Well, I am not sure about his use of the word "backfire" either. I have rarely heard a gas two stroke engine actually backfire unless it has an ignition malfunction, though a four stroke engine will readily do so if it is too lean, especially upon acceleration.

However, part of the mystery with this particular case could be this; sometimes when a carb sits for a good while, the regulator diaphragm dries out. Sometimes not all the way dried out so that it is so stiff that it won't work, but enough that the diaphragm is stiffer than it should be. This can change carb settings and/or cause erratic running. Then sometimes once the diaphragm has been wet with fuel for a while, it limbers up and the necessary carb settings change again; usually back to more "normal" settings. I've seen this fairly often, though anytime I have a suspected carb problem I just pull it right off and completely rebuild it, including verifying all the passages will flow, etc. It doesn't take much time, nor cost much, and then you know it is "good to go".

AV8TOR