Flush it on its side (after draining/catching gasoline) at the car wash with high pressure water through the fill up hole, sort of balanced horizontally on the right side pedal to pivot entire bike slightly to and fro, petcock removed and letting it drain out completely, intermittently, through petcock bung to truly wash all that loose crud out, use your finger to open and close flow. Do the $3 special long time rinse. Chase with several gallon bottles of distilled water, drain the same as before, then possibly one more night’s soak in evaporust. Drain evaporust and another quick rinse/drain with a gallon of distilled water both the same again. Hair dryer and/or compressed air through the fill hole for awhile, turn the bike upwards on front end holding the brake, with ass end towards the ceiling every so often to see how much water is still flowing around in tank. When you can’t see any more water sheen, pour some marvel mystery oil in there and swoosh it around, drain as before. Finally fill up completely with 2 stroke gas, and should be pissin clean at that point plus help keep rust from reappearing. Also make sure to have a good petcock with screen and functioning reserve setting, and only use reserve when necessary.