I tasted the beer when it came out of the fermenter and it was great. I cooled it to 4 degrees then I racked it into the keg and force carbonated - about 1 minute at 300kPa, a good few minutes of vigorous shaking later it dropped to 140kPa so left it for an hour. I then tapped the keg and it was all foam - very overcarbonated... my regulator is a bit dodgy so will go for less CO2 next time.
The thing is, when I tried the beer after the foam settled the flavour was completely different - very astringent, almost plastic-like, flavour. This is the first time I've use the lines and the CO2 cannister.
What could the off-flavour be caused by? Could it be bad gas? Or bad lines (heat-resistant PVC, only about 30cm long)? Would chlorine in the water be doing something bad with the CO2? Or just the fact that it's over-carbonated so there's excess carbonic acid?
Also, is there any way to resolve the situation? I guess just periodically release the CO2 from the head space to solve the over-carbonation issue, but is that also likely to resolve the flavour issue?