holden 179 engine in 1949 vauxhall. has extractors, holly carby, electronic distributor 4 speed manual

Engine starts first time and idles great .I can tap excellerator and it revs fine. If I excellerate slowly the engine coughs and splatters and back fires. I put new plugs, same thing happens. I tried old spare leads same thing. Checked timing and no change.The car drove fine all week until the last 100 metres to home. Any ideas? Thanks Keith