D'oh! Fueled up the 504 today. Problem? When I went to put the lead substitute in I unscrewed the bottle and tipped in the required 50ml along with the seal off the bottle! I hope it continues to float. If I get fuel starvation I will know the cause and just have to blow back down the lines into the tank. Be warned! I hope there is a filter on 504 tanks because I really don't want to pull it out to search for said seal. Neil