Try to use different brand of fuel-switch to O'Donnell and will never see it again.
What you see inside your engine-is not normal. If you can easy remove it with just a piece fo fabric-that it is "garbage" in fuel if you have to use very fine sand paper-most likely it is water damage. In any case it will make life of your engine way shorter.
I have REX (practice motor for my son's RRR) with about 8 gallons of O'Donnell through it, never use after run oil and didn't open motor since last year, Month ago finaly open it to see if engine still usefull for car tunning practice and as usualy it is perfectly clean and shiny.
Edward