Sounds like the pin on the metering unit may be jammed or sticking.
easy way to check would be...bridge the fuel pump so it runs constantly, push down gently on the metering flap, is there a resistance or does the flap just flop up and down easily? (get a feel of the flap before bridging the pump so you can tell the difference)
OR alternatively, remove the fuel distribution part (the bit all the piped bolt to, and you will see a pin in the middle poking out, this should move freely but can jam with lack of use.

HTH