I have the same blowoff valve so this is why I am bringing this up.
At ECB last year I was reving off with a buddy that has a vet, keeping the motor high in the rpm, 6-8k for a good amount of time. As I am sure you know this would be the same as a single drift. My point is my ssq faces downward so I can't see into it. I was having a huge vacuum leak and power lose after. The face on the piston, the part that pushes closed came off the piston. It was actually sitting on a funky angle and leaking like a mother.
That was a huge vacuum leak, and like I said it faces down, so I never saw it or thought to check it. till for some random reason I took it off to clean it. I think I was looking for an oil leak.
Anyway, give it a look and make sure it is closed all the way.
