The Magnum has a fairly large OD on the venturi. OS parts from the FP or LA series are too small. You can get nylon bushings from Home Depot/hardware store that are fairly close and can be filed to fit using a drill or drill press. Easier is to buy a machined venturi for 10-15 bucks. Use the OS CL needle valve it has a smaller OD and a fine thread for easier settings.