Hy tool, have a look under the other valve cover as well to check for the same problem on the other side. Take your distributor cap off and turn the engine with the balancer retaining bolt have some one watch the rotor and tell you when it is moving and when it stops, once you have turned it in one direction for a bit reverse the direction and make sure the rotor re-acts instantly to the change of direction, if it does not react instantly that indicates a worn timing chain/gears. Your backfiring could indicate an intake valve is still open when the cylinder is firing, or that an exhaust valve has not opened enough to let all the burnt charge out of the cylinder before the intake valve opens for the next cycle, hope that helps.
