Okay, I had mine do a similar thing but only at idle & when I reved it up, it done that back fire blowing thing out the exhaust. Water had gotten in when washing & blocked the pilot jet. You said yours was clean, make sure the venturi isn't blocked or restricted either. I cleared it by revving it up & sticking my hand over the right side carby, sucking the water through.
Sent from my ZTE T84 using Tapatalk