The Yamaha RD 350 YPVS, also known as the RD 350 LC, was a twin, two-stroke street bike made in 1980 and 1981. YPVS is an acronym for Yamaha Power Valve System---one of several advancements over the previous RD350s manufactured from 1973 to 1975. The YPVS version of the 350 was not sold in the US, but a mechanically similar model in race trim, known as the RZ350, was. They shared carburettor technology.
Each of the RD's two cylinders was fed by its own a 28mm Mikuni carburettor. The carburettors are essentially identical except for the choke, which only resides on one of the carbs, and affects the other by way of an air-tube.
The RD's Mikuni carburettors have a number of adjustments and means for tuning. Some adjustments can be made by replacing interchangeable parts. These include a variety of jets that control the flow of fuel and a needle that affects the flow of fuel and air. Other adjustments are made by resetting certain parts, rather than replacing them with different sizes. The adjustable items are the pilot screw, needle assembly, main jet, choke and throttle valve.
Of the adjustable items, the ones that can be tuned by setting rather than replacement are the pilot/air screw, throttle valve, needle and choke. The pilot/air screw controls rpm at idle and very low revs. In completely stock trim under normal weather conditions, it should be set one and one half turns out. In other words, tighten it clockwise until it is closed. Then begin to open it, counter-clockwise, one and one-half revolutions. The needle assembly works in conjunction with the needle jet. While both the needle and the needle jet can be replaced with varying shapes and sizes to affect performance, the needle itself will be set to one of five positions. The position affects the richness of the gas/air mixture. The factory needle setting is of little or no importance compared to weather typical in the area you ride or the elevation. It's more important to note that settings further from the tip of the needle will cause the motor to run relatively leaner and those closer to the tip will cause it to run richer. The choke system is a binary setting; it is either on or off. When the choke is on, it will temporarily lean-out the mixture, making it easier to start the motor under most circumstances. The throttle valve is the actual operation of the throttle, which opens and closes the carburettor slides, accelerating and decelerating the engine. There is an adjustment that sets the base position of the throttle cables. They should be synchronised.