Welcome Back!
I'm sure you can adjust the slack out of the throttle cable at the throttle.
As far as the sputtering, i'm positive that is because the idle mixture is probably set too lean. You will need to drill out the brass or aluminum cap plug over the idle mixture screw to richin up the idle mixture. More than likely the main jets are under sized as well.
The shifting issue is probably because of a fast idle that is engaging the auto clutch. Lower the idle a little and that should clear that issue up.