R45 firing on one pot

[mitch9]

My sons R45 1980 started firing on 1 cylinder again ! the left hand diaphragm had rip in it last time so we fitted new one from sherlocks . Started to fire just on the right pot again so we stripped the l/hand carb and now the diaphragm is swollen and will not fit back in ? whats going on ? believe them to be the correct ones for flat top carb. any ideas thanks