The fuel and nitrous jets are supplied as a matched pair, according to estimated bhp required, the fuel jet supplied normally allows the engine to run slightly rich. Providing the bottle temp is optimum, around 900/950psi if the temp is lower this effects the system and causes an excessivly rich mixture the way round this is to reduce the size of the fuel jet, a better and safer option is to use a heater that maintains a constant temp around 900/950psi.