Weber 'pump delivery valve' part number 64290.016 to fit the IDF range of carburettors.

It is a very important part, often overlooked when carrying out a service.

It fits through the pump jet & has a ball bearing fitted inside with a sealed lead plug in the top.

Very commonly this lead plug falls out, the ball goes missing & is either left open or another plug put in without the ball!

When you remove the valve & shake it you should hear the ball inside.

If you do not or its sticks, replace the valve.

Better still it should be replaced when servicing or problem flatspots occur.