The factory setting is an average setting. . . it's a starting point. You can take 5 bikes off the assembly line and each one will not run exactly the same. Racing bikes are tempermental by nature and the 426 is an excellent example. That being said, first you need to locate this screw adjuster. It's located in a recessed area of the carb between the carb and cylinder head but still a part of the carb. Get on the shifter side of the bike and look just aft(toward the rear wheel
) of where the carb connects to the cylinder head manifold. There is a round looking tunnel with the pilot screw recessed up in there. It is held in place by spring resistance. It would help if you use one of those small automotive mirrors on a telescopic wand with a flashlight to first locate it. Pro Tec makes a dandy like screwdriver just for this specific screw jet. It IS a little hard to locate at first, but once you do, it becomes second nature after that. The rule of thumb is that if you have to adjust this fuel screw from the range of 1 turn out to 2 1/2 turns out, you need to change pilot jets. In other words....less than 1...or more than 2 1/2 turns out, a different pilot jet is in order. Once you find the proper pilot jet, it's perfectly normal to need small adjustments as the ambient temperature changes. What I have found to work best is to begin leaning it out slightly(clockwise) as the outside temp rises. Conversely for the opposite.
The most common mistake most guys make is that this screw is a fuel screw...not an air screw like on a 2-stroke. This is a 4-stroke pumper-carb.