OOW,
Forgive me, I do not understand, do you not have a maintenance organisation signing off your machine, have you not contacted them? Usually, the first Immediate Action with piston engine problems is to change the plugs, I did this with a G5 which wouldn't start and it worked.
Your fuel does not need to be replaced if you have carried out water checks. What is the history of this problem, when did it last run normally? What has happened maintenance wise since then?