My new KQ just suddenly started backfiring VERY loudly sometimes when I get out of the throttle at a good speed and let the motor help me to slow down along with braking. It also will make a soft backfire if I rev it up a little in neutral, and the last symptom it has is when I get it up to temp and let it idle it will purr like a kitten and all of a sudden just shut off intermittently. I already bought a new plug, gapped it, installed it and if anything it made it worse. I did have the 100 mile service done not long ago and it just turned 400 miles today so it leads me to believe that it may be running just a little lean but I wanted to run by everyone here to see if anyone else has had this happen and how you remedied it.

 

I think your 100% correct on your lean idea. Sounds lean too me also.

 

Check for an intake leak.
Start with the carb/air box connection, and the air filter/air box connection.