Nikki 6050 carb priming issue?

hi, could somebody please advise me with a problem i have with a Nikki 6050 carb. the problem is that when i attempt to prime the carb fuel spurts out of the main jet at the needle valve rather than out of the small spill pipe on the side of the carb like it used to. I am wondering if their is some sort of check valve between the carb bowl and the main jet which is should prevent this happening but may be stuck open. could anyone please help
kind regards
mark