I'm using a former Carlsburg tap that is fitted with a sparkler. I've had the sparkler unscrewed almost off and I've tried low pressure and higher pressure but the problem persists. The line to the tap is clear so I can see the beer and as the it flows up the line, gas seems to be bubbling along with the beer. I therefore get a very frothy hissing output from the tap

I ought to say that the beer is at room temperature and pretty flat; I seemed to get a similar effect when I syphoned it from the FV to the keg - gas in the syphon tube.
I was expecting to get a steady flow of beer and then to tighten the sparkler to get a head - anyone know what I am doing wrong?