vn s1 commodore dual fuel stalling only on gas

[demiser]

i have a 90' model vn commodore that i recently got the gas tank restamped and running again. since then the car runs fine at 60km/ph + on both lpg and fuel but stalls on lower speed only on lpg. there is a round metal device fixed between the throttle body and the air filter tube that haves small adjustable screw on it. i have found that if i move the screw it seems to stop the stalling a bit. i was wondering what this component is and if i can use this to tune the car. do i only move it small amount?
any help is greatly apreciated

 

[anfo]

sounds like the gas regulator moving the screw small bits at a time is advisable move the screw bit fire her up see if she stalls keep adjusting the screw till she runs smooth