There can be several things. idle speed set too high, stuck advance rod, air leaks as well as other carb issues.
Open the right side round cover on the head. In behind this cover is the advance mechanism. Gently pull the weights open, release them, they should snap shut.
There are links to videos of this around.
If sluggish pull the rod, clean the rod and the hole through the cam and apply new grease.
Leo