If the petcock screen is clogged with rust chips, it usually means the upper part of the tank has lots of rust. Then it's time to drain the tank, remove it and start cleaning. Just throw a handful of nuts/washers/bolts into the tank and put the gas cap on. Shake the tank back and forthwith a few ounces of gasoline for a bit then rotate the tank a little and do the same. Usually the rust is on the top side of the tank. The nuts/washers/bolts will scour off any rust deposits. Be patient, take your time and do the job right. Rinse the tank with some gasoline and do it all over again. Repeat as necessary. Once the gasoline rinse is kind of clean your job is done.
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