This is an optional ProTek R/C Aluminum Samurai RM, S03 and R03 Carburetor Restrictor. Available in 6mm through 8mm diameters, ProTek R/C carb restrictors are an excellent way to tune the power delivery of your Samurai 321B for any track condition. Plus, they are also compatible with OS V-Spec, Novarossi and RB carburetors.

The carburetor restrictor (or venturi) is used to adjust the flow of air into the engine. By changing the size of the venturi, you can fine tune the power delivery and run time of the engine. A larger venturi will increase top end power and is better suited to larger tracks, but will reduce run time. A smaller venturi will improve bottom end throttle response making them a good choice for small tracks, and will help to improve run time compared to a larger venturi.  

Note: For replacement restrictor o-rings, see part PTK-2517.