How to shim the cush drive
The rear sprocket carrier is not rigidly coupled to the rear wheel; it "floats", with a piece of hard rubber taking up the slack. If there is too much slack, you'll feel excessive drive lash when applying and releasing throttle. Adding shims takes up some free play and reduces the lash.
A new cush drive is $42 plus S&H at Ron Ayers, but even a new one can have more play than desired.
With the rear wheel off the bike and properly supported (put it up on some 2x4s or an old 13" auto tire to avoid damaging the brake rotor on the other side) wiggle the sprocket carrier off. If you've never taken it off, gunk may be holding it on. Just twist and wiggle, and it'll come off.
Now, find a suitable material for the shims. Margarine tub lids, milk bottles, or anything similar should work pretty well. Cut out 12 rectangles, each roughly 5/8" x 1 1/8". Depending on how much slack you have, start out with one or (usually) two shims in each crevice, for a total of 24. Some slack may come back later; then you can add 12 more.
Another material that works great is silicone cutting board. Look at a kitchen supply store; should be around $5. It's quite a bit thicker than milk jug material, but still thin enough to cut with scissors.
Now simply insert a piece into each of the 12 crevices between rubber and steel.
Put the sprocket carrier back in place (it may be a little tight now; simply press and wiggle) and reinstall the wheel. You should notice a much smoother ride. Members report that the difference is noticeable. Acceleration and deceleration are a lot smoother and not as "jerky." It's a good modification to do and very easy to perform.