JLEMOND, I have fooled with this screw a bit on my truck, as someone already had it way out of whack, probably trying to keep the smoke down, which was caused by the plugged-SOLID air filter. You would think it would be easier to buy a $12 filter, but no. The best setting I found (probably not right, but it works) it to find the spot where the truck will smoke enough to see it in the mirror when full throttle at 3000 rpm. I have a small convex mirror on my side mirror so I can see the exhaust pipe. My goal was to give it as much fuel as it could cleanly burn under most local driving conditions. At this setting it starts to smoke pretty heavy at anything over 3000 rpm. I rarely rev it over that, as it doesn't make much difference anyway. I have tried turning it up more and it caused lots of smoke, with little increased power, and lots of annoyed looks.