It could be a leak on the actual BOV itself. Some types are known to open under idle and leak air into the intake system causing a lumpy idle but I was of the understanding that the HKS didn't do this. Alternatively, it could be a leak on one of the intake pipes that you have took off or replaced.
Idled like a bas..d when i fitted mine.
I fitted the idle control gizmo from HKS and amp; it sorted the problem.
Mine also idles like a bas..d when its been raining too.
maybe mines a bas..d full stop?
But like evoboy said prob an airleak.
Check that the stupid little inline disc filter fitted in the vacuum/pressure line 2 the dump valve isn't split , even if it isn't remove it and bypass it anyway because it will split. The HKS bov is different 2 other valves because boost pressure actually seals it more tightly , so if it is leaking it is either faulty or damaged.
It's actually a filter , intended 2 prevent any particles reaching the Valve , but as the air used by the valve is filtered anyway via the intake filter it isn't needed. The problem is that if it splits it leaks air which will mess up the mixture and prevent the BOV working correctly, and it WILL split sooner or L8r , every1 I've seen , except when new , have.