If the engine is still at factory settings it is likely it is too rich with too much idle gap....This would cause it to to want to "run On" when you let off the trigger....I would suggest closing the idle gap and leaning the LSN slightly..... Now this is just a guess from reading what you posted, a lean motor makes a sound almost similar to what a rich motor with too much idle gap ill make when letting off the trigger....How the engine idles and takes off will tell you if its lean or rich...a lean engine will have a very fast almost runaway idle...a rich engine will want to stall out at idle and will sometimes flame out as you apply trigger.... Truly the best bet is to shoot a short video of the engine....do some starts and stops in front of the camera.... varying the amount of time between starting and stopping..... Then after the stop take the machine for about a 100 foot blast wide open, then blast back wide open and stop right in the front of the camera..Vary the time of idle from a instant stop and go, all the way to a 10 second stop and go...by watching this on video we all can help you dial it in...