There is a write-up (by Chef if I remeber correctly) on how. There is a tab that has to be removed if it hasn't been already that allows the A/F screw to be adjusted. If it has been rebuilt by you or another, the screw with the tab has been replaced by a straight end with blade type slot and non-tabbed one usually. The screw is on the lower front side of the 250 carb and it takes a small stubby blade type screw driver. The write-up said to get it idling and warmed up. Be sure the choke is completely disengaged. Set the slow idle adjuster where you want it. Then turn the A/F screw either way until the sputtering/stammering barely begins. Then turn it the opposite way counting the full turns. When it begins to stammer again, stop. Then turn the opposite way One-Half the total turns you counted between "spasms". That'll get you close enough to ideal and no special tools or gauges required.