Pump Diaphragm for the Weber 40 DFAV carburetors.
Please note that the 40 DFAV uses a larger diaphragm than the other variations of the DFAV.
The cam, including the (non-compressed!) spring-loaded tip, measures 12.7 mm from the surface of the top disc.
The diameter of both the top disc and bottom disc is 25.4 mm.
The bolt hole spacing is roughly 44 mm, heart-to-heart, measured diagonally.
The other versions can be found on the Weber DFAV Parts Page.
Please note that the versions are NOT interchangeable!
This product is priced and sold as a single unit.
Required quantity per carburettor: 1
Weber Part Number: 47407.027
Operating principles of the acceleration pump and the role of the diaphragm in this:
This diaphragm is activated by a lever that is connected to the throttle lever(s).
When releasing the throttle, a spring returns the diaphragm to its initial position and by doing so, it sucks-in fuel from the float chamber.
A one-way valve prevents this fuel from leaking back into the valve chamber.
When you then activate throttle, that same lever pushes the diaphragm inwards and the pressure that is thus created makes the fuel being squirted downstream with the main airflow through the pump jet.
This extra flow of fuel will prevent "fuel starvation" from occurring when the throttle is suddenly opened which allows large quantities of air being sucked into the engine.
When you then release the throttle again, the diaphragm is again pushed back by the spring, thus priming the pump chamber with a fresh supply of fuel and making it ready to repeat the procedure.
This is an excellent system that has been employed for decades by various makes of carburetors and applied to all kinds of applications.
So, not only Weber but also Solex, Pierburg, Zenith and Mikuni used this system with more or less identical operating principles.
However, to ascertain a flawless operation of this system, the diaphragm need to be in excellent condition.
The original diaphragms are now coming of age and are suffering from the ravages of aggressive modern fuels which causes the "rubberized textile" material to harden or even crack, causing erratic running of the engine and fuel leaks.
Replacing the diaphragm is then the only sensible option which is a rather simple operation.
A complete overview of our pump diaphragms for Weber carburettors can be found HERE.