Note also the chokes (or venturi) tubes.  The choke tubes in this carburetor are frozen in place.  Obvious attempts to remove them forcibly are evident underneath the chokes.  Auxiliary venturi are inserted above the choke tubes in grooves on the barrel sidewalls.

Underside of carburetor.  The throttle shaft is visible, but the butterflies were removed.  The shaft is frozen solid in this carburetor, but would normally be free to rotate on the ball bearings.  No ball bearings are seen in between the barrels, and I'm not sure how the shafts seal or turn in the body there.  Note the cam plate in between the barrels which is an early IDF feature much hated by the tuner I spoke to.  The cam plate actuates the accelerator pump, squirting fuel into each barrel when the gas is pumped at startup and increasing fuel supply when running.  The later IDF's have a spring and lever arm in place of the cam plate, which seems to work much better.

Carb and Intake Measurements

As mentioned before, the IDF is about 4" tall.  The Aussiespeed manifold is 3" tall from base of manifold to top of carburetor pad.  A gasket is used under the manifold and under each carb, bringing the height to about 7.25" tall.  Velocity stacks can vary, but the commonly supplied stacks are 2.25" tall.

There's more to come, including assembled carburetor pictures, fuel line hardware and purchasing places, more on linkage options and linkage parts sourcing, etc.  Check back soon.

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