The boys first took to the track for the nine to under-12 85cc/150cc, with Duffy winning all three races and Grothues coming third.
They then went on to compete in the 65cc 10 to under-13 class, where Duffy had one win and two seconds to finish second overall and Grothues again took third.
Also representing the northern suburbs was Jake Harvey in the 50cc auto seven to under-nine class. He came second overall with a second place and two thirds.
The fifth and final round will head to Wanneroo on September 8.