I have a 75 280Z which supposedly had significant motor work done, the extent of which I do not know. It has custom 6-2-1 headers and what appeared to be new parts under the valve cover as there was very little to no signs of wear. I took off the valve cover and checked tappet clearance. I timed the car according to the factory spec. but it has a little backfire when the throttle is released especially at higher rpm's. I think it's out of time slightly but I can't remember which direction i should go to try n fix this. Timing? or should I look at other things as well?