COL Tom

My 1930 Ford has a problem that I cannot seem to figure out. She idles fine. When I take her out, she runs smoothly for about 5-10 minutes, but then begins to backfire constantly. I have changed the condenser and the points. The gap is good. The timing is good. I have checked the fuel mixture to ensure its not too lean. I have loosened the gas tank to ensure there is no air vacuum. I have changed carburetors. Nothing seems to work. Again, she seems to run fine initially (and also when idling, even when warmed up), but once she warms up, the backfiring begins when you are driving around. Does anyone have any other ideas?