Think of timing in a more general way. If a motor is wired and tuned perfectly a mixture of fuel and air enter the cylinder with the intake valve open, both valves close and the piston is near or at top dead center, the spark plugs arcs and ignights the gas/air mixture dirving the piston down, than the exhaust valve opens and the spent gass is released. If during the firing stroke(powder stroke) the intake valve is still open(timing) or stuck open, part of the powder is leaked back up through the carb. This is what is happening. By timing I mean something is happening out of order, which is what timing really is. A backfire through the carb occurs because an intake valve is open when it should not be. It has to be miss a adjusted intake valve, a stuck intake valve or an intake valve open when it should not be open. Have your valves adjusted. This is a part of a good tune up. Be certin the plug wires are in the right place. If you recently had a timing belt replaced I would check to see all the marks line up.
ooooooh okay. i see what your getting at now. now im sure that the wires are right, and that the all the marks are inline (was there watching the hole time) basically we did everything but touch the carb. so if the valves need to be adj, were are they located on the carb?