Stihl SR 400 Instruction Manual - Page 21

HL If you don't have a tachometer, do not set the highspeed adjusting screw any leaner by turning it beyond the standard setting. L 353BA023 Exaust emissions are controlled by the design of the fundamental engine parameters and components (e.g. carburation, ignition, timing and valve or part timing) without the addition of any major hardware. The carburetor comes with a standard setting. This is the optimum setting to ensure your machine will operate reliably with the lowest possible emission under most operating conditions. Slight correction be necessary. Use a to set the high speed adjusting screw to obtain the highest possible engine speed. From that position, turn the high speed adjusting screw 1/8 counterclockwise (richer) or as far as stop. Avoid risk of engine damage which can be caused by lack of lubrication and overheating - • Check air filter and clean it if necessary. Set idle speed correctly. •• Warm up the engine. Turn the high speed adjusting screw and low speed adjusting screw clockwise (leaner) at higher altitudes. Turn the screws very slowly and carefully - slight changes have a noticeable effect on the engine's running behavior. If it is necessary to readjust the carburetor from scratch, first carry out the standard setting. Turn high speed adjusting screw and low speed adjusting screw counterclockwise (richer) or as far as stop. The high speed adjusting screw alters the power output and maximum off-load engine speed. If you turn this screw too far clockwise and make the setting too lean, there is a risk of engine damage. LA It is usually necessary to change the setting of the idle speed adjusting screw after every correction to the low speed adjusting screw . Turn the idle speed adjusting screw clockwise until the engine runs smoothly. Idle setting is too lean. Turn the low speed adjusting screw counterclockwise until engine runs and accelerates smoothly.