The pilot screw on your carb controls the idle to about 1/8 – 1/4 throttle position.
On a multi-carb motorcycle, you’ll need to adjust all the pilot screws properly to sync the idle speed.
How to Make Air/Fuel Screw Adjustments
The fuel screw adjustment is what you’ll use to adjust your motorcycle’s idle to about 1/4 throttle range.
You’ll want to make adjustments to the fuel/air screw only after the engine has been properly warmed up.
To begin, you’ll want to start with the fuel screw in the recommended factory setting (typically about 1 1/2 to 2 turns out, but refer to the shop manual).
Use a small-head screwdriver and be gentle when adjusting these as the head can easily strip and the tips can easily break.
To adjust the pilot screw:
- Warm up the engine to normal operating temperatures.
- On air-cooled engines, it’s a good idea to run a fan when you’re doing this.
- Set the air screw to its factory position (about 1 1/2 to 2 turns out).
- Have the engine idling at a low RPM.
- Slowly turn the pilot screw about 1/4 turn at a time until you reach the smoothest idle near the recommended RPMs without stalling or stumbling.
Once you get the idle to a relatively smooth position, you can sync up the carbs at idle and at higher throttle ranges for finer tuning.
What is the Pilot Screw or Air/Fuel Screw?
The pilot screw, which is typically made of brass, is long and skinny with a tapered end and a spring. It typically sits on the outside of the carb body.
The exact location of the pilot screw will determine whether it controls the air flow or the fuel flow.
- Pilot screws on the airbox side of carb control air flow.
- Pilot screws on the cylinder side of the carb control fuel flow.
Lean or Rich Pilot Screw?
You can tell whether you need to rejet your pilot jet depending on how many turns your pilot screw needs to get your bike to the proper RPM at idle.
You make the adjustments by gently seating the pilot screw and turning it out to the factory recommended position (typically 1 1/2 to 2 turns out).
- An air pilot screw that is turned more than 1 turn in often indicates a lean pilot jet.
- A fuel screw more than 3 turns out indicates a lean pilot.
- An air screw further than 2 1/2 turns out indicates a rich pilot.
- A fuel screw more than 2 turns in indicates a rich pilot.
Learn more about lean vs. rich running conditions.