hi dnt know much about these but i got a honda its 8.5hp and its powerfull enough to pull its own water through, i wander if thats your problem, its not powerfull enough. i have to wait a couple of seconds for the pressure to build but then its fine.. and sorry to make it obvious but if it was new have u put oil in it, cos i got geni this week and that didnt run properly then i noticed it didnt have any oil inside it,
failing this get hold of the bloke that sold it to you
goodluck jonny