Turn the spring adjustment anticlockwise to reduce spring preload. Some cars, Nissans in particular, will always do this under normal driving conditions, but should still vent cleanly when driving hard – this is nothing to worry about. Pulsar GTiRs and other cars with multiple throttles, porting or ‘big’ cams (or any mods that reduce manifold vacuum) will require a softer spring (GFB 6116).
Also, check that the vacuum hose is connected to the engine inlet manifold after the throttle, that the hose is of at least 3 mm inside diameter, is less than 1m long, and that there isn’t anything else tee’d into it e.g. a boost gauge or a MAP sensor. T-joins, accessories and small diameter hoses will slow the reaction time of the valve.
Case study: A customer arrived in a 180SX with a GFB Valve that wasn’t working properly, and the car was running like a dog. A quick inspection showed a single, small diameter vacuum connection on the manifold led to the fuel pressure regulator, which was tee’d to the boost controller, which was tee’d to a boost gauge, which was tee’d to the blow off valve. The boost controller didn’t work because it should not be connected to vacuum (hence the idle was bad), the tuning was out because the fuel pressure reg was not receiving a clean signal, and the BOV had no chance at all after the signal had been weakened by all the ancillaries before it.