You should get full rpm at about 3/4 thottle only wide open on a glow carb any diesel will wander in rpm prop size on on the money
my Irvine 53 donversion will do the same thing if running wide open the engine does not need that much fuel as presnented at a wide open setting I do not thing wou need a carb restricter that engine is shipped with the glow carb larger venturi martin