How to Tip - 4 stroke fuel screw secrets
by Mike Fraser
23 June 2004

Jetting a 4 stroke can be a tedious process, especially if you don't know where to start. This may become a multi-part article because dialing in the jetting of a thumper requires a lot of know how. This week we'll explore the world of the FUEL SCREW.

The first thing you must understand when it comes to jetting a four stroke is understanding which jets control the fuel flow at a particular throttle position.

There are three main circuts in your thumper's carburetor. You have your Main jet, then the needle jet, and the Pilot/Fuel screw jet.

Each jet comes into play depending on where your throttle is positioned as you turn it to accelerate. Each circut overlaps the next by a bit. Here is how they work as a general rule.

Throttle postions : Jetting Circut
Closed to 1/4th open : Pilot Jet, which is controlled by your fuel Screw on the bottom of your carb.
1/4 - 3/4 : Needle Jet and Needle
1/2 - Full : Main Jet