To “run what you brung” is an old racing term meaning racing what you rode to the race.
Back in the late 1940s when servicemen came back from the war. They had no money for racing bikes, but they wanted to have fun. Hooligan racing was mostly a Southern California thing where they could race year round. People would ride a motorcycle to the event, race with it, and then ride the same bike home. – Bruce Brown on Hooligan racing
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