I have a Johnson 3.5 internal tank on my 525, always shut the gas off, run the gas out of the carb, raise it and tilt it , place the sunbrella cover over the engine. I've done that for over 12 yrs. It starts on the third pull and runs strong. Try to find 100% gas, we just have a few stations that still sell it in our area. Only problem to date, watch out for the mub dobbers, they will make a nest in the strangest places like air cleaners or water exit tubes...
Cal 9.2 #19 SilverSwan