I’ve had similar temperamental problems with this (& a 100 other things!!)
The only thing it can be is the needle valve that is operated by the floats (or the floats themselves!)old fuel varnish or dirt is the usual suspect...
You can get a replacement kit I think. The needle and the brass insert it fits into...
A good inspection and clean is usually good enough...
P.S. the ‘weight’ of fuel can exasperate the situation, or it has in my experience (which isn’t much!)...
G
gibboharps
@gibboharps
