How-To: Size Your Pilot Jet, Pilot Screw Explained

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How do you tell if your pilot jet is too lean, rich, or just right?

The pilot jet is sized correctly if the pilot air/fuel screw(s) setting falls in the effective range, typically 1-3 turns out, with 1.5-2.5 being the sweet spot. This jet is also known as the slow jet, where it really impacts the 0-1/8 throttle positions.

 

What does a pilot air screw look like? What does it do?

Pilot Air Screw

A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.

 

What does a pilot fuel screw look like? What does it do?

Pilot Fuel Screw

A pilot fuel screw will also be made of brass, gold in color, and will have a pointy “needle tip”. That is why you always want to “lightly” seat these, the tips can easily break! I use the smallest diameter screwdriver so I generate the least amount of torque. The pilot fuel screw meters fuel, so turning it clockwise/in will lean the fuel/air mix, while turning it counter-clockwise/out will richen the fuel air mix.

 

Where is the pilot air/fuel screw located?

Let’s define the carburetor butterfly or slide the dividing line between the air box side, and the engine side. A pilot air screw will be located on the air box side, while a fuel screw will be located on the engine side. This is true for most carburetors. I’ve seen the pilot air screw on the engine side on most Amal carburetors. When in doubt, remove the screw and see what kind of tip it has, that will determine what it meters.

 

What are some indications of a lean pilot jet and/or screw setting?

  • If your air screw is in further than 1 turn, you need a richer pilot.
  • If your fuel screw is out further than 3 turns, you need a richer plot.
  • Off idle hesitation or bog.
  • Revving the engine in neutral will result in hanging RPM’s, or RPM’s will slowly drop back to idle RPM.

What are some indications of a rich pilot jet and/or screw setting?

  • If your air screw is out further than 3 turns, you need a leaner pilot.
  • If your fuel screw is in further than 1 turn, you need a leaner pilot.
  • Off idle heavy feel or sputter.
  • Revving the engine in neutral will result in RPM dip below idle RPM, RPM’s will drop quickly.
  • Turning the fuel screw all the way in will not stall the engine.

 

How do you properly adjust a pilot air/fuel screw?

The proper way to tune a fuel/air screw is to make adjustments on a fully warmed up engine and at LOW RPM. Lightly seat the fuel/air screw and set to factory settings as a baseline. With engine running at low RPM, turn the screw in clockwise until the engine begins to stumble, then turn the screw counterclockwise until the engine stumbles, noting how many turns in/out both occurred. Now, adjust between these two settings to achieve highest RPM and smoothest running engine. Reset idle to recommended settings and test ride motorcycle. If engine bogs or hesitates during acceleration, richen idle screws 1/4 to 1/2 turn and see if that helps. Rev engine in neutral and note how RPM’s return to idle. If RPM’s hang, richen the mix. If they dip below idle, lean the mix.

Here is a short video I put together to help explain this information along with a FUEL screw adjustment procedure.

 

 

Here is an example of a an AIR screw adjustment.

 

 

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