RE: Welcome to Club SAITO !
The 4-stroke backfire is caused by a lean fuel condition. This could have been a temporary or one time thing due to a bubble in the fuel line or whatever, or it could be that the tank is now in a different position relative to the carb or something related to it's new home.
The safest thing to do might be to slightly richen the needles until it can be fully warmed up and re-tuned.
Ernie Misner