Belsø qualified a Formula One Iso-Marlboro for the Frank Williams Racing Cars team at the 1973 Swedish Grand Prix, but could not start the race because his sponsorship money did not arrive. In 1974 he tried four times to qualify for Williams, but was only successful in South Africa and Sweden. In South Africa, the clutch on Belsø's FW failed on the first lap of the race. In Sweden, he finished his only complete Grand Prix, in eighth place.