I had a 283 that didn't backfire through the carb, but (coughed). It ran fine at idle, but past idle it would (cough). It was caused by a bad exhaust lobe on the camshaft. The exhaust valve wasn't opening, so it would send the exhaust gas up the carb when the intake valve opened. Found the problem by taking the valve covers off and watching the rocker arms as the engine ran.
