Where should my fuel/air screw be?

[jsmith2232]

I did the stuff listed in my sig to my 400 ex with the jetting listed and the fuel screw 3.5 turns out the plug is a bit rich looking but it runs great, would adjusting the screw help anything, I guess i am looking for an explanation on what the mixture screw does and what part of the throttle position it is part of and where other guys are running theirs with similar mods to what I have.

[GPracer2500]

The fuel mixture screw only affects the pilot circuit. The pilot circuit is (primarily) active from 0 to about 1/8 throttle. Lets say you have a 42 pilot jet. Running the fuel screw at 1 turn out might allow the pilot to flow like a 41 pilot jet while running at 3 turns out might allow it to flow like a 43 pilot jet. Basically the fuel screw is used to fine-tune the pilot circuit. Further in (more closed) is leaner while further out (more open) is richer.

Normal fuel screw settings are in the 1 1/2 to 2 1/2 range. Anything very far oustide of that range and you could probably benefit from a larger or smaller pilot jet.

The pilot curcuit is less sensitive to air density changes (elevation, temperature, and humidity) than the other circuits (the main is the most sensitive). Once the correct size pilot jet is selected than all but the most drastic changes in the riding environment can be adjusted for using the fuel screw.

Many 400s seem to work well with a 42 pilot.

As a side note: almost all carbs have EITHER a fuel screw OR and air screw. Most four strokes have fuel screw carbs while most two strokes have air screw carbs. Fuel screws are found on the engine side of the carb while air screws are on the airbox side of the carb. Air screws work opposite from fuel screws, i.e. screwing it in richens the pilot circuit which is the opposite of how a fuel screw works.

[coryatver]

Witha 42 pilot put it at 2.5 turns out. If your jetting is off it is probley your main. Throw away that dynojunk and put in a kehien.