You've got the right idea if when you say center screw you are meaning the adjustment on the right hand side behind the little round cover.
The following is a saved post from Tim in N.Z.; in my files that has served me well.
- Mike
< > < > < > < > < > < >
"I have found the clutch adjustment very very fine... A difference of 1/8 of a turn on the adjustment nut makes the difference between slip and drag. Make sure that you cable is not kinked, well lubricated, and runs as straight as you can possible achieve. Do not over fill the primary case with oil; just enough oil to cover the bottom run of the chain on the clutch chain wheel. Do not use engine oil! Loosen off the cable adjuster, adjust the push rod clearance so that it just touches the rod, back it of 1/8 of a turn and carefully tighten the lock nut. Then take up the slack in the cable.
Assuming that your plates are still serviceable,
if it still slips, back the adjuster off another 1/8 of a turn. If it drags when hot, increase by 1/8 of a turn.
Tim N.Z. "