Is there a possibility that you are 1 tooth off on your distributor? If not, excessive wear in the cam timing (chain or gears) will cause it to either ping on backfire thru the exhaust.
To check your cam timeing for slack or excessivew gear play, take off the distributor cap, roll the engine backwards until the timing mark reads zero, then have someone watch the rotoras you slowly rotate the engine the other direction. when the rotor starts to move, stop and read the timing index. if it is more that 6 degrees, install a new timing set. This will greatly improve both power and economy.
Good luck
