The following carburetor schematic diagram apply for Mikuni HSR42, 45 and 48 series.
The HSR carburetor is divided into four interdependent systems:
1. Choke system
2. Idle system
3. Main system
4. Accelerator pump system
Each of these systems has its major effects in a different throttle range. While there may be some overlap, each system can generally be treated as though it is completely responsible for air/fuel mixtures within its range of throttle settings. Three of the systems have replaceable components that allow fine-tuning should the need arise. The choke system’s purpose is to provide the rich air/fuel mixture an engine needs to start and run reliably when cold. There are no replaceable tuning parts in the HSR choke mechanism. The HSR idle system has two tunable components: the Pilot Air Screw and the Pilot Jet. The air screw’s purpose is to fine-tune the mixture at idle. The pilot jet controls the total amount of fuel passing through the idle system.
Find more information about Mikuni HSR42|45|48 Carburetor Schematic Diagram here – http://www.mikuni.com/pdf/hsr_tuningmanual_021003.pdf – on page 16 to 19. (source: mikuni.com)