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 didn't arrive.
Belsø clasificou un Fórmula 1 Iso-Marlboro para o equipo de Frank Williams Racing Cars no Gran Premio de Suecia de 1973, pero non puido comezar a carreira porque non chegou o diñeiro doseu patrocinador.