Now that you mention it having a black Sullivan stopper turn into mush may have (very likely) happened to me... Last year I noticed that the stopper in the Sullivan tank in my 29% Aeroworks Edge 540 had turned to mush. Fortunately I just happened to catch this problem before any damage was done because of another issue... I bought this plane from an experienced builder/pilot and I remember thinking it was odd that he would have mistakenly used a glow fuel stopper... I've since replaced with a Dubro tank and brown gas stopper... Azhar -- you should seriously consider getting a brown Dubro stopper...