Driving issue after fitting Walbro pump

[Nattman82]

So fitted a walbro 255lph fuel pump which I purchased from a HJC member a few weeks back. Now I have an issue with the acceleration. Best way I can describe it is that when im accelerating the pump isnt pumping smoothly its 'pulsing', making the car accelerate in what I can only describe as 'bursts', as though im pushing the peddal an inch down then stop, inch down then stop etc. This happens no matter how im driving be it slow cruising around town or under hard acceleration.
Before I say the pump is duff, can anyone suggest why this might be happening? Just incase ive missed something stupid.
Thanks.