The only way to do it properly is to remove the tank and then flush it back out the main fill connection - that way you do not disturb the fuel pump, fuel level indicator and fuel return hose (break this and the problems really start!). If you think it is water, E10 fuel will eventually remove this (ethanol mixes with the water), but if you think it is solids check the fuel filter as it will show what has been trapped. The strainer at the end of the fuel pickup tube in the tank should strain all dirt bigger than a grain of sand but you still end up with all that crap still in the tank.
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