I know this is a thread about a Rochester B, and I think everyone knows my opinion about them
But not sure that is the culprit here. Wouldn't want the B to get unwarranted abuse
However, a backfire through the carburetor strong enough to blow off the air cleaner is virtually never the fault of the carburetor. Virtually always valve issues or ignition. Locate top dead center, then make your timing mark, and time according to specifications.
Unless you have done one recently, a compression test would be in order to check for a sticky or burned intake valve.
If you think about it; you get a backfire through the carburetor WHEN there is fire in a cylinder AND an intake valve is open.
Jon.
