My Oliver 1600 was running fine that last time I ran it.  When I went to start it a few days ago it started by when I started to drive it the engine bogged down and started to backfire through carburetor and then would stall out.  When I tried to start it yesterday, it would try over ok but would not fire.  I took the drain plug out of carb to drain anything that might be in there.  There was a good flow of gas, so I tried to start it that why and it started up without the drain plug installed.  It was still running rough, but started.  Once I tried to put plug back in it stalled out.  Any ideas??