engine backfire when slowing down

[bonanza7369]

I have a 56 with 292 engine 4 barrel tea pot carb. the carb was rebuilt last year. I am getting some backfiring thru the exhaust when engine braking.
The car has glasspack mufflers.
Could this be caused by a to rich fuel mixture or possibly timing problem.
Engine runs smooth and accelerates well. It also is hard to start after it sits for a couple days(takes a lot of pumping of the gas pedal)