am i correct when is say that the fuel mixture screw if for fine tune adjustments of jetting. So (exagerated to prove point) say im a 100 main wiht fuel screw all the way out, would that be somwhat close to a 102 wiht screw all the way in. (or vice versa) and if thats correct, for a 300ex does turning the screw IN lean the mixture or richen. thanks alot

 

The fuel mixture screw is for the idle or pilot circuit. It controls the mixture for idle and throttle just off of idle only. The needle jet and jet needle are for mid throttle openings and the main jet is for 3/4 to WOT.
Don't try jetting the main circuit with the mixture screw, you're wasting your time.

 

yes, he is correct. the fuel skrew will do next to nothing when trying to tune in a full throttle run. the fuel screw is a fine tune adjustment for the pilot jet which are used mainly from idle to 1/8 throttle. the mainjet is used mainly from 3/4 to full throttle.

 

so if the quad is hard to start its probobly lean on the mixture screw, since when u start it its idling, so on a 300ex should i turn the screw out 1/8th turn or in 1/8th, and is 1/8th good or 1/4th thanks

 

The more you turn out the screw the more air the engine is getting.

 

this is only the case if your carb has an air screw. however, the 400ex has a fuel screw. the more you turn the screw out, the more fuel the engine is getting. i think the 300ex is the same way. i would turn it 1/4 and see what happens.

how to tell if it has air or fuel screw. look and see where the screw is at. if it is on the bottom pointing down, it is an fuel screw. the air screws will be found on the side of the carb.

 

If the screw is between the carb slide and the motor, its a mixture screw, and going out is richer.
If the screw is between the carb slide and the air filter, then its an air screw, and going out is leaner.
Heres an article to help you adjust and if necessary, rejet the carb.
jetting and adjusting
And heres one on carb circuitry
carb101