Take out a pilot screw, and see there's a hole drilled thru it. THAT is the problem, if you solder up that hole and trim off the excess neatly, you will be able to get an adjustment there!! With the holes drilled in 'em, no matter where you turn the screw, air still pours thru and it won't adjust. '77's and '78's have a needle screw there so don't need ths mod. Also any other method of securely plugging the hole will do the job.