The antisiphon valve is designed to shut off the fuel supply, if the fuel line starts leaking into the bilge. The engine's fuel pump is supposed to pull the A-S valve open, against the spring, to pull the fuel.
The A-S valve is important on sterndrives, as the motor is mounted lower than an OB and siphoning of the fuel into the bilge is therefore more likely.
I removed my A-S valve, to eliminate an issue like you are having. As long as the fuel line doesn't leak, I do not expect any issue.
