1990 FLHTCU, this happens maybe 20% of the time. If I am on the bike for around an hour (or until its nice and hot) and I shut down and then restart within 10 minutes I sometimes get a nice compression fart out of the carb. I have heard it numerous times on other bikes. To me its sounds like excess fuel in the carb. The bike always starts and runs fantastic.
My motor is 10 over with S&S pistons, rings, S&S roller lifters and an EV13 cam.
I am not concerned about it. Just wondering. It only does this when the motor is fully warmed up.