Made In Japan Keihin Brass Jets available for your Keihin carburettor. When people talk about jetting a carburettor, tuning a carburettor or "breaking out the brass" they're talking about manipulating the carburettor's four main circuits to optimize petrol delivery and therefore engine performance. They might adjust the air screw, adjust the jet needle's clip position or exchanging the pilot (slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size. If you introduce power air filters, you will very probably need to increase the size of your main jets.