carburetor main jet screw and a/f screw
 

hi,

i am a user of a lancer glx 2000 sohc carb engine. i tried adjusting the main jet screw and a/f screw to the lean best idle.

i would try leaning out first the main jet screw, then adjust the a/f screw to compensate. this in turn would yield to lesser fuel delivery to the intake manifold.

would it hurt the engine if i do it this way? its like i am supplying less a/f mixture to what is prescribed by the manufacturer.

You could certainly hurt the engine. Are you monitoring A/F ratios in any way? Trying to make a "lean burn" engine out of one not designed for it will only result in overheating and severe detonation.

ok, here comes the confusion, that even i am confused with...

i am still running at an a/f mixture of approximately 14.7:1 as seen from the o2 sensor and emission tests. i do not experience preignition, knocking, pinging, and detonation or any signs of engine misfiring.

what i actually did is decrease the total amount of the a/f mixture.

let's put it this way "hypothetically". let's say my engine requires 5 grams of a/f mixture that is 14.7:1 to enter one cylinder to compress and burn. by adjusting the main jet and a/f screw appropriately, i decreased the a/f mixture which is still 14.7:1 to just 2 grams. take note that the grams that i am mentioning is just hypothetical.

will decreasing the total a/f mixture (even though 14.7:1 is maintained) hurt my engine?

What you're saying isn't possible. If you are reducing the amount of fuel, you are increasing the A/F ratio. It's as simple as that.

However, if the engine has a "feedback" carburetor that is adjusted by a computer, you're probably only adjusting the absolute limit, which the A/F isn't reaching because the computer has sufficient adjustment left.

ok, so what then can i do to improve mileage? that is besides changing my driving habits and tune up?

Those are the two main ones, but there are plenty of other suggestions here on the board, such as weight reduction, aero mods...