Furnace rough ignition.?

I have an old Tappan furnace its from the 70's lately it has started to have an ignition problem. There is the pilot light then the click from the gas valve. The first two burners ignite just fine but the 3rd one can take up to 7 seconds then it ignites when a very loud bang what can cause this and is this dangerous also there is alot of what seems like ash on the exhaust part of the furnace.