What should the pilot screw be in a Mikuni HS40?
The pilot screw is used to control idle mixture. Turn the screw out to richen the idle mixture. Turn it in to lean the mixture. The engine should have a smooth, steady idle with the screw between 1/4 and 3-1/2 turns out from fully bottomed (gently!). If the engine requires more than three turns out, the pilot or pilot air jet may be too lean.
What is purpose of fuel pump in Mikuni HS40?
The purpose of the pump is to inject fuel into the throat of the HS40 when the throttle is opened. As the throttle is opened, especially at low rpm, air velocity through the carburetor drops and the mixture naturally becomes leaner. Fuel from the pump maintains a more correct fuel/air mixture until the air velocity returns to normal.
How can I change the idle circuit on my Mikuni?
The idle circuit can be adjusted by changing either the pilot or the pilot air jet. A one-size larger pilot jet will have nearly the same effect as a one-size smaller pilot air jet. It is generally easier to change the air jet since it is more accessible.
What happens if the float level is too high on a Mikuni?
If you have doubts, check the float level and re-adjust if needed. A float level that is too high can make fine tuning of the idle circuit impossible. One that is too low will have a similar effect on mid-range tuning.
The pilot screw is used to control idle mixture. Turn the screw out to richen the idle mixture. Turn it in to lean the mixture. The engine should have a smooth, steady idle with the screw between 1/4 and 3-1/2 turns out from fully bottomed (gently!). If the engine requires more than three turns out, the pilot or pilot air jet may be too lean.
The purpose of the pump is to inject fuel into the throat of the HS40 when the throttle is opened. As the throttle is opened, especially at low rpm, air velocity through the carburetor drops and the mixture naturally becomes leaner. Fuel from the pump maintains a more correct fuel/air mixture until the air velocity returns to normal.
The idle circuit can be adjusted by changing either the pilot or the pilot air jet. A one-size larger pilot jet will have nearly the same effect as a one-size smaller pilot air jet. It is generally easier to change the air jet since it is more accessible.
If you have doubts, check the float level and re-adjust if needed. A float level that is too high can make fine tuning of the idle circuit impossible. One that is too low will have a similar effect on mid-range tuning.