usually its the linkage not closing 100% to idle...make sure you have a gap on your throttle end stop at neutral and that your linakeg spring has enough pressure

second most common is the idle gap is too large...close down the idle gap a little

third most common is the HSH is a little lean..richen the HSn a touch

4th most common is your plug is too hot...try a cooler plug

5th most common is the pipe is too restrictive, install a better flowing pipe..